Web Security Context: User Interface Guidelines

Web Security Context: User Interface Guidelines
Web
Security Context: User Interface Guidelines
W3C Recommendation 12 August 2010
This version:
Latest version:
Previous version:
Editors:
Thomas Roessler,
W3C
Anil Saldhana,
RedHat
Please refer to the
errata
for this document, which may include some normative corrections.
See also
translations
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MIT
ERCIM
Keio
), All Rights
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rules apply.
Abstract
This specification defines guidelines
and requirements for the presentation and communication of Web
security context information to end-users.
Status of this Document
This section describes the status of this document at the time of its publication. Other documents may supersede this document. A list of current W3C publications and the latest revision of this technical report can be found in the
W3C technical reports index
at http://www.w3.org/TR/.
Please see the Working Group's
implementation report
To frame its development of this specification, the Working
Group had previously published a use case note
[WSC-USECASES]
. This specification
addresses most of the use cases and issues documented in that
note by documenting best existing practice, with the following
exceptions:
This specification does not include advice for web site
authors.
This specification does not provide advice to address
the issue explained in sections
9.1.2 Visually extending the chrome
and
9.2.7 Information bar (aka: notification bar)
Additionally, section
10.4 Implementation and testing
of
[WSC-USECASES]
articulated an
expectation that the recommendations in this specification
would be subject to usability testing, at least on a low
fidelity level, and that such testing would form part of the
Candidate Recommendation exit criteria. Resources available to
the Working Group at this point will not permit the group to
conduct extensive usability testing. At the same time, the
focus of this specification has shifted toward documenting best
existing practice.
This document was developed by the
Web Security Context Working
Group
. For a list of changes to this document since its
Proposed Recommendation version, please refer to the
diff document
. All changes were editorial in
nature.
Please send comments about this document to
public-usable-authentication@w3.org
(with
public archive
).
This document has been reviewed by W3C Members, by software developers, and by other W3C groups and interested parties, and is endorsed by the Director as a W3C Recommendation. It is a stable document and may be used as reference material or cited from another document. W3C's role in making the Recommendation is to draw attention to the specification and to promote its widespread deployment. This enhances the functionality and interoperability of the Web.
This document was produced by a group operating under the
February 2004 W3C Patent Policy
. W3C maintains a
public list of
any patent disclosures
made in connection with the
deliverables of the group; that page also includes instructions
for disclosing a patent.
Table of Contents
Overview
Acknowledgments
Conformance
3.1
Product classes
3.2
Language conventions
3.3
Conformance levels
3.4
Conformance claims
Interaction and content model
4.1
Overview
4.2
Terms and
definitions
4.2.1
Common User Interface elements
Applying TLS to the Web
5.1
Certificate Handling and
Information
5.1.1
Interactively accepting trust anchors or
certificates
5.1.2
Augmented Assurance Certificates
5.1.3
Validated Certificates
5.1.4
Self-signed Certificates and Untrusted Root
Certificates
5.2
Types of
TLS
5.3
Mixed
Content
5.4
Error
conditions
5.4.1
TLS errors
5.4.2
Error Conditions based on Third Party or
Heuristic Information
5.4.3
Insecure form submission
Indicators and Interactions
6.1
Identity
and Trust Anchor Signaling
6.1.1
Identity Signal
6.1.2
Identity Signal Content
6.2
Additional Security Context
Information
6.3
TLS
indicator
6.4
Error
handling and signaling
6.4.1
Common Error Interaction Requirements
6.4.2
Warning/Caution Messages
6.4.3
Danger Messages
Robustness Best Practices
7.1
Keep Security Chrome
Visible
7.2
Do not
mix content and security indicators
7.3
Managing User Attention
7.4
APIs
Exposed To Web Content
7.4.1
Obscuring or disabling
Security User Interfaces
7.4.2
Software Installation
7.4.3
Bookmarking APIs
7.4.4
Pop-up Window APIs
Security
Considerations
8.1
Active
attacks during initial TLS interactions
8.2
Certificate Status Checking
Failures
8.3
Certificates assure identity, not
security
8.4
Binding "human readable"
names to domain names
8.5
Warning
Fatigue
8.6
Mixing Augmented Assurance and
Validated Certificates
8.7
Dynamic
content might change security properties
Terms defined in this document
10
References
1 Overview
This specification deals with the trust decisions that
users must make online, and with ways to support them in
making safe and informed decisions where possible.
In order to achieve that goal, this specification includes
recommendations on the presentation of identity information
by user agents,
6.1.1
Identity Signal
. We also include recommendations on
conveying error situations in security protocols. The error
handling recommendations both minimize the trust decisions
left to users, and represent known best practice in inducing
users toward safe behavior where they have to make these
decisions. To complement the interaction and decision related
parts of this specification,
Robustness Best Practices
addresses the question of
how the communication of context information needed to make
decisions can be made more robust against attacks.
This document specifies user interactions with a goal
toward making security usable, based on known best practice
in this area. This document is intended to provide user
interface guidelines. Most sections assume the audience has a
certain level of understanding of the core PKI (Public Key
Infrastructure) technologies as used on the Web. The
following sections are relevant to all readers and do not
require a thorough understanding of PKI:
3 Conformance
4 Interaction and content model
6 Indicators and
Interactions
6.4 Error
handling and signaling
Robustness Best Practices
and
8.5 Warning
Fatigue
. Since this document is part of the W3C
specification process, it is written to clearly lay out the
requirements and options for conforming to it as a standard.
User interface guidelines that are not intended for use as
standards do not have such a structure. Readers more familiar
with that latter form of user interface guideline are
encouraged to read this specification as a way to avoid known
mistakes in usable security.
This specification comes with two companion documents:
[WSC-USECASES]
documents the
initial assumptions about the scope of this specification. It
also includes an initial set of use cases the Working Group
discussed.
[WSC-THREATS]
documents the Working Group's initial threat analysis. This
document is based on current best practices in deployed user
agents, and covers the use cases and threats in those
documents to that extent.
2 Acknowledgments
This specification is based on text from Mary Ellen Zurko,
Stephen Farrell, Anil Saldhana, Ian Fette, Michael McCormick,
Serge Egelman, Johnathan Nightingale, Yngve N. Pettersen,
Luis Barriga, Hal Lockhart, Tyler Close, Bill Doyle, Thomas
Roessler, as well as input and discussions from the active
members of the Web Security Context Working Group, primarily
Phillip Hallam-Baker, Mike Beltzner, Joe Steele, Jan Vidar
Krey, Maritza Johnson, Rachna Dhamija and Dan Schutzer. It
has also benefited from general public and working group
commentary on earlier drafts.
3 Conformance
3.1 Product classes
This specification addresses
Web user agents
as a product class.
Web user agents and user agents are used synonymously in
this specification.
This specification also addresses products that might
incorporate changes to a user agents, such as plug-ins,
extensions, and others; they are summarily called [
Definition
plug-ins] in this section.
Such products that might incorporate changes to the user
agent, e.g. through the addition or removal of features,
can render an otherwise conforming user agent non
conforming, or vice versa.
3.2 Language conventions
Throughout the specification, the RFC 2119
[RFC2119]
keywords MUST, MUST NOT,
SHOULD, SHOULD NOT, MAY are applied, with their respective
meanings.
3.3 Conformance
levels
A user agent conforms to this specification at the
Definition
: basic level] if it honors all MUST and
MUST NOT clauses of this specification.
A user agent conforms to this specification at the
Definition
: advanced level] if it also honors
all SHOULD and SHOULD NOT clauses of this
specification.
Conformance of a plug-in is defined in terms of the
conformance of the user agent that results when the plug-in
is added to a base user agent. E.g., if a given user agent
conforms to this specification on the basic level, and a
plug-in adds features that lead to conformance on the
advanced level, then this plug-in conforms on the advanced
level
with respect to this base user agent
3.4 Conformance
claims
A claim about a Web user agent's conformance with this
specification must state:
Whether basic or advanced conformance is claimed (see
3.3 Conformance
levels
What TLS
[SSLv3]
[TLSv11]
[TLSv12]
protocol versions and
algorithms are considered as
strong TLS algorithms
, and what
protocol versions and algorithms are supported in TLS
negotiation, but not considered
strong
What user interface element is the
TLS indicator
defined in this
specification.
What user interface element is the
identity signal
defined in
this specification.
What broadly accepted practices are considered
sufficient for a trust anchor to be deemed augmented
assurance qualified (see
5.1.2
Augmented Assurance Certificates
), and what data
elements are deemed assured by those certificates.
What features beyond the claimed conformance level
the user agent conforms with.
A claim about a plug-in's conformance with this
specification must include all of the above, and also
identify the base user agent with respect to which
conformance is claimed.
4 Interaction and content model
4.1 Overview
This specification assumes a human user interacting with
Web user agent
interacting with Web resources. Many of the requirements
specified are focused on the presentation of security
context information to the user, and therefore directly
relate to user interfaces. Where requirements or techniques
are specific to certain modalities, these are made
explicit, and are part of the preconditions for applying
the requirement or technique.
When this specification speaks of a Web user agent to
describe the application through which a user interacts
with the Web, then this term is used on a conceptual level:
No assumption is made about implementation details; the
"Web user agent" may denote a combination of several
applications, extensions to such applications, operating
system features, and assistive technologies.
A common user agent will therefore be a web browser with
some number of plug-ins, extensions, call outs to external
systems which render particular document formats, and
assistive technologies.
This specification makes no specific assumption about
the content with which the user interacts, except for one:
There is a top-level
Web
page
that is identified by a URI
[RFC3986]
. This Web page might be an HTML
frameset, an application running on top of a proprietary
run-time environment, or a document in a format interpreted
by plug-ins or external systems served as part of a Web
interaction. The page's behavior might be determined by
scripting, stylesheets, or other mechanisms.
In interactive Web applications, the presentation to the
user might also depend on state that is local to the client
- be it local storage of structured data, or be it recent
interactions with the Web page. The security properties of
those data will depend on the security properties of the
client computer itself, and are out of scope for this
specification.
Some requirements are expressed in terms of user
interface components commonly found in current-generation
Web user agents
4.2 Terms and
definitions
is expected to be consistent with the
Web Content Accessibility Guidelines Version 2,
[WCAG20]
4.2 Terms and
definitions
Definition
: A
Web User Agent
is any software
that retrieves and presents Web content for users.]
Definition
: A
Web Page
is a resource that is
referenced by a URI and is not embedded in another
resource, plus any other resources that are used in the
rendering or intended to be rendered together with it.]
4.2.1 Common User
Interface elements
This section defines terms for user interface elements
commonly present in
Web User Agents
Definition
Primary User Interface
denotes the portions of a
Web user agent's
user interface
that are available to users without being solicited by a
user interaction.]
Examples of primary user interface include the
location bar in common Web user agents, the "padlock"
icon present in common Web user agents, or error pages
that take the place of a Web page that could not be
retrieved.
Definition
Secondary User Interface
denotes the portions of a
Web user agent's
user interface that are available to the user after they
are solicited by a specific user interaction.]
Examples of secondary user interface include the "Page
Information" dialog commonly found in Web user agents,
and the "Security Properties" dialog that can obtained by
clicking the padlock icon in common Web user agents.
We occasionally use the term [
Definition
chrome
] to
refer to the representation through which the user
interacts with the user agent itself, as distinct from
the accessed web content. This includes both primary and
secondary user interface.
Definition
Location Bar
is a widget in a
Web user agent's user interface which displays (and often
allows input of) the textual location (entered as a URI)
of the resource being requested (or displayed - after the
response is received).]
Definition
Identity Information
is information about the web
site which is used to present the identity signal.]
5 Applying TLS to the Web
5.1 Certificate Handling
and Information
Public key certificates (see
[PKIX]
) are widely used in TLS
[SSLv3]
[TLSv11]
[TLSv12]
, but have been the basis
for the generation of many inappropriate warnings and other
dialogs for users. This section describes some
modifications to current certificate processing aimed at
improving this aspect of handling web security context and
defines some new terms describing various cases related to
certificate handling in user agents.
User agents can base their acceptance of certificates
that are presented by Web servers on various sources,
including user action, previous interactions involving the
same certificate, or, as more traditionally assumed, on
validation of a certificate chain where each certificate is
issued by a Certification Authority (CA). The practices
used by CAs (and the information attested) vary by CA and
are not available in any useful sense to Web user
agents.
5.1.1
Interactively accepting trust anchors or
certificates
A trust anchor represents an authoritative entity
represented by a public key and associated data. The
public key is used to verify digital signatures and the
associated data is used to constrain the types of
information for which the trust anchor is authoritative.
Relying parties use trust anchors to determine if
digitally signed information objects are valid by
verifying digital signatures using the trust anchor's
public key and by enforcing the constraints expressed in
the associated certificate data.
Trust anchor installation is typically handled by user
agent vendors, systems administrators and device
manufacturers, based on out-of-band information. Note
that updating trust anchors is therefore often handled as
part of user agent or operating system software
updates.
However, current user agents sometimes support the
interactive acceptance of a trust anchor during a
session, based on user interaction. Most users cannot
sensibly decide how to handle such interactions.
Similarly, end-entity (e.g. web server) certificates
that cannot be currently verified using the Basic Path
Validation algorithm may trigger current user agents to
offer the user a choice to accept the certificate in any
case, sometimes for a single session, sometimes for all
future sessions involving that certificate, possibly
scoped to specific host and port combinations.
Definition
: A trust anchor or certificate is
interactively accepted
if the acceptance was
caused through a user interaction that happens while the
user is focused on a primary task unrelated to trust and
certificate management.]
For example, if a web server certificate is presented
for acceptance by a user during ordinary Web
interactions, and is accepted by the user, then this
matches the test for interactive acceptance. If, however,
a systems administrator (or user) adds a trust anchor's
certificate to an agent's store of trust roots, then that
certificate is not considered interactively accepted.
5.1.2 Augmented Assurance
Certificates
Some trust anchors adhere to documented broadly
accepted practices (e.g.
[EVTLSCERT]
). These involve some level of
guarantee that certificates chaining up to those roots
embody augmented assurance and can therefore be treated
more favorably in terms of the primary security
indicators. We call such certificates "Augmented
Assurance Certificates".
Definition
An
Augmented Assurance Certificate
is a public key
certificate where the issuer asserts that the subject
entity has been authenticated by means of some process
that adheres to the requirements of an augmented
assurance specification supported by the user agent. The
certificate chain for such a certificate MUST be
validated up to a trust root that is recognized as
augmented assurance qualified by the user agent.]
This specification does not define what an "augmented
assurance qualified trust root" is, except to note that
this designation is made by user agents through an out of
band mechanism consistent with the relevant underlying
augmented assurance specification.
Marking a trust anchor as augmented assurance
qualified is a security-critical step and most often will
involve the use of some application-specific out-of-band
mechanism.
Implementations MUST NOT enable users to
designate trust roots as augmented assurance qualified as
part of a unrelated interaction.
In particular,
the notions of an augmented assurance qualified trust
root and an
interactively
accepted trust
root are mutually exclusive.
In addition to the out of band designation process
described above, the trust anchor, and possibly all
certificates in a path chaining up to such a trust anchor
may need to be specially marked, e.g. through the use of
specific policy object identifiers.
The specific marking mechanisms to be used and the
special treatment to be afforded to such certificates are
out of scope of this document, but will typically be
covered by the underlying augmented assurance
specification. User agent implementers determine the set
of such standards that they support and the associated
special treatment to apply, other than as outlined below,
where we impose some consistency requirements on the
handling of this type of certificate.
To derive a human-readable subject name
from an augmented assurance certificate, user agents
SHOULD use the Subject field's Organization (O) and
Country (C) attributes.
They MUST
use information that is subject to the certificate
authority's additional assurances, as documented in the
user agent's conformance statement.
Note: Should certificates arise in the future that
provide strong assurance of the holder's identity, but do
not include an organization attribute, then user agents
can make use of the additional assurance level and
identity information without violating this
specification. Such future certificates could, for
example, include high assurance certificates for
individuals.
5.1.3
Validated Certificates
The term [
Definition
validated certificate
] is used to denote a public
key certificate that has been verified by chaining up to
a locally configured trust anchor. The set of trust
anchors used by a given Web User agent is
implementation-dependent.
Since Augmented Assurance Certificates chain up to a
"special" trust anchor, all valid Augmented Assurance
Certificates are also validated certificates.
Certificates or certificate chains that are
pinned
to a particular
destination are
not
considered validated
certificates by virtue of being pinned.
The notion of a validated certificate in this
specification corresponds to the domain validated
certificate commonly deployed on the Web. This type of
certificate attests to a binding between a domain name
registration and a key pair; additional certificate
attributes are often not validated.
5.1.4 Self-signed
Certificates and Untrusted Root Certificates
Self-signed certificates (SSC) which are not trust
anchors by themselves are commonly used for appliances
and web sites catering to small groups of users, and
essentially serve as a container for cryptographic key
material in a key exchange that is not verified by any
third party. Certificate chains that lead up to custom
root certificates which are not part of the user agent's
store of trust roots are sometimes used similarly.
In both situations, use of TLS provides
confidentiality protection services against passive
attackers. No binding of a third-party asserted identity
to the cryptographic key is achieved. In both cases, the
certificates are not considered
validated certificates
Using Key Continuity Management
[KCM]
, user agents can use
self-signed certificates (or certificates that chain up
to an untrusted root) to determine that they are
consistently communicating with the same end entity,
thereby defending against active attacks as well. Simply
put, if a Web site consistently presents the same
self-signed certificate (or the same certificate chaining
up to the same untrusted root) to a client, then this can
be strong evidence that protection against an active
attacker has been achieved as well. Conversely, a change
of certificates for the same site can be evidence that a
man in the middle attack occurs -- or it can merely
indicate that the legitimate site has changed to a
different certificate.
User agents MAY support [
Definition
pinning
] a self-signed
certificate or more generally a certificate chain that
leads to an untrusted root certificate to a particular
Web site, to enable behavior based on recorded state
about certificates shown previously by the same
site.
Such behavior includes, e.g., warning users
about changes of certificates, and not showing warning
messages if a site shows a certificate consistent with
previous visits.
The interaction
that enables
users to pin a certificate to a destination SHOULD NOT
cause a self-signed certificate to be pinned to more than
one site, identified through URI scheme, domain, and
port.
The interaction MUST NOT
cause an untrusted root certificate to be accepted
automatically for additional sites.
5.2 Types of TLS
The most common mechanism for applying TLS to the Web is
the use of the
https
URI scheme
[RFC2818]
; the alternative upgrade
mechanism
[RFC2817]
is used
rarely, if at all. For the purposes of this specification,
the most relevant property of
[RFC2818]
is that the URI used to
identify a resource includes an assertion that use of TLS
is desired.
This specification uses the term [
Definition
HTTP transaction
] regardless of
whether any kind of TLS protection was applied; in
particular, the transactions arising when an
https
URI is dereferenced are subsumed under
this term.
[RFC2616]
Definition
: An HTTP transaction is
TLS-protected
if the resource was identified through
a URI with the https URI scheme, the TLS handshake was
performed successfully, and the HTTP transaction has
occurred through the TLS channel.]
Note that an HTTP transaction may be considered
TLS protected
even though weak algorithms (including
NULL
encryption) are negotiated.
Definition
: An HTTP transaction is
strongly
TLS-protected
if it is
TLS-protected
, an https URL was
used,
strong TLS
algorithms
were negotiated for both confidentiality and
integrity protection, and at least one of the following
conditions is true:]
the server used a
validated certificate
that
matches the dereferenced URI; or
the server used a self-signed certificate that was
pinned
to the
destination; or
the server used a certificate chain leading to an
untrusted root certificate that was
pinned
to the destination.
TLS modes that do not require the server to show a
certificate (such as the
DH_anon
mode) do not
lead to a strongly TLS-protected transaction.
The ability to provide privacy and secure the connection
between a user agent and web server is in part determined
by the strength and capabilities of the TLS protocol and
underlying cryptographic mechanisms. The TLS protocol is
versioned to keep pace with protocol features and cipher
suites that are available. Cipher suites are grouped
according to algorithms and the key length used by
cryptographic functions to provide cipher strength.
When this document speaks of [
Definition
: Strong TLS
algorithms], then the following must hold:
No version of the TLS protocol that suffers known
security flaws has been negotiated.
At
the point of writing of this document, versions of SSL
prior to SSLv3
[SSLv3]
MUST NOT
be considered strong.
A cipher suite has been selected for which key and
algorithm strengths correspond to industry practice.
At the time of writing of this document,
the "export" cipher suites explicitly forbidden in
appendix A.5 of
[TLSv11]
MUST
NOT be considered strong.
What set of algorithms is considered as strong by a
given implementation must be described in any conformance
claim against this specification, see
3.4 Conformance
claims
Definition
: An HTTP transaction is
weakly
TLS-protected
if it is TLS-protected, but strong TLS
protection could not be achieved for one of the following
reasons:]
TLS handshake used an anonymous key exchange
algorithm such as
DH_anon
the cryptographic algorithms negotiated are not
considered
strong
certificates were used that are not either
validated certificates
or self-signed certificates
pinned
to the destination (see
5.1.4 Self-signed
Certificates and Untrusted Root
Certificates
Weakly
TLS-protected
interactions may provide security
services such as confidentiality protection against passive
attackers, or integrity protection against active attackers
(without confidentiality protection). These properties are
often desirable, even if
strong TLS protection
cannot be
achieved.
5.3 Mixed Content
If a given Web page consists of a single resource only,
then all content that the user interacts with has security
properties derived from the HTTP transaction used to
retrieve the content.
Definition
: A Web page is called
TLS-secured
if the top-level resource and all other resources that can
affect or control the page's content and presentation have
been retrieved through strongly TLS protected HTTP
transactions. ]
Definition
: A Web page is called
mixed
content
if the top-level resource was retrieved through
a strongly TLS protected HTTP transaction, but some
dependent resources were retrieved through a weakly
protected or unprotected HTTP transaction.]
This definition implies that inline images, stylesheets,
script content, and frame content for a secure page need to
be retrieved through
strongly TLS
protected HTTP
transactions in order for the overall page to be considered
TLS-secured.
Any UI indicator used to signal the
presence of Augmented Assurance certificates MUST NOT
signal the presence of such a certificate, unless the page
is
TLS-secured
i.e., all parts of the page are loaded from servers
presenting at least a
validated certificate
, over
strongly TLS-protected
interactions
For relevant security considerations, see
8.6 Mixing Augmented
Assurance and Validated Certificates
5.4 Error
conditions
5.4.1 TLS errors
This section covers TLS-related error conditions, and
maps them to the classes of error handling interactions
(see
6.4 Error handling and
signaling
) that are used when these conditions
arise.
If multiple error conditions apply, the
most severe signaling level currently known MUST be used,
as defined in
6.4 Error
handling and signaling
When, during TLS negotiation, the certificate chain
presented by the server does not lead to a trusted root
certificate, and the certificate chain presented was not
pinned
to the
destination, the following applies:
Error signaling of class warning or
higher (
6.4.2
Warning/Caution Messages
6.4.3 Danger Messages
) MUST
be used to signal the error condition.
User agents MAY offer a possibility
to pin newly encountered certificates to the
destination.
Note that, when untrusted certificates are accepted
without user interaction, an additional exposure to
man-in-the-middle attacks is created. See
8.1 Active attacks during initial TLS
interactions
for a more detailed discussion of
this scenario.
When certificate information is
presented in the interactions described in this section,
then human-readable information from certificates MUST
NOT be presented as trustworthy unless it is attested.
E.g., a self-signed certificate's Common Name or
Organization attribute MUST NOT be displayed, even if
that certificate is pinned to a destination.
User agents MAY display this information in
a dialog or other secondary user interfaces reachable
from the warning or error messages specified
here.
When, during TLS negotiation, the
end-entity certificate presented or one of the
intermediate certificates in the certificate chain are
found to have been revoked, error signaling of class
danger (
6.4.3 Danger
Messages
) MUST be used.
When, during TLS negotiation, the
end-entity certificate presented or one of the
intermediate certificates in the certificate chain are
found to have expired, error signaling of class danger
6.4.3 Danger
Messages
) MUST be used.
When the URI corresponding to the
transaction does not match the end-entity certificate
presented, and a
validated certificate
is used,
then error signaling of level danger (
6.4.3 Danger Messages
) MUST be
used.
If TLS negotiation otherwise fails,
error signaling of level danger (
6.4.3 Danger Messages
) MUST be
used.
When TLS error conditions occur, user
agents MAY choose to abort the connection without any
further user interaction.
The guidelines in this
section apply when user agents choose to cause a user
interaction in the case of TLS error conditions. Note
that user agents may combine both practices: E.g., an
interactive approach may be chosen for the top-level
frame of a Web page, but a non-interactive approach may
be chosen for inline content. It is expected that the
XMLHttpRequest specification
[XHR]
will include a non-interactive approach as well.
5.4.2 Error
Conditions based on Third Party or Heuristic
Information
User agents that use third party
services or heuristic approaches to assess the possible
danger of a pending Web transaction MUST use error
signaling of class danger (
6.4.3 Danger Messages
) to
signal positively identified dangers, e.g., identified
malicious downloads or exploits of user agent
vulnerabilities.
To signal risks that are identified
with high likelihood, but involve further user decisions
(e.g., phishing heuristics were triggered), error
signaling of class warning or above (
6.4.2 Warning/Caution
Messages
6.4.3
Danger Messages
) MUST be used.
5.4.3 Insecure
form submission
Users interacting with a
TLS-secured page
are likely to
develop the impression that information submitted during
these interactions will be
strongly TLS-protected
User agents MAY warn users, using an error of
class Warning or higher (
6.4.2 Warning/Caution
Messages
6.4.3
Danger Messages
), if form submissions from a
TLS-secured page are directed to an unsecured
channel.
6 Indicators and Interactions
6.1 Identity and Trust
Anchor Signaling
This section specifies practices for signaling
information about the identity of the Web site a user
interacts with. All signals specified in this section are
passive. No claim is made about the effectiveness of these
signals to counter impersonation attacks.
6.1.1 Identity Signal
User agents MUST make information about
the identity of the Web site that a user interacts with
available.
This [
Definition
identity signal
] SHOULD be
part of
primary
user interface
during usage modes which entail the
presence of signaling to the user beyond only presenting
page content. Otherwise, it MUST be available through
secondary user interface.
Note that there may be
usage modes during which this requirement does not apply:
For example, a Web agent which is interactively switched
into a presentation mode that does not display any chrome
need not preserve security indicators in primary user
interface. On the other hand, a user agent such as a
smart phone, individual entertainment screen in an
airplane seat back, or TV set which has a usage mode that
makes minimal (but visible) chrome elements available
does need to preserve security indicators in such a
mode.
User agents with a visual user
interface MUST show the Identity Signal in a consistent
visual position.
Web Content MUST
NOT obscure security user interface,
7.4.1 Obscuring
or disabling Security User Interfaces
6.1.2 Identity Signal
Content
Information displayed in the
identity
signal
MUST be derived from
validated certificates
, or from
user agent state.
Web user agents
MUST NOT use information as part of the
identity signal
that is taken
from unauthenticated or untrusted sources.
During interactions with a
TLS-secured Web page
for which the
top-level resource has been retrieved through a
strongly TLS-protected
interaction that involves an
augmented assurance
certificate
, and for which all dependent resources
have been retrieved through interactions that involve
validated
certificates
, the following applies:
The
identity signal
MUST
include human-readable information about the
certificate subject, derived as specified in
5.1.2 Augmented Assurance
Certificates
, to inform the user about the
owner of the
Web
page
During interactions with a
TLS-secured Web page
for which the
top-level resource has been retrieved through a
strongly TLS-protected
interaction that involves a
validated certificate
(including an
augmented assurance
certificate
), the following applies:
If the identity signal does not
include any other human readable information about
the identity of the certificate subject (derived,
e.g., from an augmented assurance certificate), then
it MUST include an applicable DNS name that matches
either the subject's Common Name attribute or its
subjectAltName extension.
User agents MAY shorten such a DNS name by
displaying only a suffix.
To inform the user about the
party responsible for that information, the Issuer
field's Organization attribute MUST be displayed in
the Identity Signal, or in secondary user interface
that is available through a consistent interaction
with the Identity Signal.
Subject logotypes
[RFC3709]
derived from
certificates MUST NOT be rendered, unless the
certificate used is an
augmented assurance
certificate
Note that this behavior does not apply when
self-signed certificates or certificate chains that chain
up to an untrusted root certificate are used.
During interactions with a
mixed content
Web page, the
identity
signal
MUST NOT include any site identity information
beyond that which is in use for unprotected HTTP
transactions.
In this situation,
the identity signal MAY include indicators that point out
any error conditions that occurred.
During interactions with mixed
content, user agents MUST NOT render any logotypes
[RFC3709]
derived from
certificates.
6.2 Additional Security
Context Information
This section describes additional
security context information provided by
Web user agents
Where security context information is provided in
both primary and secondary interface, the meaning of the
presented information MUST be consistent.
Best
practice will also avoid inconsistent presentation, such as
using identical or semantically similar icons for different
information in different places.
The information sources MUST make the
following security context information available:
the Web page's
domain name
Owner
information, consistent with section
6.1.2 Identity Signal
Content
Verifier
information, consistent with section
6.1.2 Identity Signal
Content
The reason
why the displayed information is trusted (or not).
This includes whether or not a certificate was
accepted interactively
whether a self-signed certificate was used, and whether
the self-signed certificate was
pinned
to the site that the user
interacts with, and whether trust relevant settings of
the user agent were otherwise overridden through user
action.
The information sources SHOULD make the
following security context information available:
An explanation of
the information represented by the
TLS indicator
, e.g.,
concerning the presence mixed content;
If the Web page is
weakly
TLS-protected, then, what
conditions cause the protection to be weak (e.g., bad
algorithms, mixed content, ...)
Whether the user
has visited the site in the past.
Whether the
user has stored credentials for this site.
Whether the site
content was encrypted in transmission.
Whether the site
content was authenticated (e.g., server authentication
via TLS).
Additionally, the information sources MAY
make the following security context information
available:
When the user
first visited the site in the past.
How often the
user visited the site in the past.
User agents that provide information
about the presence or absence of Cookies
[RFC2965]
MUST NOT make any claims that
suggest that the absence of cookies implies an absence of
any user tracking, as there are numerous tracking and
session management techniques that do not rely on
Cookies.
6.3 TLS
indicator
User agents MUST make information about
the state of TLS protection available.
The [
Definition
TLS indicator
] SHOULD be part of
primary user interface during usage modes which entail the
presence of signaling to the user beyond only presenting
page content.
Otherwise, it MUST be
available through secondary user interface.
As in
the case of
6.1.1
Identity Signal
, there may be usage modes during
which this requirement does not apply.
Web
content MUST NOT obscure security interface, see
7.4.1 Obscuring
or disabling Security User Interfaces
User agents with a visual user interface
SHOULD make the TLS indicator available in a consistent
visual position.
The TLS indicator MUST present a
distinct state that is used only for
TLS-secured
Web pages.
The User Agent SHOULD inform users when
they are viewing a page that, along with all dependent
resources, was retrieved through at least
weakly TLS protected
transactions,
including
mixed
content
The user agent MAY accomplish this by
using a third state in the TLS indicator, or via another
mechanism (such as a dialog, info bar, or other
means).
6.4 Error handling and
signaling
This section defines common error interaction
requirements and, ordered by increasing severity, practices
to signal the following classes of errors:
6.4.2 Warning/Caution
Messages
6.4.3 Danger
Messages
User agents MAY communicate additional
indicators to users. E.g., a user agent could additionally
display a persistent indicator in a "danger"
situation.
For additional security considerations concerning
frequent warning messages, see
8.5 Warning
Fatigue
6.4.1 Common Error
Interaction Requirements
Error signaling that occurs as part of
primary user
interface
SHOULD be phrased in terms of threat to
user's interests, not technical occurrence.
Primary user interface
error
messages MUST NOT be phrased solely in terms of
art.
For example, if a certificate includes a DNS
name in the subjectAltName extension that does not match
the URI of the site that the user tries to visit, an
error message can explain that the user is reaching a
different site, instead of reporting a "subjectAltName
mismatch".
They SHOULD NOT tell the user to
enter the destination site that caused the error, e.g.,
to provide feedback or obtain assistance.
For error messages that interrupt the
user's flow of interaction, user agents SHOULD enable the
user to easily return to the page that the user was
previously interacting with.
Note that this
ideally implies returning to the previous user agent
state -- including the results of user input and dynamic
processing --; however, this may not be feasible under
all circumstances.
For advanced users, error
interactions SHOULD have an option to request a detailed
description of the condition that caused the error
interaction to occur.
The error interactions discussed in this section
typically involve communication of security context
information.
6.4.2 Warning/Caution
Messages
Warning / Caution messages are
intended for situations when the system has good reason
to believe that the user may be at risk based on the
current security context information, but a determination
cannot positively be made.
Warning / Caution messages MUST
interrupt the user's current task, such that the user has
to acknowledge the message.
Warning / Caution messages MUST
provide the user with distinct options for how to proceed
(i.e., these messages MUST NOT lead to a situation in
which the only option presented to the user is to dismiss
the warning and continue).
The
options presented on these warnings MUST be descriptive
to the point that their respective meaning can be
understood in the absence of any other information
contained in the warning interaction.
One of the options offered SHOULD be recommended,
and the warning message SHOULD include a succinct text
component denoting which option is recommended.
In the absence of a recommended option,
the warning MUST present the user with a method of
finding out more information (e.g., a hyperlink,
secondary window, etc) if the options cannot be
understood.
6.4.3 Danger
Messages
Danger Messages are intended for
situations when there is a positively identified danger
to the user (i.e., not merely a risk).
The interactions communicating
these messages MUST be designed such that the user's task
is interrupted.
These interactions MUST be presented
in a way that makes it impossible for the user to go to
or interact with the destination web site that caused the
danger situation to occur, without first explicitly
interacting with the Danger Message.
7 Robustness Best Practices
7.1 Keep
Security Chrome Visible
For visual user agents, agent chrome
SHOULD always be present to signal security context
information.
This requirement does not apply when
user interface is explicitly dismissed by the user.
7.2 Do not mix content
and security indicators
To the extent to which users pay attention to passive
security indicators at all, noticing and understanding them
is made difficult to impossible when the same signal path
that is commonly used for security indicators can also be
controlled by Web content. For example, the location bar
commonly found on agents is often used to both display the
"padlock" security indicator, and a possible "favorite
icon"
[FAVICON]
, which can in
turn be a simple copy of the very padlock an informed and
attentive user might look for.
Web User Agents MUST NOT communicate
positive trust information using user interface elements
which can be mimicked within chrome under the control of
web content.
Site-controlled
content (e.g. page title, favicon) MAY be hosted in
chrome
but this content MUST
NOT be displayed in a manner that confuses hosted content
and chrome indicators, by allowing that content to mimic
chrome indicators in a position close to them.
This
requirement applies to both
primary
and
secondary
elements of visual
user interfaces.
In particular, Web User Agents SHOULD
NOT use a 16x16 image in chrome to indicate security status
if doing so would allow the favorite icon to mimic
it.
7.3 Managing User Attention
When confronted with multiple modal interactions during
a short amount of time, users are known to exercise the
default option (e.g., by pressing the Enter key repeatedly)
until the sequence of modal interactions stops blocking the
user's intended interaction.
Definition
: An
Interaction flooding attack
refers to a Web site with the malicious intent of
performing an unintended action (e.g. installing software
that would have required an user intervention such as
clicking OK on a warning dialog) or by exploiting
distraction and task-focus. The Web site opens a large
number of new windows over the desired web content and the
malicious action is performed when the user tries to close
these new windows and/or clicks on a dialog that indicates
a trust decision. ]
User interfaces
used to inform users about security critical events or to
solicit input MUST employ techniques that prevent immediate
dismissal of the user interface, e.g., by using a
temporarily disabled "OK" button on user interfaces that
make such an interaction paradigm available.
When users interact with security relevant
notifications (
6.4.2
Warning/Caution Messages
and above), Web content
MUST NOT be granted control of the user agent's
interaction.
A Web User Agent SHOULD NOT display a
modal security dialog related to a web page which does not
currently have focus.
Security dialogs include
prompts for user credentials, script errors and TLS
errors.
7.4 APIs Exposed To Web
Content
User agents commonly allow web content to perform
certain manipulations of agent UI and functionality such as
opening new windows, resizing existing windows, etc. to
permit customization of the user experience. These
manipulations need to be constrained to prevent malicious
sites from concealing or obscuring important elements of
the agent interface, or deceiving the user into performing
dangerous acts. This section includes requirements and
techniques to address known attacks of this kind.
7.4.1 Obscuring or disabling Security User
Interfaces
Web user agents
MUST prevent
web content from obscuring, hiding, or disabling user
interfaces that display security context information,
except in response to a user interaction.
User
agents MUST restrict window sizing and moving operations
consistent with section
7.1 Keep Security
Chrome Visible
This prevents attacks
wherein agent chrome is obscured by moving it off the
edges of the visible screen.
User agents
MUST NOT allow web content to open new windows with the
agent's security UI hidden.
Allowing this
operation facilitates picture-in-picture attacks, where
artificial chrome (usually indicating a positive security
state) is supplied by the web content in place of the
hidden UI.
User
agents MUST prevent web content from overlaying
chrome.
This helps to ensure that user
interactions that are perceived to target agent chrome
are not redirected to Web applications.
7.4.2
Software Installation
This section covers web user agent APIs that allow web
content to download software for later execution outside
of the web user agent context.
Web user agents
MUST NOT
expose programming interfaces which permit installation
of software without a user intervention.
User
agents MUST inform the user and request consent when the
user agent is attempting to install software outside of
the agent environment as a result of web content.
The interaction used MUST follow the
requirements in
6.4.2
Warning/Caution Messages
User agents SHOULD NOT provide features which can be
used by web content to install software outside of the
agent environment without the user's consent.
7.4.3 Bookmarking APIs
User agents often include features that enable Web
content to update the user's bookmarks, e.g. through an
ECMAScript API. If permitted unchecked, these features
can serve to confuse users by, e.g., placing a bookmark
that goes by the same name as the user's bank, but points
to an attacker's site.
Web user
agents
MUST NOT permit Web content to add bookmarks
without explicit user consent.
Web user agents
MUST NOT
permit Web content to add URIs to the user's bookmark
collection that do not match the URI of the page that the
user currently interacts with.
7.4.4 Pop-up Window APIs
With visual user
interfaces that use a windowed interaction paradigm, User
agents SHOULD restrict the opening of pop-up windows from
web content, particularly those not initiated by user
action.
Creating excessive numbers of new pop-up
windows is a technique that can be used to condition
users to rapidly dismissing dialogs. This can be employed
in
interaction flooding
attacks
User agents
which offer this restriction SHOULD offer a way to extend
permission to individual trusted sites.
Failing to
do so encourages users who desire the functionality on
certain sites to disable the feature universally.
8 Security Considerations
8.1 Active attacks during
initial TLS interactions
Section
5.4.1 TLS
errors
leads to additional exposure during the
first
TLS interaction with a site, even if that
site uses validated or extended validation certificates: An
active attacker can show a self-signed certificate, which
will cause only weak warning signals to the user.
Traditional user agents react to this scenario with a
dialog box that interrupts the user's flow of interaction,
but gives users the ability to override the security
warning. Empirical evidence shows that this ability is
typically exercised by users.
Countermeasures against this threat include the prior
designation of high-value sites, for which extended
validation or validated certificates are required (causing
a stronger warning signal during the attack scenario
described above), and communication of certification and
TLS expectations by a mechanism separate from HTTP, e.g.
through authenticated DNS records.
5.4.1 TLS errors
refers to the pinning of a new certificate to a
destination. Note that this newly pinned certificate could
be the basis for a spoofing attack, or it could represent a
refresh of an Self Signed Certificate.
8.2 Certificate
Status Checking Failures
The TLS Errors (
5.4.1 TLS
errors
) section does not document intended behavior
for user agents when a certificate status check fails for
network or other non-revocation reasons. At time of
writing, the deployment environment for OCSP
[RFC2560]
status checking is fragile and
subject to frequent failures, so it is inappropriate to
require that user agents treat such failures as warnings or
errors. However, this creates a possibility for attack:
site operators using a fraudulently obtained, and revoked,
certificate may attempt to attack a CA's revocation
infrastructure as a way to suppress revocation errors. User
agent countermeasures for this vulnerability include:
exposing failures of certificate validation checks to users
as warning (
6.4.2
Warning/Caution Messages
) or danger (
6.4.3 Danger Messages
) level
messages; or refusal to load sites that fail these
checks.
8.3 Certificates assure identity, not
security
While TLS certificates of all types (i.e. self-signed,
validated, or augmented assurance) provide the means for
strong encryption of communications, they should not be
understood to be, or treated as, blanket security
assurances. In particular, validated and augmented
assurance certificates make guarantees about some level of
owner identity verification having been performed (see
definitions) but they do not represent any guarantees that
a site is operated in a safe manner, or is not otherwise
subject to attack. Historically, issues of security and
identity have been conflated by user agent interfaces which
present SSL/TLS connections as "secure," but implementers
of this specification are advised to be cautious and
cognizant of this distinction.
8.4
Binding "human readable" names to domain names
Several recommendations in this document concern
themselves with the binding between domain names and
certificates, but equally important for users is the
binding between domain name/certificate and the actual
real-world entity involved in the transaction. It is
helpful, for example, to know not only that example.com
presents a valid certificate, but also that it is the
"Example Inc., Norway" with whom the user expects to be
interacting. In the case of augmented assurance
certificates, the identity information provided may be
considered sufficient for this purpose, but other validated
certificates do not necessarily provide this real-world
identity. User agents that wish to provide a mechanism for
users to manually establish these linkages are advised to
consider the petnames approach (see
[PETNAMES]
).
8.5 Warning Fatigue
Requirements in this document often involve warning
6.4.2 Warning/Caution
Messages
and
6.4.3
Danger Messages
) messages when warranted by
conditions in the user agent. However, it is important to
be aware, when developing user interfaces, that users will
habituate to over-frequent warnings, weakening the impact
of the messages and their ability to effectively interrupt
the user's task flow.
User agents are advised to constrain the number of
warnings and errors presented to the minimum required to
satisfy these and other security guidelines. It is also
recommended that user agents phrase options in these
messages in terms of the action taken (e.g. "Ignore this
warning," "Trust this site") rather than using generic
labels (e.g. "OK", "Cancel").
8.6 Mixing
Augmented Assurance and Validated Certificates
The Augmented Assurance indicator tells the user that
the owner and author of the Web page being displayed can be
identified using information from the associated augmented
assurance certificate. Identity signals in this
specification only directly address displaying the identity
of the party responsible for the top level resource in a
Web page. User agents may choose to make the identities of
other resources that can affect or control the page's
content available, but we do not put forward a model for
users on how they might use such information in their trust
decisions.
The identity of the top level resource vouches for the
content of all dependent resources. What resources these
are is under the control of the top-level resource, which
will typically identify these resources using URIs with
domain based authority. Therefore, this specification
requires that, in order to display any augmented assurance
related indicators, dependent resources must all be
strongly TLS protected
using validated
certificates
If an augmented assurance page includes content from
other strongly TLS-protected resources that are not
identified by augmented assurance certificates, the authors
for these third party parts of the document cannot be
identified to the same extent as for the main document.
Given that certain types of content, for example external
scripts and styling can change the containing document's
entire appearance, and framed content and plug-ins can be
where the user's main interaction occurs, the user's real
interaction may be with content under the control of a
completely different party than the one identified by the
main document's augmented assurance certificate.
Using third party content also makes the main document
reliant upon the security of the third party contributor,
and expands the available attack surface of the service,
thus giving attackers several more lines of attack.
Under the agent's Same Origin policy, separately
displayed Web pages from the same origin can freely read
and modify each other's state. A Web page's origin is
comprised of the scheme, host and port of the URI used to
retrieve the Web page. The origin does not take into
account any attributes of the TLS session or server
certificate used when retrieving a Web page. For example,
consider a user agent that has loaded two Web pages from
"https://www.example.com/". When the first page was
retrieved, an Augmented Assurance Certificate was used by
the TLS session. When the second page was retrieved, a
different certificate, such as a domain validated or
self-signed certificate, was used. Though the first page
was retrieved using an augmented assurance certificate, the
second page can freely read and write the first page.
Differing security presentations of the two pages may
obscure this relationship in the mind of the user.
8.7 Dynamic content might
change security properties
This specification is expressed in terms of the
fundamentally static indicators of existing agent security
user interfaces.
These indicators tend to assume that the security
properties "of a page" will not change in a significant way
once it has finished loading (whatever that might mean in
detail). Strictly speaking, this assumption is flawed:
Dynamic pages can load scripts and data at any time and
from any source (using techniques like the insertion of
script tags into the DOM at run time); the effect may very
well be that a page that was retrieved from a secure Web
site with an Augmented Assurance certificate could at some
point be under the control of scripts that are retrieved
insecurely. This specification does not prescribe any
specific user interaction in this kind of situation.
For TLS-protected HTTP requests caused using the
XMLHttpRequest API
[XHR]
[XHR2]
, this specification permits either
handling the error situation described above as a network
error (and leaving behavior to the Web application in
question) or causing a user interaction. It is expected
that upcoming specifications for the XMLHttpRequest API
will opt for the former choice.
Terms defined in this document
advanced
level
Augmented Assurance
Certificate
basic
level
chrome
HTTP
transaction
Identity
Information
identity
signal
Interaction flooding
attack
interactively
accepted
Location
Bar
mixed
content
pinning
plug-ins
Primary User
Interface
Secondary User
Interface
Strong TLS
algorithms
strongly
TLS-protected
TLS
indicator
TLS-protected
TLS-secured
validated
certificate
weakly
TLS-protected
Web Page
Web User
Agent
10
References
ECRYPT2009
ECRYPT2
Yearly Report on Algorithms and Key Lengths (2009
Edition)
, available at
EVTLSCERT
Guidelines for the Issuance and Management of Extended
Validation Certificates
, CA/Browser Forum, 7
June 2007, available at
FAVICON
How to Add
a Favicon to your Site
, D. Hazaël-Massieux, C.
Lilley, O. Théreaux, W3C work in progress, available at
KCM
Key Management through Key Continuity (KCM)
(Expired) Internet Draft, September 2008, Peter Gutmann.
This draft is available at
NESSIE
Portfolio of recommended cryptographic primitives,
New European Schemes for Signatures, Integrity, and
Encryption (NESSIE)
, available at
PETNAMES
Petname Systems
, HPL-2005-148, Mark Stiegler,
August 2005. This report is available at
PKIX
Internet X.509
Public Key Infrastructure Certificate and Certificate
Revocation List (CRL) Profile
, RFC 5280, D.
Cooper, S. Santesson, S. Farrell, S. Boeyen, R. Housley, W.
Polk, May 2008, available at
RFC2119
Key
words for use in RFCs to Indicate Requirement
Levels
, RFC 2119, S. Bradner, March 1997,
available at http://www.ietf.org/rfc/rfc2119.txt .
RFC2560
X.509 Internet
Public Key Infrastructure Online Certificate Status
Protocol - OCSP
, RFC 2560, M. Myers, R. Ankney,
A. Malpani, S. Galperin, C. Adams, June 1999. This
specification is available at
RFC2616
Hypertext
Transfer Protocol -- HTTP/1.1
, RFC 2616, R.
Fielding, J. Gettys, J. Mogul, H. Frystyk, L. Masinter, P.
Leach, T. Berners-Lee, June 1999, available at
RFC2817
Upgrading to
TLS Within HTTP/1.1
, RFC 2817, R. Khare, S.
Lawrence, May 2000, available at
RFC2818
HTTP Over
TLS
, RFC 2818, E. Rescorla, May 2000, available
at http://www.ietf.org/rfc/rfc2818.txt .
RFC2965
HTTP State
Management Mechanism
, RFC 2965, D. Kristol, L.
Montulli, October 2000, available at
RFC3709
Internet X.509
Public Key Infrastructure: Logotypes in X.509
Certificates
, RFC 3709, S. Santeson, R. Housley,
T. Freeman, February 2004, available at
RFC3986
Uniform
Resource Identifier (URI): Generic Syntax"
, RFC
3986, T. Berners-Lee, R. Fielding, L. Masinter, January
2005, available at http://www.ietf.org/rfc/rfc3986.txt
SSLv3
SSLv3 specification
, Netscape, November
1996. This specification is archived at
TLSv11
The
Transport Layer Security (TLS) Protocol
, RFC
4346, T. Dierks, E. Rescorla, April 2006. This
specification is available at
TLSv12
The
Transport Layer Security (TLS) Protocol Version
1.2
, RFC 5246, T. Dierks, E. Rescorla, August
2008. This specification is available at
WCAG20
Web
Content Accessibility Guidelines 2.0
, B.
Caldwell, M. Cooper, L. G. Reid, G. Vanderheiden (eds.),
W3C Recommendation 11 December 2008. This version is
latest version of
WCAG 2.0
is available at http://www.w3.org/TR/WCAG20/
WSC-THREATS
Web User Interaction: Threat Trees
, T. Roessler
(ed), W3C Working Group Note (work in progress) 1 November
2007. This version is
latest
version
is available at
WSC-USECASES
Web Security Experience, Indicators and Trust: Scope and
Use Cases
, T. Close, Editor, W3C Working Group
Note (work in progress) 06 March 2008. This version is
latest
version
is available at
XHR
XMLHttpRequest
A. van Kesteren (ed.), W3C Working Draft (Work in Progress)
19 November 2009. This version of the XMLHttpRequest
specification is at
latest
version
of this specification is available at
XHR2
XMLHttpRequest
Level 2
. A. van Kesteren (ed.), W3C Working
Draft (Work in Progress) 20 August September 2009. This
version of the XMLHttpRequest Level 2 specification is at
The
latest
version
of this specification is available at