Secure Shell - Wikipedia

Cryptographic network protocol
The
Secure Shell Protocol
SSH Protocol
) is a
cryptographic
network protocol
for operating
network services
securely over an
unsecured network
Its most notable applications are
remote login
and
command-line
execution.
SSH was designed for
Unix-like
operating systems as a replacement for
Telnet
and
unsecured
remote
Unix shell
protocols, such as the Berkeley
Remote Shell
(rsh) and the related
rlogin
and
rexec
protocols, which all use insecure,
plaintext
methods of authentication, such as
passwords
Since mechanisms like
Telnet
and
Remote Shell
are designed to access and operate remote computers, sending the
authentication
tokens (e.g.
username
and
password
) for this access to these computers across a
public network
in an unsecured way poses a great risk of third parties obtaining the password and achieving the same level of access to the remote system as the telnet user. Secure Shell mitigates this risk through the use of
encryption
mechanisms that are intended to hide the contents of the transmission from an observer, even if the observer has access to the entire
data stream
Finnish computer scientist Tatu Ylönen designed SSH in 1995 and provided an implementation in the form of two commands,
ssh
and
slogin
, as secure replacements for
rsh
and
rlogin
, respectively. Subsequent development of the protocol suite proceeded in several developer groups, producing several variants of implementation. The protocol specification distinguishes two major versions, referred to as SSH-1 and SSH-2. The most commonly implemented software stack is
OpenSSH
, released in 1999 as open-source software by the
OpenBSD
developers. Implementations are distributed for all types of operating systems in common use, including
embedded systems
SSH applications are based on a
client–server architecture
, connecting an
SSH client
instance with an
SSH server
SSH operates as a layered protocol suite comprising three principal hierarchical components: the
transport layer
provides server authentication, confidentiality, and integrity; the
user authentication protocol
validates the user to the server; and the
connection protocol
multiplexes
the encrypted tunnel into multiple logical communication channels.
SSH uses
public-key cryptography
to
authenticate
the remote computer and allow it to authenticate the user, if necessary.
SSH may be used in several methodologies. In the simplest manner, both ends of a communication channel use automatically generated public-private key pairs to encrypt a network connection, and then use a
password
to authenticate the user.
When the public-private key pair is generated by the user manually, the authentication is essentially performed when the key pair is created, and a session may then be opened automatically without a password prompt. In this scenario, the public key is placed on all computers that must allow access to the owner of the matching private key, which the owner keeps private. While authentication is based on the private key, the key is never transferred through the network during authentication. SSH only verifies that the same person offering the public key also owns the matching private key.
In all versions of SSH, it is important to verify unknown
public keys
, i.e.,
associate the public keys with identities
, before accepting them as valid. Accepting an attacker's public key without validation will authorize an unauthorized attacker as a valid user.
Authentication: OpenSSH key management
edit
On
Unix-like
systems, the list of authorized public keys is typically stored in the home directory of the user that is allowed to log in remotely, in the file
~/.ssh/authorized_keys
This file is respected by SSH only if it is not writable by anything apart from the owner and root. When the public key is present on the remote end, and the matching private key is present on the local end, typing in the password is no longer required. However, for additional security, the private key itself can be locked with a passphrase.
The private key can also be looked for in standard places, and its full path can be specified as a command-line setting (the option
-i
for ssh). The
ssh-keygen
utility produces the public and private keys, always in pairs.
SSH is typically used to log into a remote computer's
shell
or
command-line interface
(CLI) and to execute commands on a remote server. It also supports mechanisms for
tunneling
forwarding
of
TCP ports
and
X11
connections and it can be used to transfer files using the associated
SSH File Transfer Protocol
(SFTP) or
Secure Copy Protocol
(SCP).
SSH uses the
client–server model
. An SSH
client
program is typically used for establishing connections to an SSH
daemon
, such as sshd, accepting remote connections. Both are commonly present on most modern
operating systems
, including
macOS
, most distributions of
Linux
OpenBSD
FreeBSD
NetBSD
Solaris
and
OpenVMS
. Notably, versions of
Windows
prior to Windows 10 version 1709 do not include SSH by default, but
proprietary
freeware
and
open source
versions of various levels of complexity and completeness did and do exist (see
Comparison of SSH clients
). In 2018
Microsoft
began porting the
OpenSSH
source code to Windows
and in
Windows 10 version 1709
, an official Win32 port of OpenSSH is now available.
File managers for UNIX-like systems (e.g.,
Konqueror
) can use the
FISH
protocol to provide a split-pane GUI with drag-and-drop. The open source Windows program
WinSCP
provides similar file management (synchronization, copy, remote delete) capability using
PuTTY
as a back-end. Both WinSCP
and PuTTY
are available packaged to run directly off a USB drive, without requiring installation on the client machine. Crostini on
ChromeOS
comes with OpenSSH by default. Setting up an SSH server in Windows typically involves enabling a feature in the Settings app.
SSH is important in
cloud computing
to solve connectivity problems, avoiding the security issues of exposing a cloud-based virtual machine directly on the Internet. An SSH tunnel can provide a secure path over the Internet, through a firewall to a virtual machine.
The
IANA
has assigned
TCP
port
22,
UDP
port 22 and
SCTP
port 22 for this protocol.
10
IANA had listed the standard TCP port 22 for SSH servers as one of the
well-known ports
as early as 2001.
11
SSH can also be run using
SCTP
rather than TCP as the connection-oriented transport layer protocol.
12
Historical development
edit
In 1995,
Tatu Ylönen
, a researcher at
Helsinki University of Technology
in Finland designed the first version of the protocol (now called
SSH-1
) prompted by a password-
sniffing
attack at his
university network
13
The goal of SSH was to replace the earlier
rlogin
TELNET
FTP
14
and
rsh
protocols, which did not provide strong authentication nor guarantee confidentiality. He chose the port number 22 because it is between
telnet
(port 23) and
ftp
(port 21).
15
Ylönen released his implementation as
freeware
in July 1995, and the tool quickly gained in popularity. Towards the end of 1995, the SSH user base had grown to 20,000 users in fifty countries.
16
In December 1995, Ylönen founded SSH Communications Security to market and develop SSH. The original version of the SSH software used various pieces of
free software
, such as
GNU libgmp
, but later versions released by SSH Communications Security evolved into increasingly
proprietary software
It was estimated that by 2000, the number of users had grown to 2 million.
17
In 2006, after being discussed in a working group named "secsh",
18
a revised version of the SSH protocol,
SSH-2
was adopted as a standard.
19
This version offers improved security and new features, but is not compatible with SSH-1. For example, it introduces new key-exchange mechanisms like
Diffie–Hellman key exchange
, improved
data integrity
checking via
message authentication codes
like
MD5
or
SHA-1
, which can be negotiated between client and server. SSH-2 also adds stronger encryption methods like
AES
which eventually replaced weaker and compromised ciphers from the previous standard like
3DES
20
21
19
New features of SSH-2 include the ability to run any number of
shell
sessions over a single SSH connection.
22
Due to SSH-2's superiority and popularity over SSH-1, some implementations such as libssh (v0.8.0+),
23
Lsh
24
and
Dropbear
25
eventually supported only the SSH-2 protocol.
In January 2006, well after version 2.1 was established,
RFC
4253
specified that an SSH server supporting 2.0 as well as prior versions should identify its protocol version as 1.99.
26
This version number does not reflect a historical software revision, but a method to identify
backward compatibility
In 1999, developers, desiring availability of a free software version, restarted software development from the 1.2.12 release of the original SSH program, which was the last released under an
open source license
27
This served as a code base for Björn Grönvall's OSSH software.
28
Shortly thereafter,
OpenBSD
developers
forked
Grönvall's code and created
OpenSSH
, which shipped with Release 2.6 of OpenBSD. From this version, a "portability" branch was formed to port OpenSSH to other operating systems.
29
As of 2005
[update]
OpenSSH
was the single most popular SSH implementation, being the default version in a large number of operating system distributions. OSSH, meanwhile, has become obsolete.
30
OpenSSH continues to be maintained and supports the SSH-2 protocol, having expunged SSH-1 support from the codebase in the OpenSSH 7.6 release.
In 2023, an alternative to traditional SSH was proposed under the name
SSH3
31
32
33
by PhD student François Michel and Professor Olivier Bonaventure and its code has been made open source.
34
This new version implements the original SSH Connection Protocol but operates on top of
HTTP/3
, which runs on
QUIC
. It offers multiple features such as:
Faster session establishment, reducing the number of
Round-trip delays
from 5-7 to 3.
High security: while SSHv2 relies on its own protocols, SSH3 leverages
TLS 1.3
QUIC
, and
HTTP
UDP port forwarding
X.509 certificates
OpenID Connect
However, the name SSH3 is under discussion, and the project aims to rename itself to a more suitable name.
35
The discussion stems from the fact that this new implementation significantly revises the SSH protocol, suggesting it should not be called SSH3.
Example of tunneling an
X11
application over SSH: the user 'josh' has "SSHed" from the local machine 'foofighter' to the remote machine 'tengwar' to run
xeyes
Logging into
OpenWrt
via SSH using
PuTTY
running on
Windows
SSH is a protocol that can be used for many applications across many platforms including most
Unix
variants (
Linux
, the
BSDs
including
Apple
's
macOS
, and
Solaris
), as well as
Microsoft Windows
. Some of the applications below may require features that are only available or compatible with specific SSH clients or servers. For example, using the SSH protocol to implement a
VPN
is possible, but presently only with the
OpenSSH
server and client implementation.
For login to a shell on a remote host (replacing
Telnet
and
rlogin
For executing a single command on a remote host (replacing
rsh
For setting up automatic (passwordless) login to a remote server (for example, using
OpenSSH
36
In combination with
rsync
to back up, copy and mirror files efficiently and securely
For
forwarding
a port
For
tunneling
(not to be confused with a
VPN
, which
routes
packets between different networks, or
bridges
two
broadcast domains
into one).
For use as a full-fledged encrypted VPN. Note that only
OpenSSH
server and client support this feature.
For forwarding
from a remote
host
(possible through multiple intermediate hosts)
For browsing the web through an encrypted proxy connection with SSH clients that support the
SOCKS protocol
For securely mounting a directory on a remote server as a
file system
on a local computer using
SSHFS
For automated remote monitoring and management of servers through one or more of the mechanisms discussed above.
For development on a mobile or embedded device that supports SSH.
For securing file transfer protocols.
File transfer protocols
edit
The Secure Shell protocols are used in several file transfer mechanisms.
Diagram of the SSH-2 binary packet.
The SSH protocol has a layered architecture with three separate components:
The
transport layer
RFC
4253
) typically uses the
Transmission Control Protocol
(TCP) of
TCP/IP
, reserving
port number
22 as a server listening port. This layer handles initial key exchange as well as server authentication, and sets up encryption, compression, and integrity verification. It exposes to the upper layer an interface for sending and receiving plaintext packets with a size of up to 32,768 bytes each, but more can be allowed by each implementation. The transport layer also arranges for key re-exchange, usually after 1 GB of data has been transferred or after one hour has passed, whichever occurs first.
The
user authentication layer
RFC
4252
) handles client authentication, and provides a suite of authentication algorithms. Authentication is
client-driven
: when one is prompted for a password, it may be the SSH client prompting, not the server. The server merely responds to the client's authentication requests. Widely used user-authentication methods include the following:
password
: a method for straightforward password authentication, including a facility allowing a password to be changed. Not all programs implement this method.
publickey
: a method for
public-key-based authentication
, usually supporting at least
DSA
ECDSA
or
RSA
keypairs, with other implementations also supporting
X.509
certificates.
keyboard-interactive
RFC
4256
): a versatile method where the server sends one or more prompts to enter information and the client displays them and sends back responses keyed in by the user. Used to provide
one-time password
authentication such as
S/Key
or
SecurID
. Used by some OpenSSH configurations when
PAM
is the underlying host-authentication provider to effectively provide password authentication, sometimes leading to an inability to log in with a client that supports just the plain
password
authentication method.
GSSAPI
authentication methods which provide an extensible scheme to perform SSH authentication using external mechanisms such as
Kerberos 5
or
NTLM
, providing
single sign-on
capability to SSH sessions. These methods are usually implemented by commercial SSH implementations for use in organizations, though OpenSSH does have a working GSSAPI implementation.
The
connection layer
RFC
4254
) defines the concept of channels, channel requests, and global requests, which define the SSH services provided. A single SSH connection can be multiplexed into multiple logical channels simultaneously, each transferring data bidirectionally. Channel requests are used to relay out-of-band channel-specific data, such as the changed size of a terminal window or the exit code of a server-side process. Additionally, each channel performs its own flow control using the receive window size. The SSH client requests a server-side port to be forwarded using a global request. Standard channel types include:
shell
for terminal shells, SFTP and exec requests (including SCP transfers)
direct-tcpip
for client-to-server forwarded connections
forwarded-tcpip
for server-to-client forwarded connections
The
SSHFP
DNS record (RFC 4255) provides the public host key fingerprints in order to aid in verifying the authenticity of the host.
This open architecture provides considerable flexibility, allowing the use of SSH for a variety of purposes beyond a secure shell. The functionality of the transport layer alone is comparable to
Transport Layer Security
(TLS); the user-authentication layer is highly extensible with custom authentication methods; and the connection layer provides the ability to multiplex many secondary sessions into a single SSH connection, a feature comparable to
BEEP
and not available in TLS.
In 1998, a vulnerability was described in SSH 1.5 which allowed the unauthorized insertion of content into an encrypted SSH stream due to insufficient data integrity protection from
CRC-32
used in this version of the protocol.
42
43
A fix known as SSH Compensation Attack Detector
44
was introduced into most implementations. Many of these updated implementations contained a new
integer overflow
vulnerability
45
that allowed attackers to execute arbitrary code with the privileges of the SSH daemon, typically root.
In January 2001 a vulnerability was discovered that allows attackers to modify the last block of an
IDEA
-encrypted session.
46
The same month, another vulnerability was discovered that allowed a malicious server to forward a client authentication to another server.
47
Since SSH-1 has inherent design flaws that make it vulnerable, it is now generally considered obsolete and should be avoided by explicitly disabling fallback to SSH-1. Most modern servers and clients support SSH-2.
47
CBC plaintext recovery
edit
In November 2008, a theoretical vulnerability was discovered for all versions of SSH which allowed recovery of up to 32 bits of plaintext from a block of ciphertext that was encrypted using what was then the standard default encryption mode,
CBC
48
The most straightforward solution is to use
CTR
, counter mode, instead of CBC mode, since this renders SSH resistant to the attack.
48
Suspected decryption by NSA
edit
On December 28, 2014
Der Spiegel
published classified information
49
leaked by whistleblower
Edward Snowden
which suggests that the
National Security Agency
may be able to decrypt some SSH traffic. The technical details associated with such a process were not disclosed. A 2017 analysis of the
CIA
hacking tools
BothanSpy
and
Gyrfalcon
suggested that the SSH protocol was not compromised.
50
A novel man-in-the-middle attack against most current ssh implementations was discovered in 2023. It was named the
Terrapin attack
by its discoverers.
51
52
However, the risk is mitigated by the requirement to intercept a genuine ssh session, and that the attack is restricted in its scope, fortuitously resulting mostly in failed connections.
53
54
The ssh developers have stated that the major impact of the attack is to degrade the
keystroke timing
obfuscation features of ssh.
54
The vulnerability was fixed in OpenSSH 9.6, but requires both client and server to be upgraded for the fix to be fully effective.
Standards documentation
edit
The following
RFC
publications by the
IETF
"secsh"
working group
document SSH-2 as a proposed
Internet standard
RFC
4250
The Secure Shell (SSH) Protocol Assigned Numbers
RFC
4251
The Secure Shell (SSH) Protocol Architecture
RFC
4252
The Secure Shell (SSH) Authentication Protocol
RFC
4253
The Secure Shell (SSH) Transport Layer Protocol
RFC
4254
The Secure Shell (SSH) Connection Protocol
RFC
4255
Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints
RFC
4256
Generic Message Exchange Authentication for the Secure Shell Protocol (SSH)
RFC
4335
The Secure Shell (SSH) Session Channel Break Extension
RFC
4344
The Secure Shell (SSH) Transport Layer Encryption Modes
RFC
4345
Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer Protocol
The protocol specifications were later updated by the following publications:
RFC
4419
Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol
(March 2006)
RFC
4432
RSA Key Exchange for the Secure Shell (SSH) Transport Layer Protocol
(March 2006)
RFC
4462
Generic Security Service Application Program Interface (GSS-API) Authentication and Key Exchange for the Secure Shell (SSH) Protocol
(May 2006)
RFC
4716
The Secure Shell (SSH) Public Key File Format
(November 2006)
RFC
4819
Secure Shell Public Key Subsystem
(March 2007)
RFC
5647
AES Galois Counter Mode for the Secure Shell Transport Layer Protocol
(August 2009)
RFC
5656
Elliptic Curve Algorithm Integration in the Secure Shell Transport Layer
(December 2009)
RFC
6187
X.509v3 Certificates for Secure Shell Authentication
(March 2011)
RFC
6239
Suite B Cryptographic Suites for Secure Shell (SSH)
(May 2011)
RFC
6594
Use of the SHA-256 Algorithm with RSA, Digital Signature Algorithm (DSA), and Elliptic Curve DSA (ECDSA) in SSHFP Resource Records
(April 2012)
RFC
6668
SHA-2 Data Integrity Verification for the Secure Shell (SSH) Transport Layer Protocol
(July 2012)
RFC
7479
Ed25519
SSHFP Resource Records
(March 2015)
RFC
5592
Secure Shell Transport Model for the Simple Network Management Protocol (SNMP)
(June 2009)
RFC
6242
Using the NETCONF Protocol over Secure Shell (SSH)
(June 2011)
RFC
8332
Use of RSA Keys with SHA-256 and SHA-512 in the Secure Shell (SSH) Protocol
(March 2018)
RFC
8709
Ed25519 and Ed448 Public Key Algorithms for the Secure Shell (SSH) Protocol
(February 2020)
draft-gerhards-syslog-transport-ssh
SSH transport mapping for SYSLOG
(July 2006)
draft-ietf-secsh-filexfer
SSH File Transfer Protocol
(July 2006)
draft-ietf-sshm-ssh-agent
- SSH Agent Protocol (March 2025)
In addition, the
OpenSSH
project includes several vendor protocol specifications/extensions:
T. Ylonen; C. Lonvick (January 2006).
The Secure Shell (SSH) Protocol Architecture
. IETF Trust.
doi
10.17487/RFC4251
RFC
4251
"Missouri University S&T: Secure Telnet"
T. Ylonen; C. Lonvick (January 2006).
The Secure Shell (SSH) Authentication Protocol
. IETF Trust.
doi
10.17487/RFC4252
RFC
4252
"How To Set Up Authorized Keys"
Archived
from the original on 2011-05-10.
Win-32 OpenSSH
"WinSCP home page"
Archived
from the original on 2014-02-17.
"WinSCP page for PortableApps.com"
Archived
from the original on 2014-02-16.
"PuTTY page for PortableApps.com"
Archived
from the original on 2014-02-16.
Amies, A; Wu, C F; Wang, G C; Criveti, M (2012).
"Networking on the cloud"
IBM developerWorks
Archived
from the original on 2013-06-14.
"Service Name and Transport Protocol Port Number Registry"
"Service Name and Transport Protocol Port Number Registry"
iana.org
Archived
from the original on 2001-06-04.
Seggelmann, R.; Tuxen, M.; Rathgeb, E.P. (18–20 July 2012).
SSH over SCTP — Optimizing a multi-channel protocol by adapting it to SCTP
. 8th International Symposium on Communication Systems, Networks & Digital Signal Processing (CSNDSP). pp.
1–
6.
doi
10.1109/CSNDSP.2012.6292659
ISBN
978-1-4577-1473-3
S2CID
8415240
Tatu Ylönen.
"The new skeleton key: changing the locks in your network environment"
. Archived from
the original
on 2017-08-20.
Tatu Ylönen.
"SSH Port"
Archived
from the original on 2017-08-03.
Ylönen, Tatu.
"The story of the SSH port is 22"
www.ssh.com
. Retrieved
2023-11-30
Barrett, Daniel J.; Silverman, Richard E. (2001).
SSH, the secure shell: the definitive guide
(1st ed.). Cambridge [Mass.]: O'Reilly. p. 11.
ISBN
978-0-596-00011-0
Nicholas Rosasco and David Larochelle.
"How and Why More Secure Technologies Succeed in Legacy Markets: Lessons from the Success of SSH"
(PDF)
Quoting
Barrett
and Silverman, SSH, the Secure Shell: The Definitive Guide, O'Reilly & Associates (2001)
. Dept. of Computer Science, Univ. of Virginia.
Archived
(PDF)
from the original on 2006-06-25
. Retrieved
2006-05-19
IETF (Internet Engineering Task Force): datatracker for secsh
RFC4252: The Secure Shell (SSH) Authentication Protocol, Jan 2006
O'Reily: Secure Shell, The Definitive Guide
RFC4250: The Secure Shell (SSH) Protocol: Assigned names, Jan 2006, page 16
"SSH Frequently Asked Questions"
Archived
from the original on 2004-10-10.
"libssh"
"A GNU implementation of the Secure Shell protocols"
Archived
from the original on 2012-02-04.
"Dropbear SSH"
Archived
from the original on 2011-10-14.
Ylonen, T.; Lonvick, C.
"Old Client, New Server"
The Secure Shell (SSH) Transport Layer Protocol
. IETF. sec. 5.1.
doi
10.17487/RFC4253
RFC
4253
ssh-1.2.13 now available: copying policy changed (permission now required to sell ssh commercially, use is still permitted for any purpose)
OSSH sources
"OpenSSH: Project History and Credits"
. openssh.com. 2004-12-22.
Archived
from the original on 2013-12-24
. Retrieved
2014-04-27
"OSSH Information for VU#419241"
CERT Coordination Center
. 2006-02-15.
Archived
from the original on 2007-09-27.
Either way ossh is old and obsolete and I don't recommend its use.
"Remote terminal over HTTP/3 connections"
datatracker.ietf.org
. 2024-08-01.
"Secure shell over HTTP/3 connections"
www.ietf.org
. 2024-02-28.
Michel, François; Bonaventure, Olivier (2023-12-12). "Towards SSH3: how HTTP/3 improves secure shells".
arXiv
2312.08396
cs.NI
].
"ssh3"
github.com
. 2024-07-12.
"Secure shell over HTTP/3 connections"
datatracker.ietf.org
. 2024-02-28.
Sobell, Mark (2012).
A Practical Guide to Linux Commands, Editors, and Shell Programming
(3rd ed.). Upper Saddle River, NJ: Prentice Hall. pp.
702–
704.
ISBN
978-0133085044
Harris, B.; Velvindron, L. (February 2020).
Ed25519 and Ed448 Public Key Algorithms for the Secure Shell (SSH) Protocol
IETF
doi
10.17487/RFC8709
RFC
8709
Stebila, D.; Green, J. (December 2009).
Elliptic Curve Algorithm Integration in the Secure Shell Transport Layer
IETF
doi
10.17487/RFC5656
RFC
5656
. Retrieved
12 November
2012
Miller, D.; Valchev, P. (September 3, 2007).
The use of UMAC in the SSH Transport Layer Protocol
IETF
. I-D draft-miller-secsh-umac-00.
Ylonen, T.; Lonvick, C.
The Secure Shell (SSH) Transport Layer Protocol
. IETF.
doi
10.17487/RFC4253
RFC
4253
Igoe, K.; Solinas, J. (August 2009).
AES Galois Counter Mode for the Secure Shell Transport Layer Protocol
IETF
doi
10.17487/RFC5647
RFC
5647
"SSH Insertion Attack"
Core Security Technologies
Archived
from the original on 2011-07-08.
"Vulnerability Note VU#13877 - Weak CRC allows packet injection into SSH sessions encrypted with block ciphers"
US CERT
Archived
from the original on 2010-07-10.
"SSH CRC-32 Compensation Attack Detector Vulnerability"
SecurityFocus
Archived
from the original on 2008-07-25.
"Vulnerability Note VU#945216 - SSH CRC32 attack detection code contains remote integer overflow"
US CERT
Archived
from the original on 2005-10-13.
"Vulnerability Note VU#315308 - Weak CRC allows last block of IDEA-encrypted SSH packet to be changed without notice"
US CERT
Archived
from the original on 2010-07-11.
"Vulnerability Note VU#684820 - SSH-1 allows client authentication to be forwarded by a malicious server to another server"
US CERT
Archived
from the original on 2009-09-01.
"Vulnerability Note VU#958563 - SSH CBC vulnerability"
US CERT
Archived
from the original on 2011-06-22.
"Prying Eyes: Inside the NSA's War on Internet Security"
Spiegel Online
. December 28, 2014.
Archived
from the original on January 24, 2015.
Ylonen, Tatu (3 August 2017).
"BothanSpy & Gyrfalcon - Analysis of CIA hacking tools for SSH"
ssh.com
. Retrieved
15 July
2018
"Terrapin Attack"
terrapin-attack.com
. Retrieved
2023-12-20
Jones, Connor.
"SSH shaken, not stirred by Terrapin downgrade vulnerability"
www.theregister.com
. Retrieved
2023-12-20
Jones, Connor.
"SSH shaken, not stirred by Terrapin downgrade vulnerability"
www.theregister.com
. Retrieved
2023-12-20
"OpenSSH 9.6 release notes"
openssh.com
. 2023-12-18.
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