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Secure Shell (SSH) is a "cryptographic "network protocol for operating network services securely over an unsecured network.[1] The best known example application is for remote "login to computer systems by users.

SSH provides a "secure channel over an unsecured network in a "client-server architecture, connecting an "SSH client application with an "SSH server.[2] Common applications include remote "command-line "login and remote command execution, but any "network service can be secured with SSH. The protocol specification distinguishes between two major versions, referred to as SSH-1 and SSH-2.

The most visible application of the protocol is for access to "shell accounts on "Unix-like operating systems, but it sees some limited use on "Windows as well. In 2015, Microsoft announced that they would include native support for SSH in a future release.[3]

SSH was designed as a replacement for "Telnet and for "unsecured remote "shell protocols such as the Berkeley "rlogin, "rsh, and "rexec protocols. Those protocols send information, notably "passwords, in "plaintext, rendering them susceptible to interception and disclosure using "packet analysis.[4] The "encryption used by SSH is intended to provide confidentiality and integrity of data over an unsecured network, such as the "Internet, although files leaked by "Edward Snowden indicate that the "National Security Agency can sometimes decrypt SSH, allowing them to read the contents of SSH sessions.[5]

On 6 July 2017, the government transparency organization "WikiLeaks confirmed that the US "Central Intelligence Agency had developed tools that can be installed on computers running "Microsoft "Windows or "GNU/Linux operating systems to intercept SSH connections started by SSH clients on the compromised systems.[6]



SSH uses "public-key cryptography to "authenticate the remote computer and allow it to authenticate the user, if necessary.[2] There are several ways to use SSH; one is to use automatically generated public-private key pairs to simply encrypt a network connection, and then use "password authentication to log on.

Another is to use a manually generated public-private key pair to perform the authentication, allowing users or programs to log in without having to specify a password. In this scenario, anyone can produce a matching pair of different keys (public and private). The public key is placed on all computers that must allow access to the owner of the matching private key (the owner keeps the private key secret). While authentication is based on the private key, the key itself is never transferred through the network during authentication. SSH only verifies whether 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.

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.[7] 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 (some software like "Message Passing Interface (MPI) stack may need this password-less access to run properly). 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 also supports password-based authentication that is encrypted by automatically generated keys. In this case, the attacker could imitate the legitimate server side, ask for the password, and obtain it ("man-in-the-middle attack). However, this is possible only if the two sides have never authenticated before, as SSH remembers the key that the server side previously used. The SSH client raises a warning before accepting the key of a new, previously unknown server. Password authentication can be disabled.


SSH is typically used to log into a remote machine and execute commands, but it also supports "tunneling, "forwarding "TCP ports and "X11 connections; it can transfer files using the associated "SSH file transfer (SFTP) or "secure copy (SCP) protocols.[2] SSH uses the "client-server model.

The "standard TCP port 22 has been assigned for contacting SSH servers.[8]

An SSH "client program is typically used for establishing connections to an SSH "daemon 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, "Windows is one of the few modern desktop/server operating systems that does not include SSH by default. "Proprietary, "freeware and "open source (e.g. "PuTTY,[9] and the version of "OpenSSH which is part of "Cygwin[10]) versions of various levels of complexity and completeness exist. Native Linux file managers (e.g. "Konqueror) can use the "FISH protocol to provide a split-pane GUI with drag-and-drop. The open source Windows program "WinSCP[11] provides similar file management (synchronization, copy, remote delete) capability using PuTTY as a back-end. Both WinSCP[12] and PuTTY[13] are available packaged to run directly off a USB drive, without requiring installation on the client machine. Setting up an SSH server in Windows typically involves installation (e.g. via installing Cygwin[14]). In "Windows 10 version 1709, an official Win32 port of OpenSSH is available.

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.[15]

History and development[edit]

Version 1.x[edit]

In 1995, "Tatu Ylönen, a researcher at "Helsinki University of Technology, Finland, designed the first version of the protocol (now called SSH-1) prompted by a password-"sniffing attack at his "university network.[16] The goal of SSH was to replace the earlier "rlogin, "TELNET, ftp[17] and "rsh protocols, which did not provide strong authentication nor guarantee confidentiality. 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.

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 the year 2000 the number of users had grown to 2 million.[18]

Version 2.x[edit]

"Secsh" was the official "Internet Engineering Task Force's (IETF) name for the IETF working group responsible for version 2 of the SSH protocol.[19] In 2006, a revised version of the protocol, SSH-2, was adopted as a standard. This version is incompatible with SSH-1. SSH-2 features both security and feature improvements over SSH-1. Better security, for example, comes through "Diffie–Hellman key exchange and strong "integrity checking via "message authentication codes. New features of SSH-2 include the ability to run any number of "shell sessions over a single SSH connection.[20] Due to SSH-2's superiority and popularity over SSH-1, some implementations such as "Lsh[21] and "Dropbear[22] support only the SSH-2 protocol.

Version 1.99[edit]

In January 2006, well after version 2.1 was established, RFC 4253 specified that an SSH server which supports both 2.0 and prior versions of SSH should identify its protoversion as 1.99.[23] This is not an actual version but a method to identify "backward compatibility.

OpenSSH and OSSH[edit]

In 1999, developers, wanting a free software version to be available, went back to the older 1.2.12 release of the original SSH program, which was the last released under an "open source license. Björn Grönvall's OSSH was subsequently developed from this codebase. Shortly thereafter, "OpenBSD developers "forked Grönvall's code and did extensive work on it, creating "OpenSSH, which shipped with the 2.6 release of OpenBSD. From this version, a "portability" branch was formed to port OpenSSH to other operating systems.[24]

As of 2005, "OpenSSH was the single most popular SSH implementation, coming by default in a large number of operating systems. OSSH meanwhile has become obsolete.[25] OpenSSH continues to be maintained and supports the SSH-2 protocol, having expunged SSH-1 support from the codebase with the OpenSSH 7.6 release.


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.

File transfer protocols[edit]

The Secure Shell protocols are used in several file transfer mechanisms.


Diagram of the SSH-2 binary packet.

The SSH-2 protocol has an internal architecture (defined in RFC 4251) with well-separated layers, namely:

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.


These are intended for performance enhancements of SSH products:



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.[30][31] A fix known as SSH Compensation Attack Detector[32] was introduced into most implementations. Many of these updated implementations contained a new integer overflow vulnerability[33] 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.[34] The same month, another vulnerability was discovered that allowed a malicious server to forward a client authentication to another server.[35]

Since SSH-1 has inherent design flaws which make it vulnerable, it is now generally considered obsolete and should be avoided by explicitly disabling fallback to SSH-1.["citation needed] Most modern servers and clients support SSH-2.["citation needed]

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.[36] The most straightforward solution is to use "CTR, counter mode, instead of CBC mode, since this renders SSH resistant to the attack.[36]

Undisclosed vulnerabilities[edit]

On December 28, 2014 "Der Spiegel published classified information[5] 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.

US government hack of SSH clients[edit]

On 6 July, 2017, the government transparency activist organization "WikiLeaks released US "Central Intelligence Agency documents that revealed how the CIA's Information Operations Center bypassed security in communications that utilize popular SSH clients on both "Windows and "Linux operating systems. The release included official user guides for the CIA's BothanSpy and "Gyrfalcon programs which "are designed to intercept and exfiltrate SSH credentials but work on different operating systems with different attack vectors" WikiLeaks reported.

"BothanSpy is an implant that targets the SSH client program Xshell on the Microsoft Windows platform and steals user credentials for all active SSH sessions. These credentials are either username and password in case of password-authenticated SSH sessions or username, filename of private SSH key and key password if public key authentication is used. BothanSpy can exfiltrate the stolen credentials to a CIA-controlled server (so the implant never touches the disk on the target system) or save it in an encrypted file for later exfiltration by other means. BothanSpy is installed as a Shellterm 3.x extension on the target machine.

"Gyrfalcon is an implant that targets the OpenSSH client on Linux platforms (centos,debian,rhel,suse,ubuntu). The implant can not only steal user credentials of active SSH sessions, but is also capable of collecting full or partial OpenSSH session traffic. All collected information is stored in an encrypted file for later exfiltration. It is installed and configured by using a CIA-developed root kit (JQC/KitV) on the target machine."[37]

Standards documentation[edit]

The following "RFC publications by the "IETF "secsh" "working group document SSH-2 as a proposed "Internet standard.

It was later modified and expanded by the following publications.

In addition, the "OpenSSH project includes several vendor protocol specifications/extensions:

See also[edit]


  1. ^ Network Working Group of the IETF, January 2006, RFC 4251, The Secure Shell (SSH) Protocol Architecture
  2. ^ a b c Network Working Group of the IETF, January 2006, RFC 4252, The Secure Shell (SSH) Authentication Protocol
  3. ^ Peter Bright (June 2, 2015). "Microsoft bringing SSH to Windows and PowerShell". "Ars Technica. Archived from the original on June 9, 2017. 
  4. ^ "SSH Hardens the Secure Shell". Serverwatch.com. Archived from the original on 2008-12-23. 
  5. ^ a b "Prying Eyes: Inside the NSA's War on Internet Security". "Spiegel Online. December 28, 2014. Archived from the original on January 24, 2015. 
  6. ^ "BothanSpy". wikileaks.org. 2017-07-06. Archived from the original on 2017-09-21. Retrieved 2017-09-25. 
  7. ^ "How To Set Up Authorized Keys". Archived from the original on 2011-05-10. 
  8. ^ "Service Name and Transport Protocol Port Number Registry". iana.org. Archived from the original on 2001-06-04. 
  9. ^ "Download PuTTY - a free SSH and telnet client for Windows". Putty.org. Archived from the original on 2014-05-27. Retrieved 2014-04-28. 
  10. ^ "Cygwin Package List". Retrieved January 5, 2016. 
  11. ^ "WinSCP home page". Archived from the original on 2014-02-17. 
  12. ^ "WinSCP page for PortableApps.com". Archived from the original on 2014-02-16. 
  13. ^ "PuTTY page for PortableApps.com". Archived from the original on 2014-02-16. 
  14. ^ "Installing Cygwin and Starting the SSH Daemon". Archived from the original on 2014-02-22. Retrieved 2014-02-17. 
  15. ^ 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. 
  16. ^ Tatu Ylönen. "The new skeleton key: changing the locks in your network environment". Archived from the original on 2017-08-20. 
  17. ^ "Tatu Ylönen. "SSH Port". Archived from the original on 2017-08-03. 
  18. ^ 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. 
  19. ^ "Secsh Protocol Documents". VanDyke Software, Inc. Archived from the original on 2010-01-13. 
  20. ^ "SSH Frequently Asked Questions". Archived from the original on 2004-10-10. 
  21. ^ "A GNU implementation of the Secure Shell protocols". Archived from the original on 2012-02-04. 
  22. ^ "Dropbear SSH". Archived from the original on 2011-10-14. 
  23. ^ "RFC 4253". Section 5. Compatibility With Old SSH Versions. Archived from the original on 2010-07-04. , IETF
  24. ^ "OpenSSH: Project History and Credits". openssh.com. 2004-12-22. Archived from the original on 2013-12-24. Retrieved 2014-04-27. 
  25. ^ "OSSH Information for VU#419241". Archived from the original on 2007-09-27. 
  26. ^ Sobell, Mark (2012). A Practical Guide to Linux Commands, Editors, and Shell Programming (3rd Edition). Upper Saddle River, NJ: Prentice Hall. pp. 702–704. "ISBN "978-0133085044. 
  27. ^ Seggelmann, R.; Tuxen, M.; Rathgeb, E.P. (18–20 July 2012). "SSH over SCTP — Optimizing a multi-channel protocol by adapting it to SCTP". Communication Systems, Networks & Digital Signal Processing (CSNDSP), 2012 8th International Symposium on: 1–6. "doi:10.1109/CSNDSP.2012.6292659. "ISBN "978-1-4577-1473-3. 
  28. ^ a b Stebila, D.; Green J. (December 2009). "RFC5656 - Elliptic Curve Algorithm Integration in the Secure Shell Transport Layer". Archived from the original on 19 July 2012. Retrieved 12 November 2012. 
  29. ^ Miller, D.; Valchev, P. (September 3, 2007). "The use of UMAC in the SSH Transport Layer Protocol / draft-miller-secsh-umac-00.txt". Archived from the original on 19 August 2014. Retrieved 12 November 2012. 
  30. ^ "SSH Insertion Attack". "Core Security Technologies. Archived from the original on 2011-07-08. 
  31. ^ "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. 
  32. ^ "SSH CRC-32 Compensation Attack Detector Vulnerability". "SecurityFocus. Archived from the original on 2008-07-25. 
  33. ^ "Vulnerability Note VU#945216 - SSH CRC32 attack detection code contains remote integer overflow". US CERT. Archived from the original on 2005-10-13. 
  34. ^ "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. 
  35. ^ "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. 
  36. ^ a b "Vulnerability Note VU#958563 - SSH CBC vulnerability". US CERT. Archived from the original on 2011-06-22. 
  37. ^ "BothanSpy". www.wikileaks.org. 2017-07-06. Archived from the original on 2017-07-08. Retrieved 2017-07-09. 

Further reading[edit]

External links[edit]

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