Secure Shell

"SSH" redirects here. For other uses, seeSSH (disambiguation).

TheSecure Shell (SSH) Protocolis acryptographicnetwork protocolfor operatingnetwork servicessecurely over an unsecured network.Its most notable applications are remoteloginandcommand-lineexecution.

Secure Shell

Protocol stack
Purpose	secure connection, remote access
Developer(s)	Tatu Ylönen,Internet Engineering Task Force(IETF)
Introduction	1995
OSI layer	Transport layerthroughapplication layer
Port(s)	22
RFC(s)	RFC 4250, RFC 4251, RFC 4252, RFC 4253, RFC 4254

SSH was designed forUnix-likeoperating systems as a replacement forTelnetandunsecuredremoteUnix shellprotocols, such as the BerkeleyRemote Shell(rsh) and the relatedrloginandrexecprotocols, which all use insecure,plaintextmethods of authentication, likepasswords.

Since mechanisms likeTelnetandRemote Shellare designed to access and operate remote computers, sending the authentication tokens (e.g. username andpassword) for this access to these computers across a public network in an unsecured way, poses a great risk of 3rd 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,sshandslogin,as secure replacements forrshandrlogin,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 isOpenSSH,released in 1999 as open-source software by theOpenBSDdevelopers. Implementations are distributed for all types of operating systems in common use, including embedded systems.

SSH applications are based on aclient–serverarchitecture, connecting anSSH clientinstance with anSSH server.SSH operates as a layered protocol suite comprising three principal hierarchical components: thetransport layerprovides server authentication, confidentiality, and integrity; theuser authentication protocolvalidates the user to the server; and theconnection protocolmultiplexes the encrypted tunnel into multiple logical communication channels.

Definition

SSH usespublic-key cryptographytoauthenticatethe 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 apasswordto 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 unknownpublic 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

OnUnix-likesystems, 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-ifor ssh). Thessh-keygenutility produces the public and private keys, always in pairs.

Use

SSH is typically used to log into a remote computer'sshellorcommand-line interface(CLI) and to execute commands on a remote server. It also supports mechanisms fortunneling,forwardingofTCP portsandX11connections and it can be used to transfer files using the associatedSSH File Transfer Protocol(SFTP) orSecure Copy Protocol(SCP).

SSH uses theclient–server model.An SSHclientprogram is typically used for establishing connections to an SSHdaemon,such as sshd, accepting remote connections. Both are commonly present on most modernoperating systems,includingmacOS,most distributions ofLinux,OpenBSD,FreeBSD,NetBSD,SolarisandOpenVMS.Notably, versions ofWindowsprior to Windows 10 version 1709 do not include SSH by default, butproprietary,freewareandopen sourceversions of various levels of complexity and completeness did and do exist (seeComparison of SSH clients). In 2018Microsoftbegan porting theOpenSSHsource code to Windowsand inWindows 10 version 1709,an official Win32 port of OpenSSH is now available.

File managers for UNIX-like systems (e.g.Konqueror) can use theFISHprotocol to provide a split-pane GUI with drag-and-drop. The open source Windows programWinSCPprovides similar file management (synchronization, copy, remote delete) capability using PuTTY as a back-end. Both WinSCPand PuTTYare available packaged to run directly off a USB drive, without requiring installation on the client machine. Crostini onChromeOScomes with OpenSSH by default. Setting up an SSH server in Windows typically involves enabling a feature in the Settings app.

SSH is important incloud computingto 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.

TheIANAhas assignedTCPport22,UDPport 22 andSCTPport 22 for this protocol.IANA had listed the standard TCP port 22 for SSH servers as one of thewell-known portsas early as 2001.SSH can also be run usingSCTPrather than TCP as the connection oriented transport layer protocol.

Historical development

Version 1

In 1995,Tatu Ylönen,a researcher atHelsinki University of Technologyin Finland designed the first version of the protocol (now calledSSH-1) prompted by a password-sniffingattack at hisuniversity network.The goal of SSH was to replace the earlierrlogin,TELNET,FTPandrshprotocols, which did not provide strong authentication nor guarantee confidentiality. He chose the port number 22 because it is betweentelnet(port 23) andftp(port 21).

Ylönen released his implementation asfreewarein July 1995, and the tool quickly gained in popularity. Towards the end of 1995, the SSH user base had grown to20000users in fifty countries.

In December 1995, Ylönen foundedSSH Communications Securityto market and develop SSH. The original version of the SSH software used various pieces offree software,such asGNU libgmp,but later versions released by SSH Communications Security evolved into increasinglyproprietary software.

It was estimated that by 2000 the number of users had grown to 2 million.

Version 2

In 2006, after being discussed in a working group named "secsh",a revised version of the SSH protocol,SSH-2was adopted as a standard.This version offers improved security and new features, but is not compatible with SSH-1. For example, it introduces new key-exchange mechanisms likeDiffie–Hellman key exchange,improveddata integritychecking viamessage authentication codeslikeMD5orSHA-1,which can be negotiated between client and server. SSH-2 also adds stronger encryption methods likeAESwhich eventually replaced weaker and compromised ciphers from the previous standard like3-des.New features of SSH-2 include the ability to run any number ofshellsessions over a single SSH connection.Due to SSH-2's superiority and popularity over SSH-1, some implementations such as libssh (v0.8.0+),LshandDropbeareventually supported only the SSH-2 protocol.

Version 1.99

In January 2006, well after version 2.1 was established,RFC4253specified that an SSH server supporting 2.0 as well as prior versions should identify its protocol version as 1.99.This version number does not reflect a historical software revision, but a method to identifybackward compatibility.

OpenSSH and OSSH

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 anopen source license.This served as a code base for Björn Grönvall's OSSH software.Shortly thereafter,OpenBSDdevelopersforkedGrönvall's code and createdOpenSSH,which shipped with Release 2.6 of OpenBSD. From this version, a "portability" branch was formed to port OpenSSH to other operating systems.

As of 2005,OpenSSHwas the single most popular SSH implementation, being the default version in a large number of operating system distributions. OSSH meanwhile has become obsolete.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.

Uses

Example of tunneling anX11application over SSH: the user 'josh' has "SSHed" from the local machine 'foofighter' to the remote machine 'tengwar' to runxeyes.

Logging intoOpenWrtvia SSH usingPuTTYrunning onWindows.

SSH is a protocol that can be used for many applications across many platforms including mostUnixvariants (Linux,theBSDsincludingApple'smacOS,andSolaris), as well asMicrosoft 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 aVPNis possible, but presently only with theOpenSSHserver and client implementation.

• For login to a shell on a remote host (replacingTelnetandrlogin)
• For executing a single command on a remote host (replacingrsh)
• For setting up automatic (passwordless) login to a remote server (for example, usingOpenSSH)
• In combination withrsyncto back up, copy and mirror files efficiently and securely
• Forforwardinga port
• Fortunneling(not to be confused with aVPN,whichroutespackets between different networks, orbridgestwobroadcast domainsinto one).
• For using as a full-fledged encrypted VPN. Note that onlyOpenSSHserver and client supports this feature.
• For forwardingXfrom a remotehost(possible through multiple intermediate hosts)
• For browsing the web through an encrypted proxy connection with SSH clients that support theSOCKS protocol.
• For securely mounting a directory on a remote server as afilesystemon a local computer usingSSHFS.
• 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

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

• Secure copy(SCP), which evolved fromRCPprotocol over SSH
• rsync,intended to be more efficient than SCP. Generally runs over an SSH connection.
• SSH File Transfer Protocol(SFTP), a secure alternative toFTP(not to be confused withFTP over SSHorFTPS)
• Files transferred over shell protocol(FISH), released in 1998, which evolved fromUnix shellcommands over SSH
• Fast and Secure Protocol(FASP), akaAspera,uses SSH for control and UDP ports for data transfer.

Architecture

Diagram of the SSH-2 binary packet.

The SSH protocol has a layered architecture with three separate components:

• Thetransport layer(RFC4253) typically uses theTransmission Control Protocol(TCP) ofTCP/IP,reservingport number22 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.
• Theuser authentication layer(RFC4252) handles client authentication, and provides a suite of authentication algorithms. Authentication isclient-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 forpublic-key-based authentication,usually supporting at leastDSA,ECDSAorRSAkeypairs, with other implementations also supportingX.509certificates.
  • keyboard-interactive(RFC4256): 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 provideone-time passwordauthentication such asS/KeyorSecurID.Used by some OpenSSH configurations whenPAMis the underlying host-authentication provider to effectively provide password authentication, sometimes leading to inability to log in with a client that supports just the plainpasswordauthentication method.
  • GSSAPIauthentication methods which provide an extensible scheme to perform SSH authentication using external mechanisms such asKerberos 5orNTLM,providingsingle sign-oncapability to SSH sessions. These methods are usually implemented by commercial SSH implementations for use in organizations, though OpenSSH does have a working GSSAPI implementation.
• Theconnection layer(RFC4254) 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:
  • shellfor terminal shells, SFTP and exec requests (including SCP transfers)
  • direct-tcpipfor client-to-server forwarded connections
  • forwarded-tcpipfor server-to-client forwarded connections
• TheSSHFPDNS 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 toTransport 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 toBEEPand not available in TLS.

Algorithms

• EdDSA,ECDSA,RSAandDSAforpublic-key cryptography.
• ECDHandDiffie–Hellmanforkey exchange.
• HMAC,AEADandUMACforMAC.
• AES(and deprecatedRC4,3DES,DES) forsymmetric encryption.
• AES-GCMandChaCha20-Poly1305forAEADencryption.
• SHA(and deprecatedMD5) forkey fingerprint.

Vulnerabilities

SSH-1

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 fromCRC-32used in this version of the protocol.A fix known as SSH Compensation Attack Detectorwas introduced into most implementations. Many of these updated implementations contained a newinteger overflowvulnerabilitythat 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 anIDEA-encrypted session.The same month, another vulnerability was discovered that allowed a malicious server to forward a client authentication to another server.

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.Most modern servers and clients support SSH-2.

CBC plaintext recovery

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.The most straightforward solution is to useCTR,counter mode, instead of CBC mode, since this renders SSH resistant to the attack.

Suspected decryption by NSA

On December 28, 2014Der Spiegelpublished classified informationleaked by whistleblowerEdward Snowdenwhich suggests that theNational Security Agencymay be able to decrypt some SSH traffic. The technical details associated with such a process were not disclosed. A 2017 analysis of theCIAhacking toolsBothanSpyandGyrfalconsuggested that the SSH protocol was not compromised.

Terrapin attack

Main article:Terrapin attack

A novel man-in-the-middle attack against most current ssh implementations was discovered in 2023. It was named theTerrapin attackby its discoverers.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.The ssh developers have stated that the major impact of the attack is to degrade thekeystroke timingobfuscation features of ssh.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

The followingRFCpublications by theIETF"secsh"working groupdocument SSH-2 as a proposedInternet standard.

• RFC4250–The Secure Shell (SSH) Protocol Assigned Numbers
• RFC4251–The Secure Shell (SSH) Protocol Architecture
• RFC4252–The Secure Shell (SSH) Authentication Protocol
• RFC4253–The Secure Shell (SSH) Transport Layer Protocol
• RFC4254–The Secure Shell (SSH) Connection Protocol
• RFC4255–Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints
• RFC4256–Generic Message Exchange Authentication for the Secure Shell Protocol (SSH)
• RFC4335–The Secure Shell (SSH) Session Channel Break Extension
• RFC4344–The Secure Shell (SSH) Transport Layer Encryption Modes
• RFC4345–Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer Protocol

The protocol specifications were later updated by the following publications:

• RFC4419–Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol(March 2006)
• RFC4432–RSA Key Exchange for the Secure Shell (SSH) Transport Layer Protocol(March 2006)
• RFC4462–Generic Security Service Application Program Interface (GSS-API) Authentication and Key Exchange for the Secure Shell (SSH) Protocol(May 2006)
• RFC4716–The Secure Shell (SSH) Public Key File Format(November 2006)
• RFC4819–Secure Shell Public Key Subsystem(March 2007)
• RFC5647–AES Galois Counter Mode for the Secure Shell Transport Layer Protocol(August 2009)
• RFC5656–Elliptic Curve Algorithm Integration in the Secure Shell Transport Layer(December 2009)
• RFC6187–X.509v3 Certificates for Secure Shell Authentication(March 2011)
• RFC6239–Suite B Cryptographic Suites for Secure Shell (SSH)(May 2011)
• RFC6594–Use of the SHA-256 Algorithm with RSA, Digital Signature Algorithm (DSA), and Elliptic Curve DSA (ECDSA) in SSHFP Resource Records(April 2012)
• RFC6668–SHA-2 Data Integrity Verification for the Secure Shell (SSH) Transport Layer Protocol(July 2012)
• RFC7479–Ed25519SSHFP Resource Records(March 2015)
• RFC5592–Secure Shell Transport Model for the Simple Network Management Protocol (SNMP)(June 2009)
• RFC6242–Using the NETCONF Protocol over Secure Shell (SSH)(June 2011)
• RFC8332–Use of RSA Keys with SHA-256 and SHA-512 in the Secure Shell (SSH) Protocol(March 2018)
• draft-gerhards-syslog-transport-ssh-00 –SSH transport mapping for SYSLOG(July 2006)
• draft-ietf-secsh-filexfer-13 –SSH File Transfer Protocol(July 2006)

In addition, theOpenSSHproject includes several vendor protocol specifications/extensions:

• OpenSSH PROTOCOL overview
• OpenSSH certificate/key overview
• draft-miller-ssh-agent-04- SSH Agent Protocol (December 2019)

See also

• Brute-force attack
• Comparison of SSH clients
• Comparison of SSH servers
• Corkscrew
• Ident
• OpenSSH
• Secure Shell tunneling
• Web-based SSH

Further reading

• Barrett, Daniel J.;Silverman, Richard E.; Byrnes, Robert G. (2005).SSH: The Secure Shell (The Definitive Guide)(2nd ed.). O'Reilly.ISBN0-596-00895-3.
• Stahnke, Michael (2005).Pro OpenSSH.Apress.ISBN1-59059-476-2.
• Tatu Ylönen (12 July 1995)."Announcement: Ssh (Secure Shell) Remote Login Program".comp.security.unix.Original announcement of Ssh
• Dwivedi, Himanshu (2003).Implementing SSH.Wiley.ISBN978-0-471-45880-7.

External links

• SSH Protocols
• M. Joseph; J. Susoy (November 2013).P6R's Secure Shell Public Key Subsystem.doi:10.17487/RFC7076.RFC7076.
• Original SSH source tarball