CORE SDI SSH1 CRC-32 compensation attack detector allows remote attackers to execute arbitrary commands on an SSH server or client via an integer overflow.

A race condition in the authentication agent mechanism of sshd 1.2.17 allows an attacker to steal another user's credentials.

Buffer overflow in the URL catcher feature for SSH Secure Shell for Workstations client 3.1 to 3.2.0 allows remote attackers to execute arbitrary code via a long URL.

The SSH USERAUTH CHANGE REQUEST feature in SSH Tectia Server 6.0.4 through 6.0.20, 6.1.0 through 6.1.12, 6.2.0 through 6.2.5, and 6.3.0 through 6.3.2 on UNIX and Linux, when old-style password authentication is enabled, allows remote attackers to bypass authentication via a crafted session involving entry of blank passwords, as demonstrated by a root login session from a modified OpenSSH client with an added inputuserauthpasswdchangereq call in sshconnect2.c.

PrivX before 34.0 allows data exfiltration and denial of service via the REST API. This is fixed in minor versions 33.1, 32.3, 31.3, and later, and in major version 34.0 and later,

SSH Tectia Client and Server before 6.4.19 on Windows have weak key generation. ConnectSecure on Windows is affected.

Stolen credentials from SSH clients via ssh-agent program, allowing other local users to access remote accounts belonging to the ssh-agent user.

The random number generator in the Crypto application before 2.0.2.2, and SSH before 2.0.5, as used in the Erlang/OTP ssh library before R14B03, uses predictable seeds based on the current time, which makes it easier for remote attackers to guess DSA host and SSH session keys.

SSH Tectia Client and Server before 6.4.19 on Windows allow local privilege escalation. ConnectSecure on Windows is affected.

The SSH-1 protocol allows remote servers to conduct man-in-the-middle attacks and replay a client challenge response to a target server by creating a Session ID that matches the Session ID of the target, but which uses a public key pair that is weaker than the target's public key, which allows the attacker to compute the corresponding private key and use the target's Session ID with the compromised key pair to masquerade as the target.

SSH before 2.0, with RC4 encryption and the "disallow NULL passwords" option enabled, makes it easier for remote attackers to guess portions of user passwords by replaying user sessions with certain modifications, which trigger different messages depending on whether the guess is correct or not.

SSH before 2.0, when using RC4 and password authentication, allows remote attackers to replay messages until a new server key (VK) is generated.

The SSH protocols 1 and 2 (aka SSH-2) as implemented in OpenSSH and other packages have various weaknesses which can allow a remote attacker to obtain the following information via sniffing: (1) password lengths or ranges of lengths, which simplifies brute force password guessing, (2) whether RSA or DSA authentication is being used, (3) the number of authorizedkeys in RSA authentication, or (4) the lengths of shell commands.

SSH daemon version 1 (aka SSHD-1 or SSH-1) 1.2.30 and earlier does not log repeated login attempts, which could allow remote attackers to compromise accounts without detection via a brute force attack.

SSH server (sshd2) before 2.0.12 does not properly record login attempts if the connection is closed before the maximum number of tries, allowing a remote attacker to guess the password without showing up in the audit logs.

SSH Tectia Server 5.0.0 (A, F, and T), when allowing host-based authentication only, allows users to log in with the wrong credentials.

SSH 1.2.25 on HP-UX allows access to new user accounts.

SSH Secure Shell for Servers 3.0.0 to 3.1.1 allows remote attackers to override the AllowedAuthentications configuration and use less secure authentication schemes (e.g. password) than configured for the server.

SSH Tectia Server before 6.6.6 sometimes allows attackers to read and alter a user's session traffic.

SSH Secure Shell 3.0.0 on Unix systems does not properly perform password authentication to the sshd2 daemon, which allows local users to gain access to accounts with short password fields, such as locked accounts that use "NP" in the password field.

SSH 1.2.27 with Kerberos authentication support stores Kerberos tickets in a file which is created in the current directory of the user who is logging in, which could allow remote attackers to sniff the ticket cache if the home directory is installed on NFS.

SSH Secure Shell for Servers and SSH Secure Shell for Workstations 2.0.13 through 3.2.1, when running without a PTY, does not call setsid to remove the child process from the process group of the parent process, which allows attackers to gain certain privileges.

SSH 1 through 3, and possibly other versions, allows local users to bypass restricted shells such as rbash or rksh by uploading a script to a world-writeable directory, then executing that script to gain normal shell access.

Unquoted Windows search path vulnerability in multiple SSH Tectia products, including Client/Server/Connector 5.0.0 and 5.0.1 and Client/Server before 4.4.5, and Manager 2.12 and earlier, when running on Windows, might allow local users to gain privileges via a malicious program file under "Program Files" or its subdirectories.

SSH Tectia Management Agent 2.1.2 allows local users to gain root privileges by running a program called sshd, which is obtained from a process listing when the "Restart" action is selected from the Management server GUI, which causes the agent to locate the pathname of the user's program and restart it with root privileges.

ssh-signer in SSH Tectia Client and Server 5.x before 5.2.4, and 5.3.x before 5.3.6, on Unix and Linux allows local users to gain privileges via unspecified vectors.

SSH Tectia Client and Server before 6.4.19 on Windows allow local privilege escalation in nonstandard conditions. ConnectSecure on Windows is affected.

Summary

Terrapin is a prefix truncation attack targeting the SSH protocol. More precisely, Terrapin breaks the integrity of SSH's secure channel. By carefully adjusting the sequence numbers during the handshake, an attacker can remove an arbitrary amount of messages sent by the client or server at the beginning of the secure channel without the client or server noticing it.

Mitigations

To mitigate this protocol vulnerability, OpenSSH suggested a so-called "strict kex" which alters the SSH handshake to ensure a Man-in-the-Middle attacker cannot introduce unauthenticated messages as well as convey sequence number manipulation across handshakes.

Warning: To take effect, both the client and server must support this countermeasure.

As a stop-gap measure, peers may also (temporarily) disable the affected algorithms and use unaffected alternatives like AES-GCM instead until patches are available.

Details

The SSH specifications of ChaCha20-Poly1305 (chacha20-poly1305@openssh.com) and Encrypt-then-MAC (-etm@openssh.com MACs) are vulnerable against an arbitrary prefix truncation attack (a.k.a. Terrapin attack). This allows for an extension negotiation downgrade by stripping the SSHMSGEXTINFO sent after the first message after SSHMSGNEWKEYS, downgrading security, and disabling attack countermeasures in some versions of OpenSSH. When targeting Encrypt-then-MAC, this attack requires the use of a CBC cipher to be practically exploitable due to the internal workings of the cipher mode. Additionally, this novel attack technique can be used to exploit previously unexploitable implementation flaws in a Man-in-the-Middle scenario.

The attack works by an attacker injecting an arbitrary number of SSHMSGIGNORE messages during the initial key exchange and consequently removing the same number of messages just after the initial key exchange has concluded. This is possible due to missing authentication of the excess SSHMSGIGNORE messages and the fact that the implicit sequence numbers used within the SSH protocol are only checked after the initial key exchange.

In the case of ChaCha20-Poly1305, the attack is guaranteed to work on every connection as this cipher does not maintain an internal state other than the message's sequence number. In the case of Encrypt-Then-MAC, practical exploitation requires the use of a CBC cipher; while theoretical integrity is broken for all ciphers when using this mode, message processing will fail at the application layer for CTR and stream ciphers.

For more details see https://terrapin-attack.com.

Impact

This attack targets the specification of ChaCha20-Poly1305 (chacha20-poly1305@openssh.com) and Encrypt-then-MAC (-etm@openssh.com), which are widely adopted by well-known SSH implementations and can be considered de-facto standard. These algorithms can be practically exploited; however, in the case of Encrypt-Then-MAC, we additionally require the use of a CBC cipher. As a consequence, this attack works against all well-behaving SSH implementations supporting either of those algorithms and can be used to downgrade (but not fully strip) connection security in case SSH extension negotiation (RFC8308) is supported. The attack may also enable attackers to exploit certain implementation flaws in a man-in-the-middle (MitM) scenario.

The credentials for the local user "user-app" may be exposed in log files, potentially enabling a low-privileged local attacker with access to the logs to authenticate via SSH as the limited user "user-app". Charging could be interrupted.

SSH 1.2.25, 1.2.23, and other versions, when used in in CBC (Cipher Block Chaining) or CFB (Cipher Feedback 64 bits) modes, allows remote attackers to insert arbitrary data into an existing stream between an SSH client and server by using a known plaintext attack and computing a valid CRC-32 checksum for the packet, aka the "SSH insertion attack."