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.

When writing data larger than 4GB in a single Write call on an SSH channel, an integer overflow in the internal payload size calculation caused the write loop to spin indefinitely, sending empty packets without making progress. The size comparison now uses int64 to prevent truncation.

SSH Secure Shell before 3.2.9 allows remote attackers to cause a denial of service via malformed BER/DER packets.

Race condition in SSH Tectia Server 4.0.3 and 4.0.4 for Unix, when the password change plugin (ssh-passwd-plugin) is enabled, allows local users to obtain the server's private key.

The default configuration of SSH allows X forwarding, which could allow a remote attacker to control a client's X sessions via a malicious xauth program.

The SSH protocol server sshd allows local users without shell access to redirect a TCP connection through a service that uses the standard system password database for authentication, such as POP or FTP.

ssh 2.0.12, and possibly other versions, allows valid user names to attempt to enter the correct password multiple times, but only prompts an invalid user name for a password once, which allows remote attackers to determine user account names on the server.

SSH 2.0.11 and earlier allows local users to request remote forwarding from privileged ports without being root.

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."

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.

The SSH authentication agent follows symlinks via a UNIX domain socket.

In some instances of SSH 1.2.27 and 2.0.11 on Linux systems, SSH will allow users with expired accounts to login.

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

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

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

Error handling in the SSH protocol in (1) SSH Tectia Client and Server and Connector 4.0 through 4.4.11, 5.0 through 5.2.4, and 5.3 through 5.3.8; Client and Server and ConnectSecure 6.0 through 6.0.4; Server for Linux on IBM System z 6.0.4; Server for IBM z/OS 5.5.1 and earlier, 6.0.0, and 6.0.1; and Client 4.0-J through 4.3.3-J and 4.0-K through 4.3.10-K; and (2) OpenSSH 4.7p1 and possibly other versions, when using a block cipher algorithm in Cipher Block Chaining (CBC) mode, makes it easier for remote attackers to recover certain plaintext data from an arbitrary block of ciphertext in an SSH session via unknown vectors.

SSH Tectia Client/Server/Connector 5.1.0 and earlier, Manager 2.2.0 and earlier, and other products, when using an RSA key with exponent 3, removes PKCS-1 padding before generating a hash, which allows remote attackers to forge a PKCS #1 v1.5 signature that is signed by that RSA key and prevents Tectia from correctly verifying X.509 and other certificates that use PKCS #1, a similar issue to CVE-2006-4339.

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

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.

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,

Directory traversal vulnerability in scp in sshd 1.2.xx allows a remote malicious scp server to overwrite arbitrary files via a .. (dot dot) attack.

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.

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 disables host key checking when connecting to the localhost, which allows remote attackers to silently redirect connections to the localhost by poisoning the client's DNS cache.

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.

The RC4 stream cipher as used by SSH1 allows remote attackers to modify messages without detection by XORing the original message's cyclic redundancy check (CRC) with the CRC of a mask consisting of all the bits of the original message that were modified.

The IDEA cipher as implemented by SSH1 does not protect the final block of a message against modification, which allows remote attackers to modify the block without detection by changing its cyclic redundancy check (CRC) to match the modifications to the message.

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