CVE-2023-48795

Published: 18 December 2023

The SSH transport protocol with certain OpenSSH extensions, found in OpenSSH before 9.6 and other products, allows remote attackers to bypass integrity checks such that some packets are omitted (from the extension negotiation message), and a client and server may consequently end up with a connection for which some security features have been downgraded or disabled, aka a Terrapin attack. This occurs because the SSH Binary Packet Protocol (BPP), implemented by these extensions, mishandles the handshake phase and mishandles use of sequence numbers. For example, there is an effective attack against SSH's use of ChaCha20-Poly1305 (and CBC with Encrypt-then-MAC). The bypass occurs in chacha20-poly1305@openssh.com and (if CBC is used) the -etm@openssh.com MAC algorithms. This also affects Maverick Synergy Java SSH API before 3.1.0-SNAPSHOT, Dropbear through 2022.83, Ssh before 5.1.1 in Erlang/OTP, PuTTY before 0.80, AsyncSSH before 2.14.2, golang.org/x/crypto before 0.17.0, libssh before 0.10.6, libssh2 through 1.11.0, Thorn Tech SFTP Gateway before 3.4.6, Tera Term before 5.1, Paramiko before 3.4.0, jsch before 0.2.15, SFTPGo before 2.5.6, Netgate pfSense Plus through 23.09.1, Netgate pfSense CE through 2.7.2, HPN-SSH through 18.2.0, ProFTPD before 1.3.8b (and before 1.3.9rc2), ORYX CycloneSSH before 2.3.4, NetSarang XShell 7 before Build 0144, CrushFTP before 10.6.0, ConnectBot SSH library before 2.2.22, Apache MINA sshd through 2.11.0, sshj through 0.37.0, TinySSH through 20230101, trilead-ssh2 6401, LANCOM LCOS and LANconfig, FileZilla before 3.66.4, Nova before 11.8, PKIX-SSH before 14.4, SecureCRT before 9.4.3, Transmit5 before 5.10.4, Win32-OpenSSH before 9.5.0.0p1-Beta, WinSCP before 6.2.2, Bitvise SSH Server before 9.32, Bitvise SSH Client before 9.33, KiTTY through 0.76.1.13, the net-ssh gem 7.2.0 for Ruby, the mscdex ssh2 module before 1.15.0 for Node.js, the thrussh library before 0.35.1 for Rust, and the Russh crate before 0.40.2 for Rust.

Notes

Per upstream openssh developers:
While cryptographically novel, the security impact of this attack
is fortunately very limited as it only allows deletion of
consecutive messages, and deleting most messages at this stage of
the protocol prevents user user authentication from proceeding and
results in a stuck connection.
The most serious identified impact is that it lets a MITM to
delete the SSH2_MSG_EXT_INFO message sent before authentication
starts, allowing the attacker to disable a subset of the keystroke
timing obfuscation features introduced in OpenSSH 9.5. There is no
other discernable impact to session secrecy or session integrity.

putty fixes include some refactoring commits that are used
by the added strict kex mode support

openssh-ssh1 is provided for compatibility with old devices
that cannot be upgraded to modern protocols. Thus we may not
provide security support for this package if doing so would
prevent access to equipment.

snapd contains an embedded copy of golang-go.crypto
lxd in 18.04 LTS and earlier contains an embedded copy of
golang-go.crypto

proftpd-dfsg contains an ssh implementation in mod-sftp
(distributed in proftpd-mod-crypto/proftpd-basic)

per Debian, libssh2 older than mantic isn't vulnerable as it
doesn't support ChaCha20-Poly1305 and CBC-EtM.

libssh will be marked as not affected for bionic and earlier as per
the above note. ChaCha20-Poly1305 and CBC-EtM are present only in
libssh 0.8.0 (released 2018-08-10. Bionic is version 0.8.0, but
based on commit 94fa1e38, which was released 2017-08-25) and later.
Patches provided by upstream do not apply cleanly to versions in
bionic and earlier as several functionalities are not present in
these versions of the code (making backports a more likely to
introduce regressions) further supporting the defined not affected
status.

Status

Severity score breakdown

References

• https://terrapin-attack.com/
• https://www.openwall.com/lists/oss-security/2023/12/18/3
• https://github.com/ronf/asyncssh/security/advisories/GHSA-hfmc-7525-mj55
• https://ubuntu.com/security/notices/USN-6560-1
• https://ubuntu.com/security/notices/USN-6561-1
• https://ubuntu.com/security/notices/USN-6560-2
• https://ubuntu.com/security/notices/USN-6585-1
• https://ubuntu.com/security/notices/USN-6589-1
• https://ubuntu.com/security/notices/USN-6598-1
• https://www.cve.org/CVERecord?id=CVE-2023-48795
• https://ubuntu.com/security/notices/USN-6738-1
• NVD
• Launchpad
• Debian

Bugs

• https://github.com/golang/go/issues/64784
• https://github.com/paramiko/paramiko/issues/2337
• https://github.com/libssh2/libssh2/issues/1290