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MEDIUM

ssh: Prefix truncation attack on Binary Packet Protocol (BPP)

Published Dec 18, 2023

Description

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.

Affected products

Remediation

Red Hat statement

This CVE is classified as moderate because the attack requires an active Man-in-the-Middle (MITM) who can intercept and modify the connection's traffic at the TCP/IP layer. Although the attack is cryptographically innovative, its security impact is fortunately quite limited. It only allows the deletion of consecutive messages, and deleting most messages at this protocol stage prevents user authentication from proceeding, leading to a stalled connection. The most significant identified impact is that it enables a MITM to delete the SSH2_MSG_EXT_INFO message sent before authentication begins. This allows the attacker to disable a subset of keystroke timing obfuscation features. However, there is no other observable impact on session secrecy or session integrity.

Red Hat mitigation

Update to the last version and check that client and server provide kex pseudo-algorithms indicating usage of the updated version of the protocol which is protected from the attack. If "kex-strict-c-v00@openssh.com" is provided by clients and "kex-strict-s-v00@openssh.com" is in the server's reply, no other steps are necessary. Disabling ciphers if necessary: If "kex-strict-c-v00@openssh.com" is not provided by clients or "kex-strict-s-v00@openssh.com" is absent in the server's reply, you can disable the following ciphers and HMACs as a workaround on RHEL-8 and RHEL-9: 1. chacha20-poly1305@openssh.com 2. hmac-sha2-512-etm@openssh.com 3. hmac-sha2-256-etm@openssh.com 4. hmac-sha1-etm@openssh.com 5. hmac-md5-etm@openssh.com To do that through crypto-policies, one can apply a subpolicy with the following content: ``` cipher@SSH = -CHACHA20-POLY1305 ssh_etm = 0 ``` e.g., by putting these lines into `/etc/crypto-policies/policies/modules/CVE-2023-48795.pmod`, applying the resulting subpolicy with `update-crypto-policies --set $(update-crypto-policies --show):CVE-2023-48795` and restarting openssh server. One can verify that the changes are in effect by ensuring the ciphers listed above are missing from both `/etc/crypto-policies/back-ends/openssh.config` and `/etc/crypto-policies/back-ends/opensshserver.config`. For more details on using crypto-policies, please refer to https://access.redhat.com/documentation/en-us/red_hat_enterprise_linux/9/html/security_hardening/using-the-system-wide-cryptographic-policies_security-hardening Note that this procedure does limit the interoperability of the host and is only suggested as a temporary mitigation until the issue is fully resolved with an update. For RHEL-7: We can recommend to use strict MACs and Ciphers on RHEL7 in both files /etc/ssh/ssh_config and /etc/ssh/sshd_config. Below strict set of Ciphers and MACs can be used as mitigation for RHEL 7. ``` Ciphers aes128-ctr,aes192-ctr,aes256-ctr,aes128-gcm@openssh.com,aes256-gcm@openssh.com MACs umac-64@openssh.com,umac-128@openssh.com,hmac-sha2-256,hmac-sha2-512 ``` - For Openshift Container Platform 4: Please refer the KCS[1] document for verifying the fix in RHCOS. [1] https://access.redhat.com/solutions/7071748 ~~~ NOTE: The crypto-policies workaround requires RHEL 8.5 or newer. Customers (for older RHEL 8 releases) who cannot immediately update crypto-policies packages may manually configuring Ciphers and MACs directly in /etc/ssh/sshd_config and ssh_config (as used for "RHEL-7") method documented https://access.redhat.com/solutions/7066001 ~~~~

Metrics

Weaknesses (3)

CWE-222

CWE-345

CWE-354

References (149)

Change history (0)

No recorded changes yet.

Sources
CVE.org / MITRE
Status PUBLISHED
Assigner mitre
Published Dec 18, 2023
Updated Jul 14, 2026
Reserved Nov 20, 2023
Link CVE-2023-48795
CISA Vulnrichment
Updated Dec 22, 2023
NVD
Status Modified
Modified Jun 17, 2026
Link CVE-2023-48795
Red Hat
Severity Moderate
Public date Dec 18, 2023
Links CVE-2023-48795 - Bugzilla
GHSA-45X7-PX36-X8W8