CVE-2026-66033

libssh2 through 1.11.1, fixed in commit a2ed82d, contains a pre-authentication integer underflow vulnerability in the ssh2_cipher_crypt() function in src/openssl.c that allows a malicious SSH server to crash any connecting client by negotiating AES-GCM ciphers during handshake. Attackers can exploit the underflow in the expression computing blocksize minus aadlen minus authentication tag length to trigger an out-of-bounds read and a memcpy call with a near-SIZE_MAX length argument, causing immediate process crash before any authentication occurs.

• libssh2 1.11.1-5 (bug https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1142856) [trixie] - libssh2 (Minor issue; can be fixed via point release) [bookworm] - libssh2 (Vulnerable code not present) [bullseye] - libssh2 (Vulnerable code not present) https://github.com/libssh2/libssh2/pull/2401 Fixed by: https://github.com/libssh2/libssh2/commit/a2ed82d40964bbc0d64cd717aa0a5a892117d2e6