CVE-2026-66033

libssh2

Published 24 Jul 2026 · updated 30 Jul 2026 · Analyzed

8.7 High · CVSS 4.0, vulncheck.com

Description

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.

References

github.com/libssh2/libssh2/commit/a2ed82d40964bbc0d64cd717aa0a5a892117d2e6 · Patch
github.com/libssh2/libssh2/pull/2401 · Issue Tracking, Patch
www.vulncheck.com/advisories/libssh2-integer-underflow-dos-via-aes-gcm-cipher-negotiation · Patch, Third Party Advisory