CVE-2026-66033: LIBSSH2

High severity, CVSS 7.5. EPSS: 0.6% chance of exploitation in the next 30 days.

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.

Affected products

• LIBSSH2 LIBSSH2: up to and including 1.11.1

Published 2026-07-24. Last modified 2026-07-30.