CVE-2026-55199: LIBSSH2

High severity, CVSS 7.5. EPSS: 0.9% chance of exploitation in the next 30 days.

libssh2 through 1.11.1, fixed in commit 1762685, contains a pre-authentication denial of service vulnerability in the SSH_MSG_EXT_INFO handler in src/packet.c that allows a malicious SSH server to cause a client CPU exhaustion loop by sending a crafted extension count value. A malicious server can set nr_extensions to 0xFFFFFFFF during key exchange, causing the client to spin in a tight CPU loop for over 60 seconds because return values from _libssh2_get_string() are unchecked and the session timeout does not apply to CPU-bound loops.

Affected products

• LIBSSH2 LIBSSH2: up to and including 1.11.1

Published 2026-06-17. Last modified 2026-07-14.