CVE-2026-55199 – libssh2 – Pre-Authentication DoS via SSH_MSG_EXT_INFO Handler

CVE ID :CVE-2026-55199

Published : June 17, 2026, 6:44 p.m. | 58 minutes ago

Description :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.

Severity: 8.2 | HIGH

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