libssh2 <=1.11.1 SSH_MSG_EXT_INFO Pre-auth CPU DoS
CVE-2026-55199 Published on June 17, 2026
libssh2 - Pre-Authentication DoS via SSH_MSG_EXT_INFO Handler
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.
Vulnerability Analysis
CVE-2026-55199 can be exploited with network access, and does not require authorization privileges or user interaction. This vulnerability is consided to have a high level of attack complexity. The potential impact of an exploit of this vulnerability is considered to have no impact on confidentiality and integrity, and a high impact on availability.
Weakness Type
What is an Infinite Loop Vulnerability?
The program contains an iteration or loop with an exit condition that cannot be reached, i.e., an infinite loop. If the loop can be influenced by an attacker, this weakness could allow attackers to consume excessive resources such as CPU or memory.
CVE-2026-55199 has been classified to as an Infinite Loop vulnerability or weakness.
Products Associated with CVE-2026-55199
Want to know whenever a new CVE is published for Libssh2? stack.watch will email you.
Affected Versions
libssh2:- Before and including 1.11.1 is affected.
- Version 17626857d20b3c9a1addfa45979dadcee1cd84a4 is unaffected.
Exploit Probability
EPSS (Exploit Prediction Scoring System) scores estimate the probability that a vulnerability will be exploited in the wild within the next 30 days. The percentile shows you how this score compares to all other vulnerabilities.