CVE-2026-55199
MediumCVSS 5.9Exploitation Probability (EPSS)
Low risk33th percentile — higher than 33% of all known CVEs
Summary
A vulnerability in libssh2 (up to version 1.1.1.1) allows a pre-authentication denial of service attack. A malicious SSH server can send a crafted extension count value (nr_extensions=0xFFFFFFFF) during key exchange, causing a client CPU exhaustion loop for over 60 seconds due to unchecked return values from _libssh2_get_string() and session timeout not applying to CPU-bound loops.
Risk Assessment
An attacker-controlled SSH server can cause client CPU exhaustion, leading to denial of service (DoS) and preventing normal operation of applications using libssh2.
Recommendation
Update libssh2 to a version containing commit 1762685 or later. If updating is not possible, restrict trust to SSH servers or apply firewall rules to block suspicious connections.
Original NVD description (English source)
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.