Vulnerability in Libssh2
CVE-2026-55199
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.
Published · last modified .
CVSS v3 metric
CVSS v3 base score 5.9 (Medium). Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H.
EPSS exploit prediction
EPSS: 0.007 (51.0th percentile), scored .
Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 51st percentile — 51.0% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.
EPSS over last 30 days · oldest: 0.009 · newest: 0.007 · change: -0.002
Affected products
- Libssh2 — versions 0, 17626857d20b3c9a1addfa45979dadcee1cd84a4
Weakness classification (CWE)
References
- github.com/libssh2/libssh2/pull/1864 (issue-tracking, Patch)
- github.com/libssh2/libssh2/commit/17626857d20b3c9a1addfa45979dadcee1cd84a4 (Patch, patch)
- www.vulncheck.com/advisories/libssh2-pre-authentication-dos-via-ssh-msg-ext-inf… (Third Party Advisory, third-party-advisory)
Frequently asked questions
- What is CVE-2026-55199?
- CVE-2026-55199 is a medium-severity vulnerability in Libssh2, classified under Loop with Unreachable Exit Condition (Infinite Loop). CVSS score: 5.9/10. Published 2026-06-17.
- How severe is CVE-2026-55199?
- Medium severity. CVSS v3 base score is 5.9 out of 10.