Vulnerability · Published Jun 17, 2026 · Assigned by VulnCheck
CVE-2026-55199libssh2 - Pre-Authentication DoS via SSH_MSG_EXT_INFO Handler
Medium 5.9
In plain English
A medium-severity infinite loop in libssh2 from libssh2. There are no confirmed reports of exploitation so far. The chance it is exploited in the next 30 days is estimated at 0.9% (higher than 60% of all known vulnerabilities). Fixed versions are available - upgrade the affected package.
Official 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.
Affected packages & fixed versions
| Ecosystem | Package | Fixed in | Advisory |
|---|---|---|---|
| Alpine:v3.24 | chromium-rust | 1.96.1-r0 | ALPINE-CVE-2026-55199 |
| Alpine:v3.21 | libssh2 | 1.11.1-r2 | ALPINE-CVE-2026-55199 |
| Alpine:v3.22 | libssh2 | 1.11.1-r2 | ALPINE-CVE-2026-55199 |
| Alpine:v3.23 | libssh2 | 1.11.1-r3 | ALPINE-CVE-2026-55199 |
| Alpine:v3.24 | libssh2 | 1.11.1-r3 | ALPINE-CVE-2026-55199 |
| Alpine:v3.24 | rust | 1.96.1-r0 | ALPINE-CVE-2026-55199 |
| Debian:13 | libssh2 | 1.11.1-1+deb13u1 | DEBIAN-CVE-2026-55199 |
| Debian:14 | libssh2 | 1.11.1-4 | DEBIAN-CVE-2026-55199 |
| Ubuntu:24.04:LTS | libssh2 | 1.11.0-4.1ubuntu0.24.04.2 | UBUNTU-CVE-2026-55199 |
| Ubuntu:25.10 | libssh2 | 1.11.1-1ubuntu0.25.10.2 | UBUNTU-CVE-2026-55199 |
| Ubuntu:26.04:LTS | libssh2 | 1.11.1-1ubuntu0.26.04.2 | UBUNTU-CVE-2026-55199 |
Affected products (as reported)
Weakness: CWE-835 · Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H
References
- Pull Request issue-tracking
- Patch Commit patch
- https://www.vulncheck.com/advisories/libssh2-pre-authentication-dos-via-ssh-msg-ext-info-handler third-party-advisory