PatchTheCVECVE-2026-55199: libssh2 through 1.11.1, fixed in commit 1762685, contains a pre-authentication denial of service vulnerability
CVE-2026-55199 is a medium-severity flaw (CVSS 5.9) affecting libssh2. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.
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 _lib
Why this priority
- Remotely exploitable, no login, no user action
A fixed release or distro patch exists.
Risk facts
| CVSS score | 5.9 (Medium) |
| Attack vector | Network (remote) |
| Attack complexity | High |
| Privileges needed | None |
| User interaction | None |
| EPSS exploit chance | 0.9% (top 40%) |
| CISA exploited list | No |
| Weakness | CWE-835 |
| Published | 2026-06-17 |
| Last modified | 2026-07-14 |
Last updated 2026-07-14 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score
Am I affected?
Timeline
- Published CVE record published
- Updated Record last modified
Scores and fixes here are computed automatically from those and other public sources. Confirm with the vendor advisory before changing production systems.
Quick answers
How do I fix CVE-2026-55199?
libssh2: upgrade to newer than 1.11.1. Exact commands for each OS are on this page.
Is CVE-2026-55199 being exploited?
CVE-2026-55199 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.
What does CVE-2026-55199 affect?
CVE-2026-55199 affects libssh2. Severity is medium (CVSS 5.9). 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 sendi
How urgent is CVE-2026-55199?
Priority 41/100. Remotely exploitable, no login, no user action
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