CVE-2026-55199
libssh2 - Pre-Authentication DoS via SSH_MSG_EXT_INFO Handler
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.
INFO
Published Date :
June 17, 2026, 6:44 p.m.
Last Modified :
June 17, 2026, 6:44 p.m.
Remotely Exploit :
Yes !
Source :
VulnCheck
Affected Products
The following products are affected by CVE-2026-55199
vulnerability.
Even if cvefeed.io is aware of the exact versions of the
products
that
are
affected, the information is not represented in the table below.
No affected product recoded yet
CVSS Scores
| Score | Version | Severity | Vector | Exploitability Score | Impact Score | Source |
|---|---|---|---|---|---|---|
| CVSS 3.1 | MEDIUM | 83251b91-4cc7-4094-a5c7-464a1b83ea10 | ||||
| CVSS 4.0 | HIGH | 83251b91-4cc7-4094-a5c7-464a1b83ea10 |
Solution
- Update libssh2 to version 1.11.1 or later.
- Apply the fix from commit 1762685.
- Ensure return values from _libssh2_get_string() are checked.
- Verify session timeout applies to CPU-bound loops.
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CVE-2026-55199 vulnerability anywhere in the article.