CVE-2026-55199
libssh2 - Pre-Authentication DoS via SSH_MSG_EXT_INFO Handler
- State
- Published
- Severity
- 8.2 High
- Published
- Jun 17, 2026
- Updated
- Jul 14, 2026
- Reserved
- Jun 16, 2026
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.
Scores
| CVSS | Score | Vector | Source |
|---|---|---|---|
| 4.0 | 8.2 High | CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N | VulnCheck |
| 3.1 | 5.9 Medium | CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H | VulnCheck |
Weaknesses
- CWE-835 Loop with Unreachable Exit Condition ('Infinite Loop')
Affected Products
| Product | Versions |
|---|---|
libssh2libssh2 |
|
References
- Pull Requestissue-tracking
- Patch Commitpatch
- https://www.vulncheck.com/advisories/libssh2-pre-authentication-dos-via-ssh-msg-ext-info-handlerthird-party-advisory
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