CVE-2026-55199
libssh2 - Pre-Authentication DoS via SSH_MSG_EXT_INFO Handler
CVSS Score
5.9
EPSS Score
0.0%
EPSS Percentile
0th
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.
| CWE | CWE-835 |
| Vendor | libssh2 |
| Product | libssh2 |
| Published | Jun 17, 2026 |
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CVSS v3 Breakdown
CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H Attack Vector
Network
Attack Complexity
High
Privileges Required
None
User Interaction
None
Scope
Unchanged
Confidentiality
None
Integrity
None
Availability
High
Affected Versions
libssh2 / libssh2
0 ≤ 1.11.1
References
Credits
Tristan Madani (@TristanInSec)