CVE-2026-55199 Details
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.
A pre-authentication denial-of-service vulnerability has been identified in libssh2 versions through 1.11.1. The issue arises in the SSH_MSG_EXT_INFO handler within the packet.c file, where a malicious SSH server can cause CPU exhaustion on the client side. This is achieved by sending a crafted extension count value that forces the client to enter a CPU-bound loop for over 60 seconds. The vulnerability exists because the return values from the _libssh2_get_string() function are not properly checked, allowing the session to be exploited without triggering the usual timeout mechanisms.
Users can upgrade to libssh2 version 1.11.1 or later to address this vulnerability.
Metrics
CVSS 4.0 Severity and Vector Strings:
CVSS 3.x Severity and Vector Strings:
No data available for CVSS Version 2.0 on this CVE.
CISA-ADP
Assessed Jun 18, 2026References to Advisories, Solutions, and Tools
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Weakness Enumeration
| CWE-ID | CWE Name | Source |
|---|---|---|
| CWE-835 | Loop with Unreachable Exit Condition ('Infinite Loop') | [email protected] |
Affected Products
| Product | Versions |
|---|---|
| libssh2 libssh2 | <= 1.11.1 |
CPE
Remediation
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Change History
4 change records found show changes
| Date | Action | Recorded By |
|---|---|---|
| Jul 14, 2026 | CVE Modified | [email protected] |
| Jun 26, 2026 | Initial Analysis | [email protected] |
| Jun 18, 2026 | CVE Modified | CISA-ADP |
| Jun 17, 2026 | New CVE Received | [email protected] |