Medium [CVE-2026-55199] Denial of Service via crafted SSH_MSG_EXT_INFO message
This medium-severity Red Hat Linux advisory covers CVE-2026-55199 affecting Red Hat Hardened Images, Red Hat Enterprise Linux 6, Red Hat Enterprise Linux 7.
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Summary
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 vulnerability in libssh2 allows a malicious SSH server to freeze connected clients during the handshake process. By triggering an infinite CPU loop during the initial connection handshake, the server can render the client unresponsive.
Note: Red Hat Enterprise Linux (RHEL) 8 and newer are not affected by this flaw, as they do not ship the libssh2 package. Red Hat severity: Moderate — CVSS 5.9 (CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H).
Weakness: CWE-606. Affected Red Hat products: Red Hat Hardened Images; Red Hat Enterprise Linux 6; Red Hat Enterprise Linux 7.
Will not fix / out of support: Red Hat Enterprise Linux 6; Red Hat Enterprise Linux 7. Red Hat fixing advisory: RHSA-2026:29950.
Affected products named by the advisory: Red Hat package: libssh2.
Affected versions
No affected-version range was extracted from the source record. The vendor advisory is authoritative — check it before change work.
Official advisory · high-confidence parse· fetched 2 days ago·verify at source
- 1.11.1
- libssh2-main-1.11.1-8.hum1
- RHSA-2026:29950
Official advisory · high-confidence parse· fetched 2 days ago·verify at source
Mitigation checklist
- To mitigate this issue, ensure your libssh2 clients only connect to trusted SSH servers. You can enforce this by implementing outbound network access controls (egress filtering) to block applications from initiating connections to unknown or untrusted external hosts.
Official advisory · high-confidence parse· fetched 2 days ago·verify at source
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