Denial of Service via crafted SSH_MSG_EXT_INFO message

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 sending a malformed packet, the server triggers a loop that exhausts the client's CPU, resulting in a denial of service. A Moderate-rated denial of service vulnerability in the libssh2 client allows a malicious SSH server to freeze the connecting application. 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. Fixed by RHSA-2026:29950 — update the affected packages (`sudo dnf update`). Affected Red Hat products: Red Hat Hardened Images. Will not fix / out of support: Red Hat Enterprise Linux 6; Red Hat Enterprise Linux 7.