Medium [CVE-2026-55655] Local MITM of X11 forwarding via abstract UNIX socket pre-binding in Red Hat Enterprise Linux OpenSSH client versions

A flaw was found in OpenSSH. A local unprivileged attacker on a Linux client host can hijack client-side X11 forwarding connections.

This is possible by pre-binding the preferred abstract X socket name when X11 forwarding is enabled and a local UNIX-domain X socket is used. A successful attack can compromise the confidentiality of forwarded X11 traffic, including sensitive window contents and input, and may allow some manipulation of the forwarded session.

This is a Moderate severity flaw. The OpenSSH client in Red Hat Enterprise Linux is vulnerable to a local man-in-the-middle attack on X11 forwarding connections.

Exploitation requires an attacker to have local unprivileged access on the client system and for X11 forwarding to be explicitly enabled and in use, which is not a default configuration. This vulnerability doesn't affect the upstream OpenSSH versions and is restricted to the versions as shipped with Red Hat Enterprise Linux.

Red Hat severity: Moderate — CVSS 5 (CVSS:3.1/AV:L/AC:H/PR:L/UI:R/S:U/C:H/I:L/A:N). Weakness: CWE-923.

Affected Red Hat products: Red Hat Enterprise Linux 10; Red Hat Enterprise Linux 8; Red Hat Enterprise Linux 9; Red Hat Hardened Images; Red Hat Update Infrastructure 5; Red Hat Enterprise Linux 6; Red Hat Enterprise Linux 7. Under investigation: Red Hat OpenShift Container Platform 4.