CVE-2025-32728

Vulnerability

In sshd in OpenSSH before version 10.0, the DisableForwarding configuration directive does not adhere to its documentation that states it disables X11 and agent forwarding. The server's session handling code in session.c does not check options.disable_forwarding when processing X11 forwarding requests (function session_setup_x11fwd) or agent forwarding requests (function session_auth_agent_req). As a result, setting DisableForwarding yes in sshd_config has no effect on these forwarding types. Affected are all OpenSSH versions prior to 10.0; the issue is fixed in OpenSSH 10.0 and in the OpenBSD 7.6 errata 013 patch [1][3][4].

Exploitation

An attacker who can authenticate to an affected OpenSSH server and then request X11 or agent forwarding (for example, via ssh -X or ssh -A, or through authentication agent forwarding) will be granted those forwarding capabilities even though the administrator has explicitly disabled them with DisableForwarding. The attacker needs valid SSH credentials and network access to the server. No additional race condition or user interaction beyond the normal SSH session establishment is required [1][4].

Impact

Successful exploitation allows an authenticated attacker to bypass the intended security policy of DisableForwarding, gaining the ability to forward X11 connections or the SSH authentication agent. This can lead to unauthorized access to the user's local X11 display (potentially leaking screen contents or keyboard input) or to the authentication agent (which could be used to authenticate as the user to other hosts). The impact is a breach of confidentiality and potentially further lateral movement within the network [1][4].

Mitigation

The vulnerability is fixed in OpenSSH 10.0, released on 2025-04-09 [1][3]. Administrators should upgrade to OpenSSH 10.0 or apply the corresponding patch for OpenBSD 7.6 (errata 013, also dated April 9, 2025) [4]. There is no known workaround besides applying the fix. Users unable to upgrade can consider blocking X11 and agent forwarding at the application or firewall layer, but this does not address the root cause.