Description

In sshd in OpenSSH before 10.5, the restrict keyword (in authorized_keys) was supposed to be applicable to tunnel forwarding but was not.

Executive Summary

Technical Details

Mitigations

References

Additional Metadata

{
  "score": "2.5",
  "pubDate": "2026-08-11T20:18:49.960Z",
  "pubdate": "2026-08-11T20:18:49.960Z",
  "executiveSummary": "An access control enforcement flaw exists within the sshd component of OpenSSH before version 10.5. Specifically, the restrict keyword defined within authorized_keys files fails to correctly apply its intended restrictions to tunnel forwarding operations.\nThis vulnerability impacts the OpenSSH server daemon across systems running vulnerable versions prior to 10.5. The security implication centers on the breakdown of policy enforcement, where administrators utilizing the restrict directive to lock down authorized keys and limit user capabilities expect tunnel forwarding to be explicitly denied.\nAn authenticated user possessing a configured authorized_keys entry with the restrict keyword can leverage this logic failure to establish unauthorized tunnel forwarding connections, effectively bypassing administrative security boundaries. Exploitation requires the attacker to have an existing authorized key configuration where restrictions are bypassed due to the flawed parsing and enforcement logic in sshd.\nThe risk involves unauthorized network tunneling and potential pivoting through the SSH daemon despite explicit configuration directives designed to prevent such functionality.",
  "technicalDetails": "The root cause of the vulnerability resides in the authorization constraint parsing and enforcement logic within the sshd component of OpenSSH. When processing public keys specified in authorized_keys files, administrators can append the restrict keyword to apply a broad set of default restrictions, including the disabling of port, agent, X11, and tunnel forwarding.\nIn affected versions prior to 10.5, the implementation fails to correctly propagate or enforce this restriction specifically regarding tunnel forwarding channels requested by the client. Consequently, while other restrictions governed by the restrict keyword may operate as intended, the subsystem responsible for handling tunnel allocation erroneously permits tunnel forwarding requests even when the restrict directive is explicitly active.\nThe attack flow proceeds as follows: First, an authenticated client initiates an SSH connection utilizing a key that is associated with an authorized_keys entry containing the restrict keyword. During the session establishment or channel request phase, the client issues a request for tunnel forwarding (e.g., tun device forwarding). Because the vulnerable sshd component fails to validate the restrict policy against tunnel forwarding capabilities, the server grants the request instead of rejecting it.\nThe affected component is the sshd server daemon, specifically the authorization constraint enforcement module responsible for interpreting keywords in authorized_keys. The vulnerability affects OpenSSH versions prior to 10.5. Authentication is required, as the vulnerability is triggered via an authenticated SSH session using a specific authorized key configuration. Privilege requirements are limited to the permissions of the authenticated user whose key is being evaluated. The network exposure is inherent to any system running a vulnerable sshd daemon exposed to network connections. The post-exploitation impact includes unauthorized tunnel establishment, potentially facilitating lateral movement or proxying traffic through the target host contrary to administrative intent."
}