Executive brief
OpenSSH is a widely used tool for secure remote access to servers. A configuration flaw was discovered where the setting intended to disable all forms of traffic forwarding did not correctly override tunnel permissions. This could allow an authorized user to establish network tunnels even when an administrator explicitly intended to block such activity.
Technical details
A logic error in sshd's configuration parsing causes the 'DisableForwarding=yes' directive to fail to override 'PermitTunnel=yes', contrary to documented behavior. This vulnerability allows a remote user with valid authentication to establish Layer 2 or Layer 3 network tunnels if 'PermitTunnel' is enabled, even if the administrator has globally or specifically set 'DisableForwarding' to block such traffic. The attack requires 'PermitTunnel' to be manually enabled, as it is disabled by default. The issue is resolved in OpenSSH version 10.4.
Affected products
- OpenBSD OpenSSH before 10.4
Timeline
- 2026-07-06: patched: OpenSSH 10.4 released with fix.
- 2026-07-08: disclosed: CVE-2026-59999 published.