CVE-2026-33263: Medium severity vulnerability
When mailmaxuseripconnections is set (default 10) and reached, submission-login can crash with epoll() panic caused by file descriptor handling issues. If running in high-security mode (default for community releases), only the new submission connection gets terminated. If running in high-performance mode (default for Pro releases), all connections handled by the submission-login process will be terminated. The crashes can cause failure for user to send a message, or it can cause duplicate messages to be sent. If TLS is not used (in the backend server processing the submission), duplicate deliveries cannot happen, because the crash can only happen at AUTH stage. Limit the number of connections handled by single submission-login process. This has a performance impact though. Update to non-vulnerable version. No publicly available exploits are known.
Remediation
Recommended actions to resolve this vulnerability, in priority order.
- Configuration
Limit the number of connections handled by a single submission-login process by reducing mail_max_userip_connections (default shown as 10) so the configured maximum is not reached.
submission-login mail_max_userip_connections = a lower value than the default 10
Event History
Frequently Asked Questions
Which deployments are most affected by the connection termination behavior?
High-security mode, the default for community releases, terminates only the new submission connection. High-performance mode, the default for Pro releases, terminates all connections handled by the affected submission-login process.
What must occur for the crash to be triggered?
mail_max_userip_connections must be configured and its limit reached; the stated default limit is 10. The issue occurs in submission-login during file descriptor handling.
Can this lead to duplicate message delivery?
Yes, crashes can cause duplicate messages to be sent. Duplicate deliveries cannot occur when TLS is not used for backend submission processing, because the crash can only happen during the AUTH stage.
What mitigation is available if an update cannot be applied immediately?
Limit the number of connections handled by a single submission-login process. This mitigation has a performance impact.
Are public exploits known?
No publicly available exploits are known.