CVE-2026-33263 Denial of service in OX Dovecot CE
Attackers can crash the submission-login process by exhausting file descriptors, causing service disruption or duplicate message deliveries in specific configurations.
- Exploited in the wild
- Not known
- Login needed
- Any signed-in user
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-33263?
CVE-2026-33263 is a medium-severity denial of service vulnerability in Open-Xchange GmbH OX Dovecot CE, published on 28 Aug 2026. Any signed-in user can exploit it over the network, with no action from the victim.
Official description
When mail_max_userip_connections 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.
Weakness: CWE-403
Which versions are affected by CVE-2026-33263?
How do I fix CVE-2026-33263?
Upgrade to a fixed version.
Upgrade, then put limits on request size, rate and nesting depth so one client cannot exhaust the service. More about Denial of service.
Is CVE-2026-33263 being exploited?
There is no known exploitation of CVE-2026-33263 in the wild and no public exploit code.
Sources
Also known as ALPINE-CVE-2026-33263, AZL-98313, DEBIAN-CVE-2026-33263, GHSA-9rqj-cvwq-v6qh, UBUNTU-CVE-2026-33263.