High [CVE-2026-33263] Denial of Service and potential message duplication via connection limit exhaustion
This high-severity Red Hat Linux advisory covers CVE-2026-33263 affecting Red Hat Enterprise Linux 10, Red Hat Enterprise Linux 6, Red Hat Enterprise Linux 7.
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Summary
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.
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.
A flaw was found in the Dovecot mail server's `submission-login` component. This vulnerability occurs when the server reaches its maximum allowed user IP connections, leading to a crash due to file descriptor handling issues.
This can result in a denial of service, preventing users from sending emails, and potentially causing duplicate message delivery under certain conditions. Red Hat severity: Important — CVSS 7.1 (CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:L/A:H).
Weakness: CWE-910. Affected Red Hat products: Red Hat Enterprise Linux 10; Red Hat Enterprise Linux 6; Red Hat Enterprise Linux 7; Red Hat Enterprise Linux 8; Red Hat Enterprise Linux 9.
Affected versions
No affected-version range was extracted from the source record. The vendor advisory is authoritative — check it before change work.
Official advisory · high-confidence parse· fetched 1 hour ago·verify at source
Fixed versions
No fixed release is recorded yet. That does not prove no patch exists — confirm against the vendor advisory.
Official advisory · high-confidence parse· fetched 1 hour ago·verify at source
Mitigation checklist
- To mitigate this issue, limit the number of connections handled by a single submission-login process. This can be achieved by adjusting the `process_limit` setting for the `submission-login` service in the Dovecot configuration. For example, add or modify the following in a Dovecot configuration file (e.g., `/etc/dovecot/conf.d/10-master.conf`): ``` service submission-login { process_limit = <lower_value> } ``` Reducing this limit may impact performance. After modifying the configuration, restart the Dovecot service for the changes to take effect.
Official advisory · high-confidence parse· fetched 1 hour ago·verify at source
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