high CVSS 7.5 EPSS 0.55%

CVE-2026-33263·Dovecot vulnerability

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.

Severity
high
Software
Dovecot
Fixed in
1:2.4.1+dfsg1-6+deb13u7
Published

How likely is exploitation

FIRST's Exploit Prediction Scoring System puts the probability of this CVE being exploited in the next 30 days at 0.55% — higher than 45% of all published CVEs. EPSS is an independent forecast of exploitation, not a severity rating: it says nothing about how much damage a successful attack does, which is what a CVSS score measures.

Read the two together. CVE-2026-33263 scores 7.5 on CVSS, so a successful attack would be serious — but at 0.55%, almost nobody is currently attempting it. That is a different situation from a flaw under mass exploitation, and it is usually the difference between patching on your next maintenance window and patching tonight.

Affected versions

Until: 1:2.4.1+dfsg1-6+deb13u7

Fixed in: 1:2.4.1+dfsg1-6+deb13u7

How to fix this CVE

Update Dovecot to version 1:2.4.1+dfsg1-6+deb13u7 or later to resolve a file descriptor handling issue in the submission-login service that can cause process crashes when the mail_max_userip_connections limit is reached. This vulnerability impacts message delivery reliability and can result in failed sends or duplicate messages. Apply the security patch immediately to restore stability in your mail submission pipeline.

sudo dnf update dovecot

Defensia detects this vulnerability

How to check if you are affected

  1. Check installed Dovecot version: dpkg -l | grep dovecot (Debian/Ubuntu) or rpm -q dovecot (RHEL/CentOS)
  2. Verify submission-login service is enabled: systemctl is-active dovecot && grep -i 'submission-login' /etc/dovecot/dovecot.conf
  3. Search mail logs for submission-login crashes: grep -i 'submission-login.*crash\|epoll.*panic' /var/log/mail.log* /var/log/dovecot/dovecot.log*
  4. After patching, restart the service and monitor for 24 hours: sudo systemctl restart dovecot && journalctl -u dovecot -f

FAQ

What is CVE-2026-33263?

This vulnerability affects Dovecot's submission-login service, causing kernel-level epoll() panics when the mail_max_userip_connections threshold is exceeded due to improper file descriptor lifecycle management. The crash can terminate user connections and potentially cause message duplication in TLS-enabled backends.

Is CVE-2026-33263 being actively exploited?

No, this CVE is not listed on the CISA Known Exploited Vulnerabilities catalog and no public exploits are available. However, the vulnerability can be triggered through normal operation when connection limits are reached.

What versions of Dovecot are affected by CVE-2026-33263?

All versions prior to and including 1:2.4.1+dfsg1-6+deb13u7 are affected. The fixed version is 1:2.4.1+dfsg1-6+deb13u7 and later.

How do I check if my server is vulnerable to CVE-2026-33263?

Run: dpkg -l | grep dovecot (or rpm -q dovecot on RHEL). If the version is below 1:2.4.1+dfsg1-6+deb13u7, your server is vulnerable. Additionally verify submission-login is configured: grep -i submission /etc/dovecot/dovecot.conf

Does Defensia detect CVE-2026-33263?

Yes — Defensia's CVE advisory scanner compares installed package versions against the NVD database. If Dovecot is installed on a monitored server, CVE-2026-33263 will appear in your dashboard with remediation steps.

Related Dovecot CVEs

CVE-2026-73208CVSS 9An attacker that holds a token intended for a different purpose can authenticate, because when an OAuth2 token response does not contain a scope claim, the audience claim is used in its place and checked against the configured required scopes. These are different concepts, and the audience claim does not describe what a token is allowed to do. A token that grants no relevant permissions can be accepted because its intended recipient value happens to match a configured scope name, granting access that should have been denied. It also hides an identity provider misconfiguration where scopes are not being issued at all. Ensure the identity provider issues a scope claim for all tokens used with Dovecot, and that configured scope names do not match audience values. Update to non-vulnerable version. No publicly available exploits are known.
CVE-2026-40018CVSS 9None None None No publicly available exploits are known.
CVE-2026-40019CVSS 9An unauthenticated attacker can send a truncated quoted argument to the ManageSieve login process, which makes it spin in an infinite loop consuming CPU. This can cause degradation or denial of service for Sieve script management, and repeated connections can consume all available CPU on the server. Monitor system for abnormal CPU usage and kill the offending process. Restrict network access to the ManageSieve service to trusted clients. Update to non-vulnerable version. No publicly available exploits are known.
CVE-2026-33605CVSS 9An unauthenticated attacker can crash the ManageSieve login process by sending a small malformed command before authenticating. If running in high-security mode (default for community releases), only the attacker's own connection is terminated. If running in high-performance mode (default for Pro releases), all connections handled by the same managesieve-login process are terminated. Repeating the attack can cause denial of service for Sieve script management. Restrict network access to the ManageSieve service to trusted clients. Update to non-vulnerable version. No publicly available exploits are known.
CVE-2026-42391CVSS 9An unauthenticated attacker can send an IMAP ID command with a very large number of parameters before logging in, which causes memory and CPU usage to grow disproportionately. The login process can be terminated by the out-of-memory handling, which also terminates all other connections handled by the same process. This can cause degradation or denial of service for IMAP logins. Limit the number of connections handled by a single imap-login process. This has a performance impact though. Update to non-vulnerable version. No publicly available exploits are known.

References

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