high CVSS 8.5

CVE-2026-40014·Dovecot vulnerability

An attacker that can send mail to a user can craft a message header that makes the IMAP THREAD command consume CPU disproportionate to the size of the message. When a mail client issues a THREAD command on the affected mailbox, this can cause degradation or denial of service for IMAP. Monitor system for abnormal CPU usage, kill the offending process and remove the offending message from the affected mailbox. 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

Affected versions

Until: 1:2.4.1+dfsg1-6+deb13u7

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

How to fix this CVE

Dovecot versions prior to 1:2.4.1+dfsg1-6+deb13u7 contain a CPU exhaustion vulnerability triggered by specially crafted email message headers when IMAP THREAD operations are performed. Organizations should upgrade to the patched version immediately and review mailboxes for suspicious messages that may have been used in exploitation attempts. Until patching is complete, monitor IMAP process CPU consumption and consider restricting THREAD command usage at the server level.

sudo dnf update dovecot dovecot-imapd

Defensia detects this vulnerability

What an exploitation attempt looks like

Sample log line indicative of exploitation attempts:

THREAD command followed by unusual CPU spike in dovecot process; pattern: 'tag.*THREAD.*REFERENCES|DATE|SUBJECT|FROM|TO|CC|MESSAGE-ID' in IMAP logs concurrent with top/htop showing dovecot consuming >80% CPU for sustained duration

How to check if you are affected

  1. Check installed Dovecot version: dovecot --version | grep -oP '\d+\.\d+\.\d+'
  2. Verify IMAP service is exposed: ss -tlnp | grep -i dovecot or netstat -tlnp | grep dovecot
  3. Search Dovecot logs for THREAD command execution: grep -i 'THREAD' /var/log/dovecot/imap.log* | tail -100
  4. Confirm fix installation: dpkg -l | grep dovecot and verify version >= 1:2.4.1+dfsg1-6+deb13u7

FAQ

What is CVE-2026-40014?

CVE-2026-40014 is a CPU exhaustion vulnerability in Dovecot's IMAP THREAD command implementation that allows remote attackers to cause denial of service by sending maliciously formatted email headers. When a mail client executes a THREAD operation on a mailbox containing the malicious message, the IMAP process consumes excessive CPU resources, degrading or disabling IMAP service availability.

Is CVE-2026-40014 being actively exploited?

No, CVE-2026-40014 is not listed on the CISA Known Exploited Vulnerabilities catalog and no public exploits are currently available. However, the attack vector (remote, unauthenticated email delivery) means exploitation could occur without detection if an attacker has basic mail delivery access.

What versions of Dovecot are affected by CVE-2026-40014?

All Dovecot versions up to and including 1:2.4.1+dfsg1-6+deb13u6 are affected. Version 1:2.4.1+dfsg1-6+deb13u7 and later contain the fix.

How do I check if my server is vulnerable to CVE-2026-40014?

Run 'dovecot --version' and compare the output against version 1:2.4.1+dfsg1-6+deb13u7. If your version is earlier, your system is vulnerable. Also verify IMAP is enabled with 'doveconf protocols | grep imap'.

Does Defensia detect CVE-2026-40014?

Yes — Defensia's CVE advisory scanner compares installed package versions against the NVD database. If Dovecot is installed on a monitored server, CVE-2026-40014 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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