critical CVSS 9 EPSS 0.45%

CVE-2026-42391·Dovecot vulnerability

An 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.

Severity
critical
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 0.45% — higher than 39% 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-2026-42391 scores 9 on CVSS, so a successful attack would be serious — but at 0.45%, 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

Dovecot versions prior to 1:2.4.1+dfsg1-6+deb13u7 are vulnerable to a denial-of-service attack through malformed IMAP ID commands sent before authentication. Update Dovecot to version 1:2.4.1+dfsg1-6+deb13u7 or later to patch this vulnerability. If immediate patching is not feasible, restrict the number of concurrent connections per imap-login process to limit resource exhaustion impact.

sudo dnf update dovecot -y

Defensia detects this vulnerability

What an exploitation attempt looks like

Sample log line indicative of exploitation attempts:

IMAP commands matching pattern: ^\* ID \(.*\)$ with parameter count exceeding 100 items, or rapid successive ID commands from unauthenticated connections followed by connection termination and memory spikes in imap-login process logs

How to check if you are affected

  1. Check installed Dovecot version: dpkg -l | grep dovecot (Debian/Ubuntu) or rpm -qa | grep dovecot (RHEL/CentOS)
  2. Verify IMAP service is exposed: sudo netstat -tlnp | grep 143 or sudo netstat -tlnp | grep 993 to confirm listening on IMAP ports
  3. Search Dovecot logs for IMAP ID command flooding: grep -i 'ID' /var/log/dovecot/dovecot.log | grep -v authenticated or tail -f /var/log/syslog | grep imap-login
  4. Confirm patch applied: dpkg -l | grep dovecot-core | awk '{print $3}' and verify version is 1:2.4.1+dfsg1-6+deb13u7 or later

FAQ

What is CVE-2026-42391?

CVE-2026-42391 is a critical denial-of-service vulnerability in Dovecot's IMAP implementation where unauthenticated clients can trigger excessive memory and CPU consumption by sending IMAP ID commands with an abnormally large number of parameters before login.

Is CVE-2026-42391 being actively exploited?

No, CVE-2026-42391 is not currently listed in CISA's Known Exploited Vulnerabilities catalog, and no public exploits are publicly available. However, the attack is trivial to execute and remediation should be prioritized.

What versions of Dovecot are affected by CVE-2026-42391?

All Dovecot versions up to and including 1:2.4.1+dfsg1-6+deb13u6 are affected. The vulnerability is patched in version 1:2.4.1+dfsg1-6+deb13u7 and later.

How do I check if my server is vulnerable to CVE-2026-42391?

Run 'dpkg -l | grep dovecot' and check the version number. If it is earlier than 1:2.4.1+dfsg1-6+deb13u7, your server is vulnerable. Additionally confirm IMAP is listening with 'netstat -tlnp | grep :143'.

Does Defensia detect CVE-2026-42391?

Yes — Defensia's CVE advisory scanner compares installed package versions against the NVD database. If Dovecot is installed on a monitored server, CVE-2026-42391 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-2026-27852CVSS 9An attacker that can send mail to a user can craft a message whose headers contain a very large number of email addresses or MIME parameters, which causes excessive memory usage when the message is later parsed. The message is still delivered, but reading it over IMAP can exhaust the memory limit of the process and terminate it, causing denial of service for the affected user. Update to non-vulnerable version. No publicly available exploits are known.

References

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