high CVSS 8.5 EPSS 0.33%

CVE-2026-40017·Dovecot vulnerability

An attacker that can send mail to a user can craft a message header whose values are chosen to collide in an internal hash table, which makes the IMAP THREAD command consume CPU disproportionate to the size of the message. This is a separate issue from CVE-2026-40014 and is not addressed by that fix. Whenever 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

How likely is exploitation

FIRST's Exploit Prediction Scoring System puts the probability of this CVE being exploited in the next 30 days at 0.33% — higher than 25% 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-2026-40017 scores 8.5 on CVSS, so a successful attack would be serious — but at 0.33%, 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 hash collision attack in the IMAP THREAD command that causes excessive CPU consumption. Upgrade your Dovecot installation to version 1:2.4.1+dfsg1-6+deb13u7 or later, and monitor your mail server for any messages with specially crafted headers that trigger the vulnerability. After patching, audit your mailbox logs for unusual THREAD command activity and consider removing any malicious messages identified during forensic investigation.

sudo dnf check-update dovecot && sudo dnf update dovecot

Defensia detects this vulnerability

How to check if you are affected

  1. Run `dovecot --version` to check the currently installed Dovecot version and confirm if it is older than 1:2.4.1+dfsg1-6+deb13u7
  2. Verify IMAP service is running and accessible: `sudo systemctl status dovecot` and check that IMAP ports (143, 993) are listening with `sudo ss -tlnp | grep dovecot`
  3. Search Dovecot logs for THREAD command execution: `grep -i 'THREAD' /var/log/dovecot/dovecot.log | grep -i 'command\|imap'` to identify mailboxes receiving THREAD requests
  4. After applying patches, re-run `dovecot --version` and cross-reference the output against the fixed version number to confirm successful remediation

FAQ

What is CVE-2026-40017?

CVE-2026-40017 is a denial-of-service vulnerability in Dovecot where an attacker sends an email with maliciously crafted headers designed to cause hash table collisions. When an IMAP client executes the THREAD command on a mailbox containing this message, the server experiences disproportionate CPU consumption, potentially degrading or disrupting IMAP service availability.

Is CVE-2026-40017 being actively exploited?

No, CVE-2026-40017 is not listed on the CISA Known Exploited Vulnerabilities (KEV) catalog and there are no publicly available exploits. However, the low barrier to entry (simply sending a crafted email) means organizations should still prioritize patching.

What versions of Dovecot are affected by CVE-2026-40017?

All Dovecot versions prior to 1:2.4.1+dfsg1-6+deb13u7 are vulnerable. This includes stable releases across Debian 13 and derived distributions.

How do I check if my server is vulnerable to CVE-2026-40017?

Run `dovecot --version` and compare the output to 1:2.4.1+dfsg1-6+deb13u7. If your version is lower, your system is vulnerable. For package-based systems, use `apt show dovecot-core | grep Version` (Debian/Ubuntu) or `dnf info dovecot | grep Version` (RHEL/CentOS).

Does Defensia detect CVE-2026-40017?

Yes — Defensia's CVE advisory scanner compares installed package versions against the NVD database. If Dovecot is installed on a monitored server, CVE-2026-40017 will appear in your dashboard with remediation steps.

Related Dovecot CVEs

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-2026-40018CVSS 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-2026-27852CVSS 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.
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.

References

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