high CVSS 7.5 EPSS 0.87%

CVE-2026-40013·Dovecot vulnerability

An attacker that has valid credentials can submit a Sieve script containing an extreme numeric literal, which causes an out-of-bounds write when the ManageSieve service compiles the script. This causes memory corruption and an observed crash of the ManageSieve process, resulting in denial of service for script management. This might be able to be used for remote code execution. Disable the ManageSieve service if users do not need remote Sieve script management. 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 0.87% — higher than 57% 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-2026-40013 scores 7.5 on CVSS, so a successful attack would be serious — but at 0.87%, 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

Organizations running Dovecot should update to version 1:2.4.1+dfsg1-6+deb13u7 or later to patch a memory corruption vulnerability in the ManageSieve service. The vulnerability allows authenticated attackers to trigger an out-of-bounds write through malformed Sieve scripts, leading to service crashes and potential code execution. If ManageSieve functionality is not required, disabling the service provides immediate protection while patches are staged.

sudo dnf update dovecot --refresh && sudo systemctl restart dovecot

Defensia detects this vulnerability

How to check if you are affected

  1. Run 'dovecot --version' to check the installed Dovecot version; versions prior to 1:2.4.1+dfsg1-6+deb13u7 are vulnerable
  2. Verify ManageSieve is enabled by checking if port 4190 is listening: 'sudo ss -tlnp | grep 4190' or 'sudo netstat -tlnp | grep 4190'
  3. Search Dovecot logs for ManageSieve process crashes: 'grep -i "managesieve" /var/log/dovecot/dovecot.log | grep -i "crash\|segfault\|error"'
  4. Confirm the patch was applied by running 'dovecot --version' again and verifying the version matches or exceeds 1:2.4.1+dfsg1-6+deb13u7

FAQ

What is CVE-2026-40013?

CVE-2026-40013 is a memory corruption vulnerability in Dovecot's ManageSieve service that occurs when processing Sieve scripts with extreme numeric literals. Authenticated users can exploit this flaw to crash the ManageSieve process and potentially achieve remote code execution.

Is CVE-2026-40013 being actively exploited?

No, CVE-2026-40013 is not listed on the CISA Known Exploited Vulnerabilities catalog and no public exploits are currently available, though the attack requires valid user credentials.

What versions of Dovecot are affected by CVE-2026-40013?

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

How do I check if my server is vulnerable to CVE-2026-40013?

Run 'dovecot --version' and compare the output to version 1:2.4.1+dfsg1-6+deb13u7; if your version is lower, your server is vulnerable. Additionally check that ManageSieve is enabled with 'sudo ss -tlnp | grep 4190'.

Does Defensia detect CVE-2026-40013?

Yes — Defensia's CVE advisory scanner compares installed package versions against the NVD database. If Dovecot is installed on a monitored server, CVE-2026-40013 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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