critical CVSS 9 EPSS 0.37%

CVE-2026-40019·Dovecot vulnerability

An 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.

Severity
critical
Software
Dovecot
Fixed in
1:2.4.5+dfsg1-1
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 0.37% — higher than 29% 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-2026-40019 scores 9 on CVSS, so a successful attack would be serious — but at 0.37%, 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: 1:2.4.5+dfsg1-1

Fixed in: 1:2.4.5+dfsg1-1

How to fix this CVE

Update Dovecot to version 1:2.4.5+dfsg1-1 or later to resolve a ManageSieve authentication flaw that triggers infinite CPU loops under specific conditions. Organizations running older versions should prioritize this patch to prevent denial of service attacks targeting the Sieve script management service. Implement network segmentation to restrict ManageSieve access (port 4190) to trusted administrative clients in the interim.

sudo dnf update dovecot

Defensia detects this vulnerability

What an exploitation attempt looks like

Sample log line indicative of exploitation attempts:

ManageSieve protocol logs (typically in /var/log/mail.log or journalctl) showing repeated truncated AUTHENTICATE commands with incomplete quoted strings followed by CRLF, correlating with spikes in dovecot process CPU usage and lack of subsequent client disconnect

How to check if you are affected

  1. Run 'dovecot --version' or check '/usr/lib/dovecot/dovecot --version' to identify the currently installed Dovecot release
  2. Confirm ManageSieve is enabled by checking 'grep -i sieve /etc/dovecot/dovecot.conf' or 'grep -r sieve /etc/dovecot/conf.d/'
  3. Monitor system CPU usage during off-peak hours with 'top' or 'htop' and look for dovecot processes consuming abnormally high CPU; check 'ps aux | grep dovecot' for runaway processes
  4. After patching, re-run 'dovecot --version' and verify the version is 1:2.4.5+dfsg1-1 or higher; restart Dovecot with 'sudo systemctl restart dovecot' and confirm no CPU spike occurs

FAQ

What is CVE-2026-40019?

CVE-2026-40019 is a denial of service vulnerability in Dovecot's ManageSieve authentication handler that causes the service to enter an infinite loop when processing malformed quoted arguments, leading to uncontrolled CPU consumption.

Is CVE-2026-40019 being actively exploited?

No, CVE-2026-40019 is not listed on the CISA KEV catalog and no public exploits are currently available, but the high CVSS score (9.0) warrants prompt remediation.

What versions of Dovecot are affected by CVE-2026-40019?

All Dovecot versions up to and including 1:2.4.5+dfsg1-1 are vulnerable; the fix is included in version 1:2.4.5+dfsg1-1 and later releases.

How do I check if my server is vulnerable to CVE-2026-40019?

Execute 'dovecot --version' and compare the output against version 1:2.4.5+dfsg1-1; if your version is older, the system is vulnerable.

Does Defensia detect CVE-2026-40019?

Yes — Defensia's CVE advisory scanner compares installed package versions against the NVD database. If Dovecot is installed on a monitored server, CVE-2026-40019 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-2026-33605CVSS 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.
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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