high CVSS 8.5 EPSS 0.26%

CVE-2026-40203·Dovecot vulnerability

When IMAP compression is enabled, the same compression state is reused across responses in a session, so response sizes depend on both attacker-supplied mail and other mail in the same mailbox. An attacker that can send mail to a user and can also observe the sizes of that user's IMAP traffic can confirm whether the body of a small message matches a guessed text. Recovery of arbitrary unknown content was not demonstrated, but the attack can disclose whether a secret-like message body matches a candidate. Disable IMAP compression. 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 0.26% — higher than 16% 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-2026-40203 scores 8.5 on CVSS, so a successful attack would be serious — but at 0.26%, 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's IMAP compression feature can leak information about mailbox contents through traffic analysis when an attacker sends mail to a user and monitors response sizes. Upgrade to version 1:2.4.1+dfsg1-6+deb13u7 or later to resolve this vulnerability. As an interim mitigation, disable IMAP compression in your Dovecot configuration if you cannot immediately update.

sudo dnf update dovecot

Defensia detects this vulnerability

How to check if you are affected

  1. Check installed Dovecot version: dovecot --version
  2. Verify IMAP compression is enabled by checking /etc/dovecot/conf.d/20-imap.conf for 'imap_compress' settings
  3. Review mail server logs for repeated IMAP connections from the same source followed by COMPRESS commands: grep -i 'compress' /var/log/dovecot/dovecot.log
  4. After patching, confirm the new version is active: dovecot --version && sudo systemctl restart dovecot && dovecot --version

FAQ

What is CVE-2026-40203?

This vulnerability allows an attacker to infer whether specific message content matches a guessed value by analyzing IMAP response sizes when compression is enabled, since compression ratios vary based on mailbox contents.

Is CVE-2026-40203 being actively exploited?

No, CVE-2026-40203 is not currently listed on the CISA Known Exploited Vulnerabilities catalog and no public exploits are available.

What versions of Dovecot are affected by CVE-2026-40203?

Dovecot versions prior to 1:2.4.1+dfsg1-6+deb13u7 are affected. The vulnerability exists in all versions that support IMAP compression without proper state isolation.

How do I check if my server is vulnerable to CVE-2026-40203?

Run 'dovecot --version' and compare your version to 1:2.4.1+dfsg1-6+deb13u7. If your version is older, check grep 'imap_compress' /etc/dovecot/conf.d/20-imap.conf to see if compression is active.

Does Defensia detect CVE-2026-40203?

Yes — Defensia's CVE advisory scanner compares installed package versions against the NVD database. If Dovecot is installed on a monitored server, CVE-2026-40203 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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