CVE-2026-40203 Information disclosure in OX Dovecot CE
An attacker can confirm if a message body matches a guess by observing IMAP traffic sizes when compression is enabled, disclosing sensitive information.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-40203?
CVE-2026-40203 is a low-severity vulnerability in Open-Xchange GmbH OX Dovecot CE, published on 28 Aug 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official description
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.
Weakness: CWE-200
Which versions are affected by CVE-2026-40203?
How do I fix CVE-2026-40203?
Upgrade to a fixed version.
Is CVE-2026-40203 being exploited?
There is no known exploitation of CVE-2026-40203 in the wild and no public exploit code.
Sources
Also known as ALPINE-CVE-2026-40203, AZL-98274, DEBIAN-CVE-2026-40203, GHSA-cf77-42mj-h39j, UBUNTU-CVE-2026-40203.