Quick answers
- Is CVE-2026-40203 exploited in the wild?
- No. Junglewise Threat Intelligence has no report of CVE-2026-40203 being exploited in the wild.
- How severe is CVE-2026-40203?
- Low severity, with a CVSS score of 3.7, published 28 August 2026.
- Which products and versions are affected?
- Open-Xchange Dovecot Pro 2.3.0 to <2.3.22.2, 3.0.0 to <3.0.7, 3.1.0 to <3.1.6.
- Is there a fix for CVE-2026-40203?
- No fixed version is recorded yet. Check the vendor's advisory in the references below. If you run it, Junglewise drafts a mitigation that fits your setup and applies it once you approve, with senior security engineers behind it.
- How is CVE-2026-40203 attacked?
- It is attacked over the network.
- Am I affected by CVE-2026-40203?
- Check whether you run Open-Xchange Dovecot Pro at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.
Technical details
This vulnerability is an information disclosure issue in Dovecot's IMAP compression implementation. The root cause is that compression state is reused across responses rather than being reset between messages, making response sizes dependent on both attacker-controlled content and other mail in the mailbox. The attack requires network access to observe IMAP traffic and the ability to send mail to the target user. An attacker can perform a compression side-channel attack to confirm whether a message body matches a candidate plaintext (e.g., a password or secret phrase), though recovery of arbitrary unknown content was not demonstrated. Patches are available in Dovecot 2.3.22.2, 3.0.7, and 3.1.6 or later; workaround is to disable IMAP compression.
Affected products
- Open-Xchange Dovecot Pro 2.3.0 to <2.3.22.2, 3.0.0 to <3.0.7, 3.1.0 to <3.1.6
Timeline
- 2026-08-28: disclosed