CVE-2026-40203

Severity: low (CVSS 3.7 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.3% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2026-08-28 · updated 2026-09-03
Weakness: CWE-200

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.

Affected products

Open-Xchange GmbH OX Dovecot CE: from 2.3.0 before 2.4.5 — fixed in 2.4.5
Open-Xchange GmbH OX Dovecot Pro: from 2.3.0 before 2.3.22.2; from 3.0.0 before 3.0.7; from 3.1.0 before 3.1.6 — fixed in 2.3.22.2, 3.0.7, 3.1.6

How to fix

Upgrade to a fixed version: Open-Xchange GmbH OX Dovecot CE 2.4.5; Open-Xchange GmbH OX Dovecot Pro 2.3.22.2, 3.0.7, 3.1.6. Check the vendor advisory for mitigations.

References

https://documentation.open-xchange.com/dovecot/security/advisories/csaf/2026/oxdc-adv-2026-0003.json

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