Bug 2525535 (CVE-2026-40203)

Summary: CVE-2026-40203 dovecot: Dovecot: Information disclosure via IMAP compression side-channel
Product: [Other] Security Response Reporter: OSIDB Bzimport <bzimport>
Component: vulnerabilityAssignee: Product Security DevOps Team <prodsec-dev>
Status: NEW --- QA Contact:
Severity: low Docs Contact:
Priority: low    
Version: unspecifiedCC: rhel-process-autobot, watson-tool-maintainers
Target Milestone: ---Keywords: Security
Target Release: ---   
Hardware: All   
OS: Linux   
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A flaw was found in dovecot where, when IMAP compression is enabled, the system reuses the same compression state across responses within a session. This allows a remote attacker, who can send mail to a user and monitor the sizes of their IMAP traffic, to potentially deduce the content of small messages. By observing response sizes, an attacker could confirm if a secret message body matches a guessed text, leading to information disclosure.
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oVirt Team: --- RHEL 7.3 requirements from Atomic Host:
Cloudforms Team: --- Target Upstream Version:
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Bug Depends On: 2526680    
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Description OSIDB Bzimport 2026-08-28 11:21:31 UTC 
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.