Low [CVE-2026-40203] Information disclosure via IMAP compression side-channel
This low-severity Red Hat Linux advisory covers CVE-2026-40203 affecting Red Hat Enterprise Linux 10, Red Hat Enterprise Linux 6, Red Hat Enterprise Linux 7.
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Summary
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.
Update to non-vulnerable version. No publicly available exploits are known.
By observing response sizes, an attacker could confirm if a secret message body matches a guessed text, leading to information disclosure. Red Hat severity: Low — CVSS 3.7 (CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:N/A:N).
Weakness: CWE-205. Affected Red Hat products: Red Hat Enterprise Linux 10; Red Hat Enterprise Linux 6; Red Hat Enterprise Linux 7; Red Hat Enterprise Linux 8; Red Hat Enterprise Linux 9.
Will not fix / out of support: Red Hat Enterprise Linux 6. Red Hat does not currently list a fixing RHSA for this CVE.
Affected products named by the advisory: Red Hat package: dovecot.
Affected versions
No affected-version range was extracted from the source record. The vendor advisory is authoritative — check it before change work.
Official advisory · high-confidence parse· fetched 60 minutes ago·verify at source
Fixed versions
No fixed release is recorded yet. That does not prove no patch exists — confirm against the vendor advisory.
Official advisory · high-confidence parse· fetched 60 minutes ago·verify at source
Mitigation checklist
- To mitigate this issue, disable IMAP compression in the Dovecot configuration. This typically involves setting `imap_compression = no` in the relevant Dovecot configuration file. After making this change, restart the Dovecot service for the mitigation to take effect. Restarting the service may temporarily interrupt IMAP access for users.
Official advisory · high-confidence parse· fetched 60 minutes ago·verify at source
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