CVE-2026-40203
Description
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.
Mitigation
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.
Common Vulnerability Scoring System (CVSS) Score Details
Info alert:Important note
CVSS scores for open source components depend on vendor-specific factors (e.g. version or build chain). Therefore, Red Hat's score and impact rating can be different from NVD and other vendors. Red Hat remains the authoritative CVE Naming Authority (CNA) source for its products and services (see Red Hat classifications).
The following CVSS metrics and score provided are preliminary and subject to review.
CVSS v3 Score Breakdown
| Red Hat | NVD | cve.org | |
|---|---|---|---|
| Base Score | 3.7 | N/A | 3.7 |
| Attack Vector | Network | N/A | Network |
| Attack Complexity | High | N/A | High |
| Privileges Required | None | N/A | None |
| User Interaction | None | N/A | None |
| Scope | Unchanged | N/A | Unchanged |
| Confidentiality | Low | N/A | Low |
| Integrity Impact | None | N/A | None |
| Availability Impact | None | N/A | None |
Vector
Red Hat: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:N/A:N
cve.org: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:N/A:N
Understanding the Weakness (CWE)
Confidentiality,Access Control
Technical Impact: Read Application Data; Bypass Protection Mechanism
Frequently Asked Questions
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