CVE-2026-40203: Infoleak
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.
Remediation
Recommended actions to resolve this vulnerability, in priority order.
- Upgrade
Upgrade to a fixed release to a version that resolves this vulnerability.
Fixed in non-vulnerable version - Configuration
Disable IMAP compression so that the attacker cannot confirm whether the body of a small message matches guessed text by observing IMAP response sizes.
IMAP IMAP compression = disabled
Event History
Frequently Asked Questions
What conditions must an attacker meet to exploit this issue?
The attacker must be able to send email to the target user and observe the sizes of that user's IMAP traffic. IMAP compression must also be enabled for the affected session.
Who is realistically exposed?
Users whose mailboxes can receive attacker-supplied messages and whose IMAP traffic sizes can be observed are exposed when IMAP compression is enabled. The disclosed impact is confirmation that a small message body matches attacker-guessed text.
What should be done if an update cannot be applied immediately?
Disable IMAP compression. This removes the reused compression state that makes response sizes dependent on other mailbox content.
Is exploitation known to recover arbitrary mailbox content?
No. Recovery of arbitrary unknown content was not demonstrated; the described attack confirms whether a secret-like message body matches a candidate.
Are public exploits available?
No publicly available exploits are known.