Medium [CVE-2026-40017] Denial of Service via crafted IMAP message headers
This medium-severity Red Hat Linux advisory covers CVE-2026-40017 affecting Red Hat Enterprise Linux 10, Red Hat Enterprise Linux 6, Red Hat Enterprise Linux 7.
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Summary
An attacker that can send mail to a user can craft a message header whose values are chosen to collide in an internal hash table, which makes the IMAP THREAD command consume CPU disproportionate to the size of the message. This is a separate issue from CVE-2026-40014 and is not addressed by that fix.
Whenever a mail client issues a THREAD command on the affected mailbox, this can cause degradation or denial of service for IMAP. Monitor system for abnormal CPU usage, kill the offending process and remove the offending message from the affected mailbox.
Update to non-vulnerable version. No publicly available exploits are known.
A flaw was found in Dovecot. This manipulation forces the IMAP THREAD command to consume excessive CPU resources, leading to degradation or a complete denial of service for the IMAP server.
This can disrupt email services for affected users. Red Hat severity: Moderate — CVSS 6.5 (CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:N/A:H).
Weakness: CWE-770. 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 57 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 57 minutes ago·verify at source
Mitigation checklist
- To mitigate this issue, administrators should identify and remove the specially crafted IMAP message from the affected user's mailbox. If a Dovecot process is consuming excessive CPU resources due to this vulnerability, it should be terminated to restore service. Note that terminating a process will temporarily interrupt service for affected users.
Official advisory · high-confidence parse· fetched 57 minutes ago·verify at source
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