CVE-2026-40017 Denial of service in OX Dovecot CE
An attacker can cause CPU exhaustion and denial of service in the IMAP THREAD command by sending crafted mail headers that collide in an internal hash table.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- A user has to click or open something
- Fix
- Available
What is CVE-2026-40017?
CVE-2026-40017 is a medium-severity denial of service vulnerability in Open-Xchange GmbH OX Dovecot CE, published on 28 Aug 2026. An attacker can exploit it over the internet without logging in, and only if a user opens a file or follows a link.
Official description
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.
Weakness: CWE-400
Which versions are affected by CVE-2026-40017?
How do I fix CVE-2026-40017?
Upgrade to a fixed version.
Upgrade, then put limits on request size, rate and nesting depth so one client cannot exhaust the service. More about Denial of service.
Is CVE-2026-40017 being exploited?
There is no known exploitation of CVE-2026-40017 in the wild and no public exploit code.
Sources
Also known as ALPINE-CVE-2026-40017, AZL-98286, DEBIAN-CVE-2026-40017, GHSA-gr5f-599w-cg8j, UBUNTU-CVE-2026-40017.