CVE-2026-40014: IMAP THREAD Command DoS Vulnerability

CVE-2026-40014 Overview

CVE-2026-40014 is a resource consumption vulnerability affecting the IMAP THREAD command in Open-Xchange Dovecot. An attacker who can send email to a target user can craft a message header that causes disproportionate CPU consumption when a mail client issues the THREAD command on the affected mailbox. The result is service degradation or denial of service (DoS) for the IMAP service.

The flaw is classified under CWE-400: Uncontrolled Resource Consumption. Exploitation requires no authentication on the attacker side; the attacker only needs the ability to deliver mail to a victim mailbox. No publicly available exploits are known at time of publication.

Critical Impact
A single crafted message can degrade or deny IMAP service for the recipient once a mail client triggers a THREAD operation on the mailbox.

Affected Products

Open-Xchange Dovecot (see vendor advisory for affected versions)
IMAP servers based on the affected Dovecot code branch
Mail platforms embedding vulnerable Dovecot builds

Discovery Timeline

2026-08-28 - CVE-2026-40014 published to NVD
2026-09-03 - Last updated in NVD database

Technical Details for CVE-2026-40014

Vulnerability Analysis

The IMAP THREAD extension (RFC 5256) groups messages into conversation threads using algorithms such as ORDEREDSUBJECT and REFERENCES. These algorithms parse message headers including References, In-Reply-To, Subject, and Message-ID and build a tree structure linking related messages.

CVE-2026-40014 arises when the threading logic processes an adversarially crafted header. The parsing or graph-construction work grows disproportionately to the size of the input, producing an algorithmic complexity condition. When a mail client issues THREAD against the affected mailbox, the server consumes CPU well beyond what the message size would justify.

Because the trigger occurs during a legitimate client command, the impact extends beyond the attacker's own session and can disrupt IMAP service for the affected user or process.

Root Cause

The root cause is uncontrolled resource consumption during THREAD command processing. The server does not enforce sufficient bounds on the cost of parsing or correlating message header fields relative to input size, allowing a crafted message to trigger worst-case algorithmic behavior.

Attack Vector

The attack vector is network-based and unauthenticated from the attacker's perspective. The attacker sends an email containing a maliciously crafted header to any address served by the vulnerable Dovecot instance. The DoS condition is triggered when the victim's mail client issues an IMAP THREAD command over the mailbox containing the message, which most modern threaded mail clients do automatically.

Refer to the Open-Xchange Security Advisory for header specifics and affected version ranges.

Detection Methods for CVE-2026-40014

Indicators of Compromise

Sustained high CPU usage by Dovecot IMAP worker processes tied to a specific user session or mailbox.
IMAP client timeouts or unresponsive sessions immediately following a THREAD command.
Recently delivered messages with unusually large or malformed References, In-Reply-To, or Subject header fields.

Detection Strategies

Monitor Dovecot process CPU time per-session and alert on IMAP workers exceeding a defined CPU-per-command budget.
Correlate IMAP protocol logs for THREAD commands that precede spikes in resource usage or session terminations.
Inspect mail flow logs for inbound messages with anomalously structured threading headers.

Monitoring Recommendations

Enable Dovecot verbose logging (mail_debug, auth_verbose) and forward logs to a centralized SIEM for correlation with host CPU metrics.
Track per-user IMAP command latency and failure rates to identify affected mailboxes.
Alert on repeated IMAP disconnects for the same mailbox that follow THREAD execution.

How to Mitigate CVE-2026-40014

Immediate Actions Required

Identify Dovecot processes consuming disproportionate CPU and terminate the offending worker to restore service.
Locate and remove the offending message from the affected mailbox once identified from headers or logs.
Update Dovecot to a non-vulnerable version as specified in the vendor advisory.

Patch Information

Open-Xchange has published fixed builds. Administrators should review the Open-Xchange Security Advisory for the applicable fixed versions and apply the update through the standard package channel for the deployed distribution.

Workarounds

Temporarily restrict or disable the IMAP THREAD extension for exposed users if operationally acceptable until patching is complete.
Apply strict per-process CPU limits (cgroups, ulimit) to Dovecot IMAP workers to contain the impact of a single abusive session.
Deploy inbound mail filtering to reject or normalize messages with malformed or excessively large threading headers.

bash

# Example: constrain Dovecot IMAP worker CPU via systemd override
# /etc/systemd/system/dovecot.service.d/limits.conf
[Service]
CPUQuota=50%
MemoryMax=1G
TasksMax=512