CVE-2026-40014 in Dovecot Pro
Summary
by MITRE • 08/28/2026
An attacker that can send mail to a user can craft a message header that makes the IMAP THREAD command consume CPU disproportionate to the size of the message. When 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.
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Analysis
by VulDB Data Team • 08/28/2026
The vulnerability described involves a resource exhaustion flaw within the Internet Message Access Protocol (IMAP) implementation, specifically affecting the THREAD command functionality. This issue arises when an attacker who has the ability to send email messages to a target user crafts a message header designed to trigger inefficient processing logic during thread construction. The core technical flaw lies in the algorithmic complexity of how the mail server processes and organizes message threads based on headers such as In-Reply-To or References. Instead of scaling linearly with the size of the input, the CPU consumption grows disproportionately, potentially exhibiting exponential behavior relative to specific header structures crafted by an adversary. This represents a classic case where improper handling of user-controlled data leads to excessive resource utilization, allowing for a denial-of-service condition without requiring authentication beyond the ability to deliver mail to the victim's mailbox.
From a security classification perspective, this vulnerability aligns with CWE-400, which denotes Uncontrolled Resource Consumption. The specific mechanism involves an algorithmic complexity issue where the system fails to limit or optimize processing time based on input size, leading to excessive CPU usage. In terms of offensive security frameworks, this behavior is consistent with ATT&CK technique T1496, Resource Hijacking, specifically under the sub-technique for Denial of Service via resource exhaustion. The attacker leverages the legitimate functionality of the mail server against itself, causing it to consume computational resources at a rate that degrades service availability for other users or processes on the same system. This is particularly dangerous in shared hosting environments or systems with limited CPU capacity where such spikes can cascade into broader outages.
The operational impact of this vulnerability is primarily focused on service degradation and potential denial of service for IMAP clients accessing the affected mailbox. When a mail client issues the THREAD command, which is commonly used to group related messages by conversation thread, the server enters an inefficient processing loop or performs excessive computations due to the crafted header. This results in high CPU utilization that can lock up system resources, prevent other legitimate operations from completing, and cause timeouts for users attempting to access their email. While there are no publicly known exploits currently circulating, the ease of crafting such a message means that any attacker with mail delivery capabilities poses a significant threat vector. The impact is not data exfiltration or remote code execution but rather availability disruption, which can be critical in environments where continuous email access is required for business operations.
Mitigation strategies should focus on both immediate remediation and long-term architectural improvements. Administrators are advised to monitor system metrics closely, specifically looking for abnormal spikes in CPU usage correlated with IMAP activity. Upon detection of such anomalies, the offending process should be terminated immediately to restore service stability. Furthermore, the specific message triggering the issue must be identified and removed from the affected mailbox using server-side tools or direct database manipulation if necessary. The most effective long-term solution is to update the mail server software to a version that includes patches for this vulnerability, ensuring that the THREAD command implementation has been optimized to handle edge cases in header data without excessive resource consumption. Additionally, implementing rate limiting on IMAP commands and deploying intrusion detection systems capable of identifying patterns indicative of such crafted headers can provide an additional layer of defense against potential abuse until updates are applied.