Description

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.

Executive Summary

Technical Details

Mitigations

References

Additional Metadata

{
  "score": "6.5",
  "pubDate": "2026-08-28T12:16:28.240Z",
  "pubdate": "2026-08-28T12:16:28.240Z",
  "executiveSummary": "A resource exhaustion vulnerability exists within the IMAP server implementation, specifically related to the processing of the THREAD command.\nAn unauthenticated or authenticated attacker capable of sending mail to a user can inject specifically crafted message headers designed to trigger algorithmic complexity issues.\nWhen a mail client executes the THREAD command on a mailbox containing such a message, the IMAP service experiences disproportionate CPU utilization, leading to performance degradation or a complete Denial of Service (DoS) for the affected mail service.\nThe vulnerability represents a significant risk to service availability, as it allows a remote attacker to induce resource starvation without requiring elevated privileges or complex exploitation chains.\nWhile no public exploits are currently documented, the feasibility of the attack is high given the standard nature of IMAP header parsing.\nImmediate mitigation requires identifying the malicious email, purging it from the mailbox, and ensuring the server software is updated to a version containing the appropriate patch or algorithmic optimization.",
  "technicalDetails": "The root cause of this vulnerability lies in an inefficient implementation of the IMAP THREAD command, which is responsible for organizing messages into hierarchical conversation threads based on header fields such as 'In-Reply-To' and 'References'.\nThe vulnerability is triggered when the IMAP server attempts to parse and sort mail headers that are maliciously crafted to exploit the sorting algorithm's complexity, likely causing an exponential increase in processing time relative to the number of messages or the specific structure of the headers.\nThe attack flow begins when an attacker sends an email containing a specially crafted header structure to a target user. This message is stored in the user's mailbox on the IMAP server.\nThe vulnerability remains latent until an IMAP client, such as a mail user agent (MUA), initiates a THREAD command—typically during the mailbox synchronization or folder refresh process—to display conversations. The IMAP server invokes the vulnerable parsing routine on the mailbox contents.\nAs the server processes the malicious headers, it consumes excessive CPU cycles, effectively pinning the processor. Because the IMAP command execution is often synchronous or blocks the worker process, this causes the service to become unresponsive to other legitimate requests.\nThe technical impact is a Denial of Service (DoS) condition at the application level. Because the processing logic is tied to the structure of the message headers, the attacker does not need to send large payloads; rather, the exploitation relies on the complexity of the data structure defined in the headers, leading to a 'complexity-based' resource exhaustion.\nThe vulnerability is exposed via the standard IMAP protocol (TCP/143 or TCP/993). Successful exploitation does not require the attacker to possess administrative credentials; the ability to deliver a message to the target's mailbox is sufficient to trigger the flaw upon the next synchronization event.\nPost-exploitation impact includes sustained service unavailability for the targeted account or the entire IMAP service, depending on the server's threading model and resource isolation configurations."
}