Description

An unauthenticated attacker can crash the ManageSieve login process by sending a small malformed command before authenticating. If running in high-security mode (default for community releases), only the attacker's own connection is terminated. If running in high-performance mode (default for Pro releases), all connections handled by the same managesieve-login process are terminated. Repeating the attack can cause denial of service for Sieve script management. Restrict network access to the ManageSieve service to trusted clients. Update to non-vulnerable version. No publicly available exploits are known.

Executive Summary

Technical Details

Mitigations

References

Additional Metadata

{
  "score": "7.5",
  "pubDate": "2026-08-28T12:16:27.740Z",
  "pubdate": "2026-08-28T12:16:27.740Z",
  "executiveSummary": "This vulnerability involves a crash condition within the ManageSieve login process, specifically triggered by the submission of malformed command sequences. The defect allows an unauthenticated, remote attacker to induce a denial-of-service (DoS) state on the ManageSieve service.\nThe impact is contingent upon the operational configuration of the service. In high-security mode, the impact is restricted to the individual attacker's session. Conversely, in high-performance mode, the vulnerability results in the termination of all concurrent connections processed by the affected managesieve-login process. This exposure poses a significant risk to the availability of Sieve script management services.\nNo authentication is required to initiate the attack, as the malformed command is processed prior to session authentication. While no public exploits are currently documented, the repetitive nature of the attack allows for sustained service disruption. System administrators are advised to prioritize network segmentation and prompt application of available vendor patches to mitigate the underlying defect.",
  "technicalDetails": "The vulnerability resides in the command parsing logic of the ManageSieve login component. The flaw is triggered when the service receives a malformed command string during the initial handshake or pre-authentication phase. The application fails to sanitize or properly handle the anomalous input, leading to an unhandled exception or process fault within the managesieve-login process.\nThe attack flow initiates with the attacker establishing a TCP connection to the ManageSieve service. Prior to providing valid authentication credentials, the attacker transmits a specifically crafted, malformed command designed to exploit the parsing error. Upon receipt, the service attempts to process the command, triggering the internal crash. \nThe operational impact is dictated by the process architecture defined by the deployment mode. In high-security mode—the default configuration for community releases—the service architecture typically isolates sessions, ensuring that a crash only terminates the attacker's own connection. In high-performance mode—the default for Pro releases—multiple connections are managed by a single managesieve-login process. When the process crashes due to the malformed input, it results in a cascading failure, terminating every legitimate connection handled by that specific process instance.\nBecause the service allows processing of these commands without authentication, the attack surface is exposed to any entity with network-level access to the ManageSieve port. By repeatedly sending the malicious payload, an attacker can maintain a persistent DoS, rendering Sieve script management unavailable to legitimate users. The vulnerability highlights a lack of robust input validation at the protocol-parsing layer of the managesieve-login component. The absence of proper exception handling allows the malformed input to propagate into critical execution paths, causing the termination of the service process rather than a graceful rejection of the command."
}