Description

An unauthenticated attacker can send a truncated quoted argument to the ManageSieve login process, which makes it spin in an infinite loop consuming CPU. This can cause degradation or denial of service for Sieve script management, and repeated connections can consume all available CPU on the server. Monitor system for abnormal CPU usage and kill the offending process. 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": "5.9",
  "pubDate": "2026-08-28T12:16:28.770Z",
  "pubdate": "2026-08-28T12:16:28.770Z",
  "executiveSummary": "The ManageSieve service is susceptible to a denial-of-service (DoS) vulnerability triggered by malformed input during the authentication phase.\nThe vulnerability manifests as an infinite loop when the service processes a truncated quoted argument, leading to excessive CPU consumption.\nThis flaw can be exploited by an unauthenticated remote attacker, requiring no prior system access or privileges.\nSuccessful exploitation results in severe degradation or complete exhaustion of server CPU resources, effectively disabling Sieve script management functionality.\nRepeated connection attempts can lead to persistent resource depletion, impacting the availability of the broader mail infrastructure if integrated on the same host.\nThere are currently no publicly documented exploits for this vulnerability, but the ease of execution necessitates immediate defensive measures.",
  "technicalDetails": "The vulnerability resides within the argument parsing logic of the ManageSieve protocol implementation. When the service receives a request containing a quoted argument that is improperly terminated or truncated, the internal state machine fails to handle the unexpected input EOF (End-of-File) condition gracefully.\nInstead of returning an error or closing the connection, the parser enters an infinite loop, continuously iterating over the malformed buffer or attempting to re-read the incomplete argument.\nThe attack flow begins with an unauthenticated attacker initiating a TCP connection to the ManageSieve service port. The attacker then sends a specially crafted command containing a quoted string that is intentionally truncated. Upon receipt, the ManageSieve process triggers the flawed parsing routine.\nBecause the vulnerability occurs at the protocol parsing level, it does not require authentication or elevated privileges, allowing any remote client with network reach to the service to trigger the condition.\nThe impact is a state of high CPU utilization localized to the ManageSieve process thread. If the architecture employs a single-threaded model or has limited process scaling, the CPU-bound infinite loop will block the service from responding to legitimate management requests.\nIn environments where multiple connections are permitted, an attacker can launch concurrent sessions, each causing an infinite loop, leading to rapid CPU saturation and total denial of service for the ManageSieve subsystem. This behavior persists until the individual process is manually terminated by a system administrator or process monitor.\nThis vulnerability highlights a lack of input validation and robust error handling in the protocol's state machine, specifically regarding string tokenization and buffer boundary conditions during the initial handshake and login phases."
}