CVE-2026-40016: Sieve Script DoS Vulnerability

CVE-2026-40016 Overview

CVE-2026-40016 affects the Dovecot mail server's Sieve script handling component. An attacker with the ability to upload Sieve scripts, either through the ManageSieve service or local access, can craft a malicious script that bypasses the configured CPU time limit by up to 130 times the intended threshold. The flaw maps to [CWE-400] Uncontrolled Resource Consumption and was disclosed in the Open-Xchange Security Advisory. The vulnerability degrades server performance and undermines administrative controls designed to constrain Sieve execution cost.

Critical Impact
Authenticated attackers can consume server CPU resources far beyond configured Sieve limits, causing availability degradation across mail processing infrastructure.

Affected Products

Open-Xchange Dovecot (Sieve component)
Deployments exposing the ManageSieve service to authenticated users
Deployments permitting local Sieve script upload

Discovery Timeline

2026-05-12 - CVE-2026-40016 published to NVD
2026-05-12 - Last updated in NVD database

Technical Details for CVE-2026-40016

Vulnerability Analysis

Dovecot enforces a CPU time limit on Sieve script execution to prevent any single mail filter from monopolizing server resources. The implementation contains a control-flow gap that allows specially crafted scripts to continue executing well past the configured ceiling. Measurements in the vendor advisory indicate the bypass can extend execution to roughly 130 times the configured limit.

The attacker needs authenticated access to upload a Sieve script. ManageSieve, the standard protocol for managing user filters, is the primary remote vector. Local script placement on the mail server is an equivalent vector when filesystem access exists.

The issue is classified as Uncontrolled Resource Consumption [CWE-400]. Successful exploitation does not disclose data or alter integrity, but it sustains CPU pressure that delays legitimate mail processing.

Root Cause

The Sieve interpreter's CPU time accounting fails to interrupt certain script constructs at the configured limit. Specific operations continue to consume processor time after the limit should have terminated execution. The vendor has not published the exact construct in public advisories.

Attack Vector

The attacker authenticates to the ManageSieve service, uploads a script crafted to trigger the accounting weakness, and activates it. Subsequent message delivery or explicit script execution causes the interpreter to run far beyond the configured ceiling. Repeated invocations amplify the impact and can starve mail delivery workers.

No public exploit code is available. The vendor advisory does not include a proof of concept.

Detection Methods for CVE-2026-40016

Indicators of Compromise

Sustained CPU usage by Dovecot Sieve worker processes that exceeds configured sieve_max_cpu_time boundaries.
ManageSieve PUTSCRIPT operations from accounts that do not normally manage filters.
Mail delivery latency spikes correlated with specific user accounts or Sieve script activations.

Detection Strategies

Audit recently uploaded or modified Sieve scripts in user script directories and compare against baseline content.
Correlate ManageSieve authentication events with subsequent CPU consumption on the mail server.
Review Dovecot logs for repeated Sieve execution by the same user near or beyond expected runtime ceilings.

Monitoring Recommendations

Enable verbose Sieve logging including execution time per script invocation.
Track per-user Sieve execution duration and alert on outliers relative to historical norms.
Monitor ManageSieve service connection rates and script upload frequency per account.

How to Mitigate CVE-2026-40016

Immediate Actions Required

Install the fixed Dovecot version referenced in the Open-Xchange Security Advisory.
Restrict ManageSieve exposure to trusted networks or place it behind a VPN where feasible.
Audit existing Sieve scripts for unusual constructs and remove untrusted entries.

Patch Information

Open-Xchange has issued a fixed release that corrects the Sieve CPU accounting logic. Administrators should consult the vendor advisory for exact version numbers applicable to their distribution and apply the update during the next maintenance window.

Workarounds

Disable the ManageSieve service if users do not require remote Sieve management.
Prevent local Sieve script uploads from untrusted users by tightening filesystem permissions on script directories.
Constrain operating system level CPU quotas (for example, cgroups) on Dovecot worker processes as a defense in depth measure.

bash

# Example: disable ManageSieve in dovecot.conf if not required
protocols = imap pop3 lmtp
# Remove 'sieve' from protocols and ensure managesieve-login service is not started
service managesieve-login {
  inet_listener sieve {
    port = 0
  }
}