CVE-2026-40013: ManageSieve Service DOS Vulnerability

CVE-2026-40013 Overview

CVE-2026-40013 is an out-of-bounds write vulnerability [CWE-124] in the Dovecot ManageSieve service. An authenticated attacker can submit a Sieve script containing an extreme numeric literal that triggers memory corruption during compilation. The observed impact is a crash of the ManageSieve process, producing denial of service for remote Sieve script management. The vendor advisory notes that the memory corruption might be leveraged for remote code execution, though no public exploit exists. Open-Xchange published the advisory documenting the flaw and remediation path.

Critical Impact
Authenticated attackers can crash the ManageSieve process through a crafted Sieve script, disrupting remote script management with potential for further memory corruption abuse.

Affected Products

Dovecot ManageSieve service (Open-Xchange distribution)
Refer to the Open-Xchange Security Advisory for specific affected versions
Deployments exposing ManageSieve to authenticated users

Discovery Timeline

2026-08-28 - CVE-2026-40013 published to NVD
2026-09-03 - Last updated in NVD database

Technical Details for CVE-2026-40013

Vulnerability Analysis

The vulnerability resides in the Sieve script compiler invoked by the ManageSieve service. ManageSieve is the protocol clients use to upload, edit, and manage Sieve mail filtering scripts on the server. When the compiler parses a script containing an extreme numeric literal, it writes past the bounds of an allocated buffer. This out-of-bounds write corrupts adjacent memory and crashes the ManageSieve worker process.

Exploitation requires valid credentials because ManageSieve enforces authentication before accepting script uploads. The attack vector is network-based and requires no user interaction. Successful exploitation denies script management functionality to legitimate users until the service restarts. The vendor advisory states the memory corruption might enable remote code execution, but no working exploit has been published.

Root Cause

The root cause is improper validation of numeric literal magnitudes during Sieve script compilation. The compiler accepts numeric tokens whose values exceed the internal storage type or buffer capacity, producing a boundary violation classified under [CWE-124] Buffer Underwrite / Out-of-Bounds Write. See the Open-Xchange Security Advisory for technical detail.

Attack Vector

An authenticated attacker connects to the ManageSieve service over the network and uploads a crafted script containing a malformed numeric literal. Compilation triggers the out-of-bounds write. The ManageSieve process crashes, and other authenticated users lose the ability to manage their filter scripts until service recovery.

No public proof-of-concept code is available. Readers requiring exploit specifics should consult the vendor advisory linked above.

Detection Methods for CVE-2026-40013

Indicators of Compromise

Repeated crashes or unexpected restarts of the ManageSieve process in system logs
Sieve script upload attempts containing unusually large numeric literals
Authentication events immediately followed by ManageSieve service termination
Core dumps generated by the ManageSieve worker process

Detection Strategies

Monitor Dovecot logs for managesieve process crashes and abnormal exit codes
Inspect uploaded Sieve scripts for numeric tokens exceeding expected magnitudes before compilation
Correlate authenticated ManageSieve sessions with subsequent service faults to identify malicious users
Alert on repeated PUTSCRIPT commands from a single account that trigger compilation errors

Monitoring Recommendations

Enable verbose logging on the ManageSieve service to capture script upload metadata
Track service uptime and restart frequency through host telemetry
Ingest mail server logs into a centralized analytics platform to correlate patterns across users
Set alerting thresholds for anomalous script upload sizes or failed compilations per account

How to Mitigate CVE-2026-40013

Immediate Actions Required

Update Dovecot and the ManageSieve component to the fixed version listed in the Open-Xchange Security Advisory
Disable the ManageSieve service if users do not require remote Sieve script management
Audit accounts with ManageSieve access and rotate credentials suspected of compromise
Restrict network exposure of the ManageSieve port to trusted management networks

Patch Information

Open-Xchange has published a security advisory with remediation guidance. Administrators should apply the vendor-supplied update for their Dovecot distribution. Consult the Open-Xchange Security Advisory for version-specific patch identifiers.

Workarounds

Disable the ManageSieve service in the Dovecot configuration if remote script management is not required
Restrict ManageSieve access to specific source addresses using firewall rules
Enforce strong authentication and account lockout policies to limit abuse by compromised credentials
Monitor and rate-limit script upload activity per authenticated user

bash

# Disable ManageSieve protocol in Dovecot configuration
# /etc/dovecot/dovecot.conf
protocols = imap pop3 lmtp
# Restart Dovecot to apply changes
systemctl restart dovecot