CVE-2026-42007: Sieve Editheader Use-After-Free Vulnerability

CVE-2026-42007 Overview

CVE-2026-42007 is a use-after-free vulnerability [CWE-416] in the Dovecot mail server, disclosed in Open-Xchange Security Advisory OXDC-ADV-2026-0003. An authenticated attacker can craft a Sieve script that abuses the editheader extension to trigger a use-after-free condition in the mail editing code. The flaw causes the delivery process to write memory contents beyond the intended buffer into the delivered mail message.

Exploitation leaks process memory, enables memory corruption during mail delivery, and can crash the delivery process. Under certain conditions, an attacker may achieve arbitrary code execution in the context of the delivery process.

Critical Impact
Authenticated users can leak server memory, corrupt the delivery process, and potentially execute arbitrary code by submitting a malicious Sieve script that invokes the editheader extension.

Affected Products

Dovecot (as distributed by Open-Xchange)
Deployments with the Sieve editheader extension enabled
Mail delivery pipelines using Pigeonhole Sieve

Discovery Timeline

2026-08-28 - CVE-2026-42007 published to the National Vulnerability Database
2026-09-03 - Last updated in the NVD database

Technical Details for CVE-2026-42007

Vulnerability Analysis

The vulnerability resides in the Sieve editheader extension implementation, which allows Sieve scripts to add and remove headers on a message during delivery. When the script manipulates headers in a specific sequence, the mail editing code references a memory region that has already been freed. Subsequent writes then place attacker-influenced data or stale memory contents into the buffer that is serialized into the delivered mail.

Because the corrupted or leaked bytes end up inside the recipient's mailbox, an attacker can read them back by simply retrieving the delivered message. This turns an internal memory safety flaw into an observable information disclosure primitive and provides feedback for further exploitation attempts.

The issue is triggerable over the network by any account that can submit or install Sieve scripts, which typically includes all authenticated mailbox users. Successful memory corruption can crash the delivery process, disrupt mail flow, or, depending on allocator state, lead to code execution in the delivery context.

Root Cause

The root cause is a use-after-free [CWE-416] in the header editing routines invoked by the editheader Sieve extension. The code retains a pointer to a header buffer whose backing memory is released during script execution, and later writes into that pointer without revalidating it.

Attack Vector

An authenticated user uploads or activates a Sieve script that uses editheader to add, delete, or replace headers on incoming mail. When the Mail Delivery Agent processes an inbound message against the script, the crafted sequence of header operations triggers the freed-buffer write. The result is that memory beyond the intended buffer is embedded into the delivered mail, giving the attacker both a memory disclosure channel and a corruption primitive. No user interaction from the victim is required. Consult the Open-Xchange Security Advisory OXDC-ADV-2026-0003 for vendor-provided technical details.

Detection Methods for CVE-2026-42007

Indicators of Compromise

Unexpected crashes, segmentation faults, or restarts of the Dovecot LDA/LMTP delivery processes recorded in system logs.
Delivered messages containing unusual binary content, non-printable bytes, or fragments of unrelated memory inside header fields.
Newly uploaded or modified Sieve scripts that reference the editheader extension by users who did not previously use it.

Detection Strategies

Audit all active Sieve scripts for require "editheader" declarations and review the header operations performed by each script.
Correlate mail delivery process crashes with the user account whose script was executing at the time of failure.
Alert on repeated delivery failures for a single sender or recipient, which can indicate exploitation attempts against the editheader code path.

Monitoring Recommendations

Enable verbose logging for the Pigeonhole Sieve interpreter and forward logs to a centralized SIEM for correlation.
Monitor the ManageSieve service for anomalous script upload activity, especially scripts using editheader alongside complex header manipulation.
Track process resource metrics for dovecot-lda and lmtp workers to catch memory corruption symptoms early.

How to Mitigate CVE-2026-42007

Immediate Actions Required

Update Dovecot to a non-vulnerable version as published in the Open-Xchange advisory.
If patching cannot be performed immediately, disable the Sieve editheader extension across all mail delivery configurations.
Review the list of users with ManageSieve access and remove the capability from accounts that do not require it.

Patch Information

Open-Xchange has issued fixed Dovecot packages as documented in Open-Xchange Security Advisory OXDC-ADV-2026-0003. Administrators should apply the updated packages provided through their Open-Xchange distribution channel and restart the affected delivery services. No publicly available exploits are currently known, but the advisory should be treated as the authoritative source for version-specific remediation guidance.

Workarounds

Remove editheader from the sieve_extensions setting in dovecot.conf or the Sieve plugin configuration and reload the service.
Restrict Sieve script upload privileges to trusted administrators until patched packages are deployed.
Enforce mailbox-level rate limiting and quarantine outbound crash-triggering messages pending investigation.

bash

# Configuration example: disable the editheader extension in Pigeonhole Sieve
# /etc/dovecot/conf.d/90-sieve.conf
plugin {
  sieve_extensions = +imapflags -editheader
  sieve_global_extensions = -editheader
}

# Reload Dovecot to apply changes
sudo doveadm reload