CVE-2025-53909: Mailcow SSTI RCE Vulnerability

CVE-2025-53909 Overview

CVE-2025-53909 is a Server-Side Template Injection (SSTI) vulnerability in mailcow: dockerized, an open source groupware and email suite distributed as Docker containers. The flaw exists in the notification template system that generates quota and quarantine alerts. The Jinja2 template rendering engine processes administrator-supplied templates without sandboxing, allowing template expressions to execute code during routine notification rendering. Exploitation requires admin-level access to the mailcow UI to configure malicious templates. Versions prior to 2025-07 are affected. The vendor released a patch in version 2025-07 that switches rendering to a sandboxed Jinja2 environment [CWE-1336].

Critical Impact

An authenticated administrator can inject template expressions that execute arbitrary code in the Dovecot notification container, leading to confidentiality, integrity, and availability compromise of the mail platform.

Affected Products

• mailcow: dockerized versions prior to 2025-07
• data/Dockerfiles/dovecot/quarantine_notify.py rendering component
• data/Dockerfiles/dovecot/quota_notify.py rendering component

Discovery Timeline

• 2025-07-17 - CVE-2025-53909 published to NVD
• 2025-07 - mailcow releases patched version 2025-07 with Jinja2 sandbox
• 2025-09-11 - Last updated in NVD database

Technical Details for CVE-2025-53909

Vulnerability Analysis

The vulnerability resides in the Dovecot notification subsystem that mailcow uses to deliver quota warnings and quarantine alerts. Administrators can configure the body of these notifications through the mailcow UI. The application passes those administrator-controlled strings directly to a non-sandboxed Jinja2 Template object for rendering. Jinja2 expression syntax such as {{ ... }} is evaluated server-side. An attacker with admin privileges can craft expressions that traverse Python object attributes to reach dangerous builtins, leading to arbitrary command execution inside the dovecot container.

The Exploit Prediction Scoring System currently reports an EPSS probability of 0.679%. No public proof-of-concept exploit is referenced in the advisory, and the issue is not listed in the CISA Known Exploited Vulnerabilities catalog.

Root Cause

The root cause is the use of the unrestricted jinja2.Template class for rendering user-influenced content. Standard Template objects expose the full Python object graph through attributes such as __class__, __mro__, and __subclasses__, which attackers chain to reach os.system or equivalent execution primitives. The codebase did not apply allow-listing, output escaping for code contexts, or a sandboxed environment to constrain template capabilities [CWE-1336].

Attack Vector

An attacker with administrator credentials authenticates to the mailcow UI and edits the quota or quarantine notification template. They embed a Jinja2 expression that resolves a Python subclass chain to an OS command runner. When mailcow's Dovecot worker processes a quota event or releases a quarantined message, the notification script renders the template and executes the injected payload as the container user.

The vendor fix replaces the unsafe Template import with SandboxedEnvironment from jinja2.sandbox, which blocks attribute access to unsafe builtins:

 from email.mime.text import MIMEText
 from email.utils import COMMASPACE, formatdate
 import jinja2
-from jinja2 import Template
+from jinja2 import TemplateError
+from jinja2.sandbox import SandboxedEnvironment
 import json
 import redis
 import time

Source: mailcow-dockerized commit 8c5f6c0

Detection Methods for CVE-2025-53909

Indicators of Compromise

• Unexpected child processes spawned by the dovecot-mailcow container, particularly shells (/bin/sh, /bin/bash) or interpreters launched from Python notification scripts.
• Notification template content in the mailcow database containing Jinja2 expressions referencing __class__, __mro__, __subclasses__, os.popen, or subprocess.
• Outbound network connections originating from the Dovecot container to unfamiliar hosts during quota or quarantine notification events.

Detection Strategies

• Audit the mailcow administrator activity log for template edits to quota and quarantine notification settings, especially from unrecognized sessions or IP addresses.
• Inspect persisted notification templates for Jinja2 syntax beyond simple variable substitution. Legitimate templates should not require attribute traversal.
• Compare the deployed quota_notify.py and quarantine_notify.py against the upstream 2025-07 release to confirm SandboxedEnvironment is in use.

Monitoring Recommendations

• Enable container runtime monitoring on the dovecot-mailcow service and alert on process executions outside the expected Python and Dovecot binaries.
• Forward mailcow admin UI access logs and Dovecot container stdout to a centralized log platform for retention and correlation.
• Track changes to the mailbox.quota_notification and quarantine template configuration tables and flag modifications outside scheduled change windows.

How to Mitigate CVE-2025-53909

Immediate Actions Required

• Upgrade mailcow: dockerized to version 2025-07 or later, which renders notifications inside a Jinja2 SandboxedEnvironment.
• Review every existing quota and quarantine notification template and remove any expressions that perform attribute traversal or call builtins.
• Rotate mailcow administrator credentials and enforce multi-factor authentication on the admin UI to reduce the prerequisite admin access risk.

Patch Information

The upstream fix is delivered in commit 8c5f6c03214a4b2bdbf3c78932f860eee949012b, included in release 2025-07. The patch replaces from jinja2 import Template with from jinja2.sandbox import SandboxedEnvironment in both data/Dockerfiles/dovecot/quota_notify.py and data/Dockerfiles/dovecot/quarantine_notify.py. Operators should follow the standard mailcow update procedure (git pull and ./update.sh) to apply the patched containers. Full details are available in the GitHub Security Advisory GHSA-8p7g-6cjj-wr9m.

Workarounds

• Restrict access to the mailcow admin UI to trusted management networks using firewall rules or a reverse proxy ACL until patching is complete.
• Manually replace the Template usage in the Dovecot notification scripts with SandboxedEnvironment and rebuild the affected container if immediate upgrade is not feasible.
• Reset quota and quarantine notification templates to vendor defaults to remove any pre-existing injected payloads before applying the upgrade.

# Update mailcow to a patched release
cd /opt/mailcow-dockerized
git fetch --tags
git checkout 2025-07
./update.sh
docker compose pull dovecot-mailcow
docker compose up -d dovecot-mailcow