Compliance Impact

This vulnerability allows an attacker to exfiltrate sensitive data such as administrator credentials by exploiting a second-order SQL injection in the Mailcow system. Such unauthorized access and data leakage can lead to violations of data protection regulations like GDPR and HIPAA, which mandate the confidentiality and integrity of personal and sensitive information.

Because the vulnerability impacts confidentiality, integrity, and availability with high severity, it poses a significant risk to compliance with standards that require secure handling of sensitive data, including protecting against unauthorized access and data breaches.

Useful 0 Not Useful 0

Executive Summary

CVE-2026-40871 is a high-severity second-order SQL injection vulnerability in the Mailcow dockerized package, affecting versions prior to 2026-03b.

The vulnerability occurs because the Mailcow API endpoint /api/v1/add/mailbox accepts a parameter called quarantine_category, which is stored in the database without any validation or sanitization.

Later, a script called quarantine_notify.py constructs SQL queries dynamically using unsafe string formatting (% operator) instead of parameterized queries, which leads to execution of malicious SQL code stored earlier.

This delayed execution (second-order SQL injection) allows an attacker with API access to inject arbitrary SQL commands that can exfiltrate sensitive data such as administrator credentials by embedding them into quarantine notification emails.

Useful 0 Not Useful 0

Impact Analysis

This vulnerability can have severe impacts on confidentiality, integrity, and availability of the affected system.

• An attacker with API access can inject malicious SQL payloads that are executed later, allowing extraction of sensitive data including admin credentials.
• The attacker can gain unauthorized access to internal data and potentially escalate to full system compromise.
• Sensitive information can be exfiltrated and embedded inside quarantine notification emails, making detection harder.
• The vulnerability can be combined with other attacks, such as cross-site scripting (XSS), to steal API keys and further exploit the system.

Useful 0 Not Useful 0

Detection Guidance

Detection of this vulnerability involves checking for the presence of malicious payloads in the quarantine_category field stored via the Mailcow API and monitoring the execution of the quarantine notification job that processes this field.

Since the vulnerability is a second-order SQL injection triggered when quarantine_notify.py runs, detection can include:

• Reviewing API logs for suspicious or unexpected input values in the /api/v1/add/mailbox endpoint, especially in the quarantine_category parameter.
• Querying the Mailcow database directly to inspect the quarantine_category field for unusual or SQL injection payload patterns.
• Monitoring quarantine notification emails for unexpected content that may indicate exfiltration of sensitive data.

Suggested commands might include:

• Using SQL queries to search for suspicious patterns in the quarantine_category field, for example: SELECT quarantine_category FROM mailbox WHERE quarantine_category LIKE '%UNION%';
• Checking API access logs for unusual POST requests to /api/v1/add/mailbox containing suspicious payloads.
• Reviewing the output or logs of quarantine_notify.py execution for errors or unexpected SQL query behavior.

Useful 0 Not Useful 0

Mitigation Strategies

The primary immediate mitigation step is to upgrade Mailcow dockerized to version 2026-03b or later, where this vulnerability is fixed.

Additional immediate steps include:

• Restrict API access to trusted users only, as the vulnerability requires high privileges via API access.
• Monitor and sanitize inputs to the /api/v1/add/mailbox endpoint, especially the quarantine_category parameter, to prevent injection of malicious payloads.
• Temporarily disable or restrict the quarantine notification job (quarantine_notify.py) if possible, until the patch is applied.
• Review and audit existing mailbox entries for suspicious quarantine_category values and remove or sanitize them.

Useful 0 Not Useful 0