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CVE-2025-53909: Server-Side Template Injection Vulnerability in Mailcow

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Overview

In the ever-evolving landscape of cybersecurity, vulnerabilities present a persistent challenge. One such vulnerability, identified as CVE-2025-53909, affects mailcow: dockerized, an open-source groupware/email suite based on docker. This vulnerability, a Server-Side Template Injection (SSTI), is located in the notification template system used by mailcow for sending quota and quarantine alerts. By exploiting this vulnerability, attackers can potentially execute code, compromising the system and potentially leading to data leakage. This vulnerability is particularly significant due to the widespread use of mailcow in various enterprise settings, thereby underscoring the urgency of addressing this vulnerability.

Vulnerability Summary

CVE ID: CVE-2025-53909
Severity: Critical (9.1)
Attack Vector: Network
Privileges Required: Admin
User Interaction: Required
Impact: System compromise and potential data leakage

Affected Products

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Product | Affected Versions

mailcow: dockerized | Versions prior to 2025-07

How the Exploit Works

The Server-Side template Injection (SSTI) vulnerability in mailcow functions by allowing template expressions that can be manipulated to execute code. The template rendering engine of mailcow, which is used in sending quota and quarantine alerts, does not properly sanitize user input. If an attacker with admin-level access to mailcow’s UI configures the templates, they could inject malicious code that gets executed during the template rendering process.

Conceptual Example Code

Here is a conceptual example illustrating how the vulnerability might be exploited. In this example, the attacker sends a POST request with a malicious payload to the mailcow server. The malicious payload is designed to execute arbitrary code when the template is rendered.

POST /template/configure HTTP/1.1
Host: mailcowserver.example.com
Content-Type: application/json
Authorization: Bearer {admin_token}
{ "template": "{{ malicious_code }}" }

In this payload, `malicious_code` is a placeholder for the actual code that an attacker would use to exploit the vulnerability. The server, upon receiving this payload, would process the template and execute the malicious code.

Mitigation Guidance

To protect your system from this vulnerability, apply the vendor patch available in version 2025-07 of mailcow: dockerized. If for some reason the patch cannot be applied immediately, consider employing a Web Application Firewall (WAF) or Intrusion Detection System (IDS) as a temporary mitigation measure. These tools can help identify and block suspicious activity, thereby providing an additional layer of security. However, these are only temporary solutions, and applying the vendor patch should be the ultimate goal to ensure system security.

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Disclaimer:

The information and code presented in this article are provided for educational and defensive cybersecurity purposes only. Any conceptual or pseudocode examples are simplified representations intended to raise awareness and promote secure development and system configuration practices.

Do not use this information to attempt unauthorized access or exploit vulnerabilities on systems that you do not own or have explicit permission to test.

Ameeba and its authors do not endorse or condone malicious behavior and are not responsible for misuse of the content. Always follow ethical hacking guidelines, responsible disclosure practices, and local laws.

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