Mailcow Mail Server Flaws Expose Servers to Remote Code Execution

Recent discoveries have highlighted significant security vulnerabilities in the Mailcow open-source mail server suite, potentially exposing servers to remote code execution. These vulnerabilities, which affect all versions prior to 2024-04, were responsibly disclosed by SonarSource on March 22, 2024, and have been addressed in the latest update released on April 4, 2024.

Understanding the Vulnerabilities

The two vulnerabilities, both rated as Moderate in severity, pose serious risks to the integrity and security of Mailcow servers:

  1. CVE-2024-30270 (CVSS score: 6.7): This is a path traversal vulnerability in a function named rspamd_maps(). By exploiting this flaw, a threat actor can overwrite any file that can be modified with the "www-data" user, leading to arbitrary command execution on the server.
  2. CVE-2024-31204 (CVSS score: 6.8): This cross-site scripting (XSS) vulnerability occurs via the exception handling mechanism when not operating in DEV_MODE. The flaw arises because the exception details are saved without any sanitization or encoding and are then rendered into HTML, which is executed as JavaScript within the user’s browser.

Exploitation and Impact

The exploitation of these vulnerabilities can have severe consequences. Specifically, the combination of these flaws allows a malicious actor to take control of accounts on a Mailcow server, gain access to sensitive data, and execute commands. Here’s how the exploitation could unfold:

  • Path Traversal Attack: By leveraging the path traversal vulnerability, an attacker could overwrite critical files on the server.
  • XSS Attack: Through the XSS vulnerability, an attacker can inject malicious scripts into the admin panel by triggering exceptions with specially crafted input. This can lead to session hijacking and unauthorized actions in the context of an administrator.

The Attack Scenario

In a theoretical attack scenario, an attacker could send an HTML email containing a CSS background image loaded from a remote URL. This email would trigger the XSS payload when viewed by an admin user. The admin does not need to click any link or interact with the email; merely viewing it while logged into the admin panel is sufficient. Once the XSS payload is executed, the attacker can combine it with the path traversal flaw to achieve arbitrary code execution on the server.

Paul Gerste, a vulnerability researcher at SonarSource, explained, "An attacker can combine both vulnerabilities to execute arbitrary code on the admin panel server of a vulnerable Mailcow instance. The requirement for this is that an admin user views a malicious email while being logged into the admin panel. The victim does not have to click a link inside the email or perform any other interaction with the email itself, they only have to continue using the admin panel after viewing the email."

Mitigation and Recommendations

To protect against these vulnerabilities, it is crucial to update Mailcow to the latest version (2024-04 or later). Additionally, administrators should follow these best practices:

  • Apply Security Patches: Regularly update software to incorporate the latest security patches.
  • Sanitize Input: Ensure proper sanitization and encoding of all input to prevent XSS attacks.
  • Monitor for Suspicious Activity: Keep an eye on server logs and network traffic for any signs of exploitation attempts.

By taking these steps, Mailcow users can safeguard their systems against potential attacks and maintain a secure mail server environment.