CVE-2025-53909 is a critical Server-Side Template Injection (SSTI) vulnerability affecting mailcow-dockerized, an open-source groupware and email suite deployed via Docker containers. The vulnerability exists in the notification template system used by mailcow to send quota and quarantine alerts. Specifically, the template rendering engine improperly neutralizes special elements within templates, allowing crafted template expressions to be executed as code in certain contexts. This can lead to remote code execution (RCE) on the mailcow server. Exploitation requires administrative-level access to the mailcow UI to configure or modify notification templates, which are then automatically rendered during normal system operations. The vulnerability affects all versions prior to 2025-07, with the vendor releasing a patch in version 2025-07 to address the issue. The CVSS v3.1 base score is 9.1 (critical), reflecting the high impact on confidentiality, integrity, and availability, the network attack vector, low attack complexity, and the requirement for high privileges but no user interaction. Although no known exploits are currently observed in the wild, the potential for exploitation is significant due to the ability to execute arbitrary code on the server hosting mailcow. This could allow attackers to compromise mail services, access sensitive email data, pivot within the network, or disrupt email operations.

For European organizations, the impact of this vulnerability can be severe. Mailcow is used by many enterprises, educational institutions, and government agencies as a self-hosted email and collaboration platform. Successful exploitation could lead to full system compromise, exposing sensitive communications, user credentials, and internal documents. This could result in data breaches, regulatory non-compliance (e.g., GDPR violations), and operational disruptions. Given the critical nature of email infrastructure, attackers could also use compromised mail servers to launch phishing campaigns or distribute malware internally. The requirement for admin-level access limits the attack surface but insider threats or compromised admin credentials could enable exploitation. The vulnerability's ability to execute arbitrary code means attackers can establish persistent footholds, escalate privileges, and move laterally within networks, amplifying the risk to European organizations that rely on mailcow for secure communications.

To mitigate this vulnerability, European organizations should immediately upgrade mailcow-dockerized installations to version 2025-07 or later, which contains the official patch. Until upgrading is possible, restrict administrative access to the mailcow UI using strong authentication mechanisms such as multi-factor authentication (MFA) and IP whitelisting. Conduct thorough audits of admin accounts and credentials to detect and remove any unauthorized access. Limit the number of administrators with template editing privileges to reduce risk. Additionally, monitor mailcow logs for unusual template changes or rendering errors that could indicate exploitation attempts. Network segmentation of mailcow servers can reduce lateral movement if compromise occurs. Implement strict template input validation and sanitization policies, and consider disabling custom templates if not required. Regularly back up mailcow configurations and data to enable recovery in case of compromise. Finally, maintain up-to-date vulnerability management and incident response plans tailored to mail infrastructure.