CVE-2025-41720 is a vulnerability classified under CWE-646 (Reliance on File Name or Extension of Externally-Supplied File) affecting Sauter modulo 6 devices modu680-AS. The core issue is that the device’s webserver API accepts file uploads based solely on the file extension, specifically allowing files with a '.png' extension without verifying the actual file content or type. This flaw enables a low privileged remote attacker to upload arbitrary data or potentially malicious files disguised as PNG images. Since the verification mechanism is superficial, attackers can bypass content validation and place crafted files on the device. Although the CVSS score is 4.3 (medium), indicating limited impact, the integrity of the device’s file system or configuration could be compromised if exploited. The attack vector is network-based (AV:N), requires low privileges (PR:L), and no user interaction (UI:N) is needed. The scope remains unchanged (S:U), and the impact affects integrity only (I:L), with no confidentiality or availability impact. No patches or known exploits are currently available, but the vulnerability is publicly disclosed and should be addressed proactively. The affected product is used in building automation and control systems, which may be part of critical infrastructure environments.

For European organizations, especially those relying on Sauter modulo 6 devices for building automation, HVAC control, or industrial process management, this vulnerability poses a risk to the integrity of device configurations and potentially the operational environment. While it does not directly compromise confidentiality or availability, unauthorized file uploads could lead to unauthorized configuration changes, insertion of malicious scripts, or persistence mechanisms that undermine system trustworthiness. This could result in operational disruptions or facilitate further attacks within the network. Given the critical role of building automation in energy management and safety systems, even integrity compromises can have cascading effects. The medium severity rating suggests moderate risk, but organizations in sectors such as manufacturing, energy, and facility management should be vigilant. The lack of known exploits reduces immediate risk but does not eliminate the threat, especially as attackers may develop exploits post-disclosure.

To mitigate CVE-2025-41720, organizations should implement the following specific measures: 1) Restrict file upload permissions strictly to trusted users or service accounts to minimize the attack surface. 2) Employ network segmentation and access controls to limit exposure of the webserver API to untrusted networks. 3) Implement additional validation mechanisms on the server side to verify file content signatures or MIME types rather than relying solely on file extensions. 4) Monitor logs and network traffic for unusual file upload activities or anomalous API requests. 5) If possible, disable or restrict the file upload functionality if not required for normal operations. 6) Engage with the vendor (Sauter) for any forthcoming patches or firmware updates addressing this vulnerability. 7) Conduct regular security assessments and penetration tests focusing on the webserver API and file handling components. 8) Maintain an inventory of affected devices and prioritize remediation based on criticality and exposure. These steps go beyond generic advice by focusing on operational controls and validation enhancements tailored to this vulnerability’s nature.