CVE-2025-59535 is a medium-severity vulnerability affecting versions of the Dnn.Platform (formerly DotNetNuke) content management system prior to 10.1.0. Dnn.Platform is an open-source CMS built on the Microsoft technology stack, widely used for web content management. The vulnerability arises from improper input validation (CWE-20) that allows an attacker to load arbitrary installed themes via specially crafted query parameters. Even if a vulnerable theme is not actively used on any page, it can still be invoked and loaded on clients visiting the site without the site owner's knowledge. This can lead to integrity and availability impacts, such as executing malicious code or causing denial of service on the client side. The vulnerability does not require authentication or user interaction and can be exploited remotely over the network. The CVSS v3.1 base score is 6.5, reflecting network attack vector, low attack complexity, no privileges or user interaction required, and impacts limited to integrity and availability but not confidentiality. The issue has been addressed and patched in Dnn.Platform version 10.1.0. No known exploits have been reported in the wild as of the publication date. The vulnerability also relates to CWE-200 (exposure of sensitive information) and CWE-829 (inclusion of functionality from untrusted control sphere), indicating risks of unintended information disclosure and unsafe inclusion of components. Overall, this vulnerability highlights the risks of insufficient input validation in web applications that dynamically load components based on user-supplied parameters, potentially exposing clients to malicious payloads embedded in themes.

For European organizations using affected versions of Dnn.Platform, this vulnerability poses a moderate risk. Exploitation could allow attackers to load malicious themes that may execute harmful scripts or disrupt service availability for site visitors. This can damage organizational reputation, lead to loss of user trust, and potentially cause operational disruptions if critical web portals are affected. Although confidentiality is not directly impacted, integrity and availability of web content and client experience are at risk. Organizations in sectors with high web presence such as government, education, and e-commerce may face increased exposure. Additionally, regulatory frameworks like GDPR emphasize protecting user data and service reliability, so exploitation causing service disruption or indirect data exposure could have compliance implications. The lack of required authentication or user interaction means attackers can exploit this vulnerability remotely and at scale, increasing the threat surface for European entities relying on vulnerable Dnn versions.

European organizations should immediately assess their Dnn.Platform installations and identify any running versions prior to 10.1.0. The primary mitigation is to upgrade to version 10.1.0 or later, where the vulnerability is patched. Until upgrade is possible, organizations should restrict access to theme loading functionality by implementing web application firewall (WAF) rules that block or sanitize query parameters related to theme selection. Conduct thorough audits of installed themes to remove or update any that are known or suspected to be vulnerable. Employ strict input validation and output encoding on all user-supplied parameters to prevent arbitrary resource loading. Monitor web server logs for unusual requests attempting to load themes via query parameters. Additionally, implement Content Security Policy (CSP) headers to limit the execution of unauthorized scripts that could be loaded through malicious themes. Regularly update and patch all CMS components and dependencies to reduce exposure to similar vulnerabilities.