CVE-2025-40834 is a cross-site scripting (XSS) vulnerability identified in the Siemens Mendix RichText widget, affecting all versions from 4.0.0 up to but not including 4.6.1. The root cause is improper neutralization of user input during web page generation, which allows malicious scripts to be injected and executed in the context of the victim's browser. This vulnerability is classified under CWE-79, which pertains to improper input sanitization leading to XSS. The CVSS v3.1 base score is 5.7, indicating a medium severity level, with an attack vector of network (remote exploitation), low attack complexity, requiring privileges (PR:L) and user interaction (UI:R). The impact primarily affects confidentiality, as attackers could steal session tokens, cookies, or other sensitive information accessible via the browser. Integrity and availability are not impacted. No known exploits have been reported in the wild to date. The vulnerability is particularly relevant for web applications built using Mendix that incorporate the RichText widget, which is commonly used for rich text editing and display. The widget’s failure to properly sanitize input before rendering HTML content creates an attack surface for XSS. Siemens has published the vulnerability with no direct patch links provided, but remediation involves upgrading to Mendix RichText version 4.6.1 or later where the issue is resolved.

For European organizations, this vulnerability poses a risk of confidentiality breaches through the theft of sensitive user data such as authentication tokens, personal information, or corporate data accessible via the browser session. Sectors such as finance, healthcare, and government, which often use Mendix for rapid application development, could be particularly vulnerable to data leakage and subsequent phishing or session hijacking attacks. Although the vulnerability does not directly affect system integrity or availability, the exploitation of XSS can lead to further attacks, including social engineering and malware distribution. The requirement for user interaction and some privileges limits the ease of exploitation but does not eliminate risk, especially in environments with multiple user roles and complex workflows. Non-compliance with data protection regulations such as GDPR could result if sensitive data is exposed. The absence of known exploits currently provides a window for proactive mitigation before widespread attacks occur.

1. Upgrade Mendix RichText widget to version 4.6.1 or later where the vulnerability is fixed. 2. Implement strict input validation on all user-supplied data before it reaches the RichText widget, ensuring that potentially malicious scripts are sanitized or rejected. 3. Apply output encoding techniques to neutralize any HTML or script content rendered in the browser. 4. Deploy Content Security Policy (CSP) headers to restrict the execution of unauthorized scripts and reduce the impact of potential XSS attacks. 5. Conduct regular security testing, including automated scanning and manual code reviews, focusing on input handling in Mendix applications. 6. Educate developers and administrators on secure coding practices specific to Mendix and RichText usage. 7. Monitor application logs and user activity for unusual behavior that could indicate attempted exploitation. 8. Limit user privileges where possible to reduce the attack surface, especially for roles that can input content into the RichText widget.