CVE-2026-25516 is a medium-severity cross-site scripting (XSS) vulnerability affecting the NiceGUI Python UI framework, specifically versions prior to 3.7.0. NiceGUI's ui.markdown() component relies on the markdown2 library to convert markdown content into HTML. However, markdown2 by default permits raw HTML to pass through unfiltered. When user-controlled input is rendered through ui.markdown(), this behavior allows attackers to inject malicious HTML elements containing JavaScript event handlers, leading to XSS attacks. Unlike other NiceGUI components such as ui.html(), ui.chat_message(), or ui.interactive_image(), which provide or require sanitization parameters, ui.markdown() lacks such sanitization capabilities, leaving applications vulnerable. The vulnerability impacts confidentiality and integrity by enabling attackers to execute arbitrary scripts in the context of the victim's browser, potentially stealing session tokens, performing actions on behalf of the user, or defacing content. The CVSS 3.1 vector (AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N) indicates network attack vector, low attack complexity, no privileges required, user interaction needed, scope changed, and low impact on confidentiality and integrity with no availability impact. No known exploits have been reported in the wild as of the publication date. The vulnerability was reserved on 2026-02-02 and published on 2026-02-06. The fix is included in NiceGUI version 3.7.0, which presumably implements proper sanitization or disables raw HTML passthrough in ui.markdown().

For European organizations, this vulnerability poses a risk primarily to web applications built with NiceGUI versions earlier than 3.7.0 that render user-generated markdown content without additional sanitization. Successful exploitation can lead to client-side script execution, enabling attackers to steal session cookies, perform unauthorized actions, or manipulate displayed content, thereby compromising user confidentiality and data integrity. Although the vulnerability does not affect availability, the reputational damage and potential regulatory consequences under GDPR for data leakage or unauthorized access could be significant. Organizations in sectors with heavy reliance on web-based user interfaces—such as finance, healthcare, and public administration—are particularly at risk. The requirement for user interaction (e.g., clicking a malicious link or viewing crafted content) means phishing or social engineering campaigns could be used to exploit this vulnerability. The lack of known exploits in the wild suggests limited current active threat but does not preclude future exploitation, especially as the vulnerability becomes publicly known.

European organizations should immediately upgrade any NiceGUI deployments to version 3.7.0 or later, where this vulnerability is fixed. If upgrading is not immediately feasible, implement strict input validation and sanitization on all user-generated markdown content before rendering it via ui.markdown(). Consider disabling or restricting the use of raw HTML in markdown inputs or replacing ui.markdown() with components that support sanitization parameters. Employ Content Security Policy (CSP) headers to restrict the execution of unauthorized scripts and reduce the impact of potential XSS attacks. Conduct thorough code reviews and penetration testing focusing on user input handling in NiceGUI-based applications. Educate developers about the risks of rendering unsanitized user content and enforce secure coding practices. Monitor web application logs and user reports for signs of suspicious activity or attempted exploitation. Finally, maintain an updated inventory of applications using NiceGUI to ensure all affected instances are identified and remediated.