CVE-2026-21873 is a cross-site scripting (XSS) vulnerability classified under CWE-79 found in the NiceGUI Python UI framework, specifically in versions from 2.22.0 up to but not including 3.5.0. The vulnerability stems from an unsafe implementation in the pushstate event listener used by the ui.sub_pages component, which improperly neutralizes input during web page generation. This flaw allows an attacker to manipulate the fragment identifier portion of the URL (the part after the '#' symbol) even from a cross-origin context, such as through an iframe embedded on a malicious site. Because the fragment identifier is client-side and typically not sent to the server, this manipulation can be exploited to inject and execute arbitrary JavaScript code within the context of the vulnerable NiceGUI application. Notably, the attack does not require any privileges or user interaction, increasing its risk profile. The vulnerability affects confidentiality and integrity by enabling script injection that could steal sensitive data or alter application behavior. The issue has been addressed and patched in NiceGUI version 3.5.0. No known exploits are currently reported in the wild, but the vulnerability's characteristics make it a significant risk for applications relying on affected NiceGUI versions, especially those exposing UI components to untrusted or cross-origin content.

For European organizations, this vulnerability poses a significant risk to web applications built with the affected versions of NiceGUI. The ability to inject scripts via manipulated URL fragments can lead to theft of sensitive user data such as authentication tokens, personal information, or session cookies, undermining confidentiality. It can also allow attackers to alter the user interface or application logic, impacting integrity. Although availability is not directly affected, the loss of trust and potential regulatory consequences under GDPR due to data breaches could be severe. Organizations with public-facing dashboards, internal tools accessible via browsers, or embedded iframe content are particularly vulnerable. The cross-origin exploit vector increases the attack surface, as attackers can host malicious pages that trigger the vulnerability without direct access to the target domain. This could facilitate phishing, session hijacking, or further lateral attacks within corporate networks. The high CVSS score (7.2) reflects the ease of exploitation and the potential for impactful data compromise, making timely remediation critical.

1. Upgrade all NiceGUI deployments to version 3.5.0 or later immediately to apply the official patch addressing the XSS vulnerability. 2. Implement strict Content Security Policies (CSP) that restrict script sources and disallow inline scripts to reduce the risk of script injection exploitation. 3. Review and sanitize any user-controllable inputs that may influence URL fragments or client-side routing logic beyond the framework's default handling. 4. Avoid embedding NiceGUI-based applications within iframes on untrusted domains or implement frame-ancestors directives to control framing. 5. Conduct security testing, including penetration tests and code reviews, focusing on client-side input handling and URL manipulation. 6. Monitor web application logs and user reports for suspicious activity related to URL fragment manipulation or unexpected script execution. 7. Educate developers about secure handling of client-side routing and fragment identifiers to prevent similar issues in future development.