CVE-2026-21873 is a cross-site scripting (XSS) vulnerability categorized under CWE-79, affecting the NiceGUI Python UI framework from versions 2.22.0 up to but not including 3.5.0. The vulnerability stems from an unsafe implementation in the pushstate event listener mechanism used by the ui.sub_pages component, which fails to properly neutralize input when processing the fragment identifier portion of URLs. This improper input handling allows an attacker to inject malicious scripts by manipulating the URL fragment identifier, even from a cross-site context using an iframe, without requiring user interaction or authentication. The vulnerability is significant because the fragment identifier is typically client-side and not sent to the server, but NiceGUI’s handling exposes it to script injection risks. The CVSS v3.1 base score is 7.2, reflecting network attack vector, low attack complexity, no privileges or user interaction required, and a scope change with partial confidentiality and integrity impact but no availability impact. Although no known exploits have been reported in the wild, the vulnerability can be leveraged to execute arbitrary JavaScript in the context of the vulnerable application, potentially leading to session hijacking, data theft, or UI manipulation. The issue was addressed in NiceGUI version 3.5.0 by properly sanitizing the fragment identifier input in the pushstate event listener. Organizations using affected versions should upgrade promptly to prevent exploitation.

For European organizations, the impact of CVE-2026-21873 can be significant if NiceGUI is used in internal or customer-facing web applications. Exploitation could allow attackers to execute arbitrary scripts, leading to theft of sensitive information such as authentication tokens, user data, or manipulation of the user interface to perform phishing or fraud. This undermines confidentiality and integrity of the affected applications. Since the vulnerability can be exploited remotely without authentication or user interaction, it increases the attack surface and risk of automated or large-scale attacks. Organizations in sectors with high regulatory requirements for data protection, such as finance, healthcare, and government, could face compliance violations and reputational damage if exploited. The lack of known exploits in the wild currently reduces immediate risk, but the public disclosure and availability of patches mean attackers may develop exploits soon. The vulnerability’s impact is heightened in environments where NiceGUI is integrated into critical business workflows or exposed to untrusted users or the internet.

To mitigate CVE-2026-21873, European organizations should immediately upgrade all NiceGUI deployments to version 3.5.0 or later, where the vulnerability is patched. If upgrading is not immediately feasible, implement strict Content Security Policy (CSP) headers to restrict script execution and reduce the risk of XSS exploitation. Additionally, review and sanitize any user-controllable inputs that may influence URL fragments or client-side navigation. Employ web application firewalls (WAFs) with rules targeting suspicious URL fragment manipulations or iframe injection attempts. Conduct thorough security testing and code reviews of applications using NiceGUI to identify any residual or related input validation issues. Educate developers about secure handling of client-side URL components and event listeners. Monitor application logs and network traffic for unusual iframe or URL fragment activity that could indicate exploitation attempts. Finally, maintain an inventory of NiceGUI usage across the organization to ensure all instances are updated and protected.