CVE-2026-23769 is a vulnerability classified under CWE-79, indicating improper neutralization of input during web page generation, commonly known as cross-site scripting (XSS). The issue resides in NAVER's lucy-xss-filter, a security component designed to sanitize user input and prevent XSS attacks. Prior to the commit identified as e5826c0, the default superset rule files used by lucy-xss-filter were misconfigured, leading to insufficient sanitization of malicious input. This misconfiguration allows attackers to craft input that bypasses the filter, resulting in the execution of arbitrary JavaScript code within the context of the victim's browser session. Such execution can lead to theft of sensitive information such as cookies, session tokens, or enable unauthorized actions on behalf of the user. The vulnerability affects all versions of lucy-xss-filter before the specified commit, though exact version numbers are not provided. No public exploits have been reported yet, but the nature of XSS vulnerabilities makes them relatively easy to exploit, especially in web applications that rely heavily on user-generated content. The lack of a CVSS score means severity must be assessed based on impact and exploitability factors. The flaw impacts confidentiality and integrity primarily, with potential secondary effects on availability if exploited to perform further attacks. The vulnerability does not require authentication or user interaction beyond visiting a maliciously crafted page, increasing its risk profile. NAVER lucy-xss-filter is used in various web applications, and organizations deploying it without the patch or proper configuration remain vulnerable.

For European organizations, this vulnerability poses a significant risk to web application security, particularly those that integrate NAVER lucy-xss-filter for input sanitization. Successful exploitation could lead to session hijacking, credential theft, and unauthorized actions, compromising user data confidentiality and integrity. This is especially critical for sectors handling sensitive personal data under GDPR, such as finance, healthcare, and government services. The exploitation of XSS can also facilitate phishing attacks or malware distribution, increasing the overall threat landscape. The absence of authentication requirements and the ease of injecting malicious scripts mean that attackers can target a broad user base with minimal effort. Additionally, compromised web applications can suffer reputational damage and regulatory penalties in Europe. The impact extends to the availability of services if attackers leverage XSS to conduct further attacks like defacement or denial of service. Organizations relying on NAVER lucy-xss-filter without updated configurations or patches are at heightened risk, necessitating immediate attention to mitigate potential breaches.

European organizations should first identify all instances of NAVER lucy-xss-filter in their web application environments. Immediate steps include updating the lucy-xss-filter component to versions incorporating the fix from commit e5826c0 or later. If an update is not immediately feasible, reconfiguring the default superset rule files to ensure proper sanitization is critical. Implementing additional layers of defense such as strict Content Security Policies (CSP) can help mitigate the impact of potential XSS payloads by restricting script execution contexts. Web application firewalls (WAFs) should be configured to detect and block suspicious input patterns indicative of XSS attempts. Developers should review and enhance input validation and output encoding practices beyond reliance on lucy-xss-filter. Regular security testing, including automated scanning and manual penetration testing focused on XSS vectors, should be conducted to identify residual vulnerabilities. Finally, educating developers and security teams about secure coding practices and the specifics of this vulnerability will help prevent recurrence.