CVE-2025-50864 is a vulnerability in the elysia-cors library, versions up to 1.3.0, which is used to enforce Cross-Origin Resource Sharing (CORS) policies in web applications. The vulnerability arises from an origin validation error where the library incorrectly validates the supplied origin by checking if it is a substring of any domain listed in the site's CORS policy, rather than requiring an exact match. This flawed substring matching allows malicious origins that contain the legitimate domain as a substring (e.g., "notexample.com" or "example.common.net" when the policy specifies "example.com") to bypass CORS restrictions. Consequently, unauthorized origins can gain access to resources and user data that should be restricted, violating the intended same-origin policy protections. The vulnerability is classified under CWE-178 (Improper Neutralization of Input During Web Page Generation), indicating improper input validation. The CVSS v3.1 base score is 6.5 (medium severity), with an attack vector of network (remote exploitation), low attack complexity, no privileges required, but requiring user interaction (e.g., victim visiting a malicious site). The impact is primarily on integrity, as unauthorized origins can perform actions or access data they should not. No known exploits are currently reported in the wild, and no patches have been linked yet. This vulnerability affects any web application using the vulnerable elysia-cors library for CORS validation, potentially exposing sensitive user data to malicious third-party websites.

For European organizations, this vulnerability poses a significant risk to web applications that rely on the elysia-cors library for CORS enforcement. Exploitation could lead to unauthorized access to sensitive user data, session tokens, or other protected resources, undermining user privacy and data protection obligations under regulations such as GDPR. The integrity of user interactions with affected web services could be compromised, leading to potential data leakage or unauthorized actions performed on behalf of users. This could result in reputational damage, regulatory fines, and loss of customer trust. Since the vulnerability requires user interaction (e.g., visiting a malicious site), phishing or social engineering campaigns could be used to exploit it. The medium severity score reflects that while the vulnerability is exploitable remotely without privileges, it does not directly impact confidentiality but can compromise integrity. The absence of known exploits in the wild currently reduces immediate risk but does not eliminate the threat, especially as attackers may develop exploits once the vulnerability becomes widely known.

European organizations should take the following specific steps to mitigate this vulnerability: 1) Identify all web applications and services using the elysia-cors library, particularly versions up to 1.3.0. 2) Implement strict origin validation by replacing or patching the elysia-cors library to enforce exact origin matching rather than substring matching. If no official patch is available, consider custom middleware to validate origins precisely. 3) Conduct code reviews and penetration testing focused on CORS policies to detect similar misconfigurations or vulnerabilities. 4) Educate developers and security teams about the risks of improper CORS validation and the importance of exact origin matching. 5) Monitor web traffic and logs for suspicious cross-origin requests that may indicate exploitation attempts. 6) Employ Content Security Policy (CSP) headers to restrict resource loading and mitigate impact of malicious origins. 7) Prepare incident response plans to quickly address any exploitation attempts. 8) Stay updated with vendor advisories for official patches or updates to the elysia-cors library and apply them promptly once available.