CVE-2025-50864 is a security vulnerability found in the elysia-cors library, a tool 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 validation logic allows malicious origins that contain the legitimate domain as a substring to bypass CORS restrictions. For example, if a site’s CORS policy whitelists "example.com", an attacker controlling "notexample.com" or "example.common.net" can bypass the policy and gain unauthorized access to user data. This bypass undermines the fundamental security model of CORS, which is designed to prevent unauthorized cross-origin requests and protect sensitive user information. The vulnerability affects all versions of elysia-cors up to and including 1.3.0. Although no known exploits are currently reported in the wild, the nature of the vulnerability makes it a significant risk for web applications relying on this library for CORS enforcement. The lack of a CVSS score indicates that the vulnerability is newly published and may not yet have been fully assessed for severity, but the technical details suggest a critical flaw in origin validation logic that can lead to unauthorized data access.

For European organizations, this vulnerability poses a serious risk to web applications that use the elysia-cors library to manage CORS policies. Exploitation could lead to unauthorized access to sensitive user data, including personal information protected under GDPR regulations. This could result in data breaches, loss of user trust, regulatory fines, and reputational damage. Organizations in sectors such as finance, healthcare, e-commerce, and government services are particularly vulnerable due to the sensitive nature of the data they handle. Additionally, the vulnerability could be exploited to perform cross-origin attacks that bypass same-origin policy restrictions, potentially enabling session hijacking, unauthorized API access, or data exfiltration. The impact is heightened in environments where elysia-cors is used as a primary defense mechanism for cross-origin requests, especially if no additional security controls are in place.

To mitigate this vulnerability, organizations should immediately review their use of the elysia-cors library and upgrade to a patched version once available that correctly implements exact origin matching for CORS validation. In the interim, organizations can implement strict server-side validation of the Origin header using exact string matching or whitelist validation logic that does not rely on substring checks. Employing Content Security Policy (CSP) headers to restrict allowed origins can provide an additional layer of defense. Web application firewalls (WAFs) can be configured to detect and block suspicious cross-origin requests that do not match expected origins exactly. Regular security audits and penetration testing focused on CORS configurations should be conducted to identify and remediate similar misconfigurations. Finally, educating developers about secure CORS implementation practices is critical to prevent recurrence of such vulnerabilities.