CVE-2025-66202 affects the Astro web framework, specifically versions 5.15.7 and earlier. The vulnerability stems from improper handling of URL path encoding in middleware authorization checks. Middleware typically protects routes by verifying the request path against allowed patterns. However, Astro's middleware only decodes URL paths once before performing authorization checks. Attackers can exploit this by double-encoding URL paths (e.g., encoding the percent character itself), which bypasses the single decode step and causes the middleware to misinterpret the actual requested path. This allows unauthorized access to routes that should be protected, effectively bypassing authentication and authorization controls. The issue is classified under CWE-647 (Use of Non-Canonical URL Paths for Authorization Decisions), highlighting the risk of relying on non-normalized inputs for security decisions. Although a prior CVE (2025-64765) addressed single encoding bypass, it did not fully resolve the problem because double-encoded paths remained exploitable. The vulnerability has a CVSS v3.1 base score of 6.5 (medium severity), reflecting its network attack vector, low attack complexity, no privileges or user interaction required, and impact on confidentiality and integrity but not availability. No known exploits have been reported yet, but the flaw presents a significant risk for web applications relying on Astro middleware for route protection. The fix is included in Astro version 5.15.8, which properly normalizes URL paths by fully decoding them before authorization checks.

For European organizations, this vulnerability can lead to unauthorized access to sensitive web application routes, potentially exposing confidential data or allowing unauthorized modifications. Since Astro is a modern web framework used in building performant websites and applications, organizations relying on it for internal or customer-facing services could face data breaches or integrity violations. The lack of authentication requirements for exploitation increases the risk of automated or opportunistic attacks from external threat actors. This could impact sectors such as finance, e-commerce, healthcare, and government services where web applications protect sensitive information. Additionally, unauthorized access could facilitate further attacks like privilege escalation or lateral movement within networks. The medium severity rating indicates moderate but tangible risk, especially if exploited in combination with other vulnerabilities or misconfigurations. Organizations failing to update or implement proper input validation may suffer reputational damage, regulatory penalties under GDPR, and operational disruptions.

1. Upgrade all Astro framework instances to version 5.15.8 or later immediately to apply the official fix that fully decodes URL paths before authorization checks. 2. Implement strict input validation and normalization at the web server or application gateway level to reject or properly decode double-encoded URLs before they reach the application. 3. Conduct thorough security testing, including fuzzing and penetration testing, focusing on URL encoding edge cases to verify middleware authorization logic robustness. 4. Review and harden middleware and route protection logic to ensure it does not rely solely on raw or partially decoded URL paths for access control decisions. 5. Monitor web application logs for suspicious requests containing double-encoded characters or unusual URL patterns that could indicate exploitation attempts. 6. Employ Web Application Firewalls (WAFs) with rules to detect and block double-encoded URL attacks. 7. Educate development teams on secure URL handling practices and the risks of non-canonical path usage in authorization.