CVE-2025-66202

Authentication Bypass via Double URL Encoding in Astro

Bypass for CVE-2025-64765 / GHSA-ggxq-hp9w-j794

A double URL encoding bypass allows any unauthenticated attacker to bypass path-based authentication checks in Astro middleware, granting unauthorized access to protected routes. While the original CVE-2025-64765 (single URL encoding) was fixed in v5.15.8, the fix is insufficient as it only decodes once. By using double-encoded URLs like /%2561dmin instead of /%61dmin, attackers can still bypass authentication and access protected resources such as /admin, /api/internal, or any route protected by middleware pathname checks.

CVE-2025-66202 has a CVSS score of 6.5 (Medium). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (5.15.8); upgrading removes the vulnerable code path.

Kodem intelligence

A more secure fix is just decoding once, then if the request has a %xx format, return a 400 error by using something like :

if (containsEncodedCharacters(pathname)) {
            // Multi-level encoding detected - reject request
            return new Response(
                'Bad Request: Multi-level URL encoding is not allowed',
                {
                    status: 400,
                    headers: { 'Content-Type': 'text/plain' }
                }
            );
        }

1. What is CVE-2025-66202? CVE-2025-66202 is a medium-severity security vulnerability in astro (npm), affecting versions < 5.15.8. It is fixed in 5.15.8.
2. How severe is CVE-2025-66202? CVE-2025-66202 has a CVSS score of 6.5 (Medium). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of astro are affected by CVE-2025-66202? astro (npm) versions < 5.15.8 is affected.
4. Is there a fix for CVE-2025-66202? Yes. CVE-2025-66202 is fixed in 5.15.8. Upgrade to this version or later.
5. Is CVE-2025-66202 exploitable, and should I be worried? Whether CVE-2025-66202 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2025-66202 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2025-66202? Upgrade astro to 5.15.8 or later.