Authentication Bypass via Double URL Encoding in Astro
Summary
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.
Fix
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' } } )}
References
zomaxsecReporter
Authentication Bypass via Double URL Encoding in Astro
Bypass for CVE-2025-64765 / GHSA-ggxq-hp9w-j794
Summary
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
/%2561dmininstead of/%61dmin, attackers can still bypass authentication and access protected resources such as/admin,/api/internal, or any route protected by middleware pathname checks.Fix
A more secure fix is just decoding once, then if the request has a %xx format, return a 400 error by using something like :
References