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 /%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' }
}
);
}