CVE-2025-66202 is a vulnerability in the Astro web framework, specifically affecting versions 5.15.7 and earlier. Astro uses middleware pathname checks to enforce authorization on protected routes. However, these checks are vulnerable due to improper handling of URL encoding. The root cause is that the middleware decodes URL paths only once, which allows attackers to craft double-encoded URLs that bypass the authorization logic. For example, an attacker can encode a protected path twice, and when the middleware decodes it once, it still appears encoded and thus bypasses the path-based authentication checks. This vulnerability is classified under CWE-647 (Use of Non-Canonical URL Paths for Authorization Decisions), indicating that the system fails to normalize URL paths before making security decisions. The original fix for a related CVE (CVE-2025-64765) in version 5.15.8 was insufficient because it only performed a single decoding step. The fully patched version 5.15.8 corrects this by properly handling double-encoded URLs, ensuring that all encoded forms are normalized before authorization checks. The CVSS v3.1 score is 6.5 (medium), reflecting that the vulnerability is remotely exploitable without authentication or user interaction, impacting confidentiality and integrity but not availability. No known exploits have been reported in the wild yet, but the vulnerability poses a significant risk to applications relying on Astro's middleware for access control.

For European organizations, this vulnerability can lead to unauthorized access to protected web application routes, potentially exposing sensitive data or allowing unauthorized actions within applications built on vulnerable Astro versions. The impact primarily affects confidentiality and integrity, as attackers can bypass authentication controls without credentials or user interaction. This can result in data leakage, unauthorized data modification, or access to administrative functions. Since Astro is a modern web framework used in building performant websites and applications, organizations in sectors such as finance, e-commerce, government, and technology that rely on Astro middleware for access control are at risk. The vulnerability does not directly impact availability but could facilitate further attacks that degrade service or compromise systems. The lack of known exploits currently provides a window for proactive mitigation, but the ease of exploitation and remote nature of the vulnerability mean that attackers could develop exploits rapidly once the vulnerability becomes widely known.

European organizations should immediately upgrade all Astro framework instances to version 5.15.8 or later, where the double URL encoding bypass is fully fixed. In addition to patching, developers should audit middleware and authorization logic to ensure that URL paths are fully normalized and canonicalized before any security decisions are made. Implement strict input validation and decoding routines that handle multiple layers of encoding. Employ web application firewalls (WAFs) with rules to detect and block suspicious double-encoded URL patterns. Conduct thorough security testing, including fuzzing and penetration testing focused on URL encoding edge cases. Monitor web server logs for unusual URL encoding patterns that could indicate exploitation attempts. Finally, maintain an inventory of applications using Astro to prioritize patching and risk assessment efforts.