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🛡️ CVE-2025-66202 — astro

🟡 CVSS 6.5 — Medium ✅ No Known Exploit CWE-647 OSV
6.5
CVSS Score
0 Low4 Medium7 High9 Critical10

Description

Astro has an Authentication Bypass via Double URL Encoding, a bypass for CVE-2025-64765

# 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' }

}

);

}

```

How this vulnerability can be exploited

This issue can be reached over the network, attack complexity is low, an attacker needs no privileges on the target. No user interaction is required. The scope is unchanged, so the impact stays within the vulnerable component. Rated impact: confidentiality low, integrity low, availability none.

CVSS metrics in full

The score comes from this vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:N

  • Attack vector: Network — reachable from anywhere that can route to the service.
  • Attack complexity: Low — the attack works reliably, with no preparation.
  • Privileges required: None — an unauthenticated stranger can try it.
  • User interaction: None — nobody has to be tricked into anything.
  • Scope: Unchanged — the damage stays inside the vulnerable component.
  • Confidentiality impact: Low — limited, and the attacker does not choose what is affected.
  • Integrity impact: Low — limited, and the attacker does not choose what is affected.
  • Availability impact: None.

Weakness class

CVE-2025-66202 is classified as CWE-647: Use of Non-Canonical URL Paths for Authorization Decisions. The product defines policy namespaces and makes authorization decisions based on the assumption that a URL is canonical. This can allow a non-canonical URL to bypass the authorization.

Affected software

CVE-2025-66202 is recorded against 1 package.

  • astro

Timeline and source

Published on 8 December 2025 and last revised on 17 June 2026. No public exploit is currently recorded for this entry. A vendor advisory or fix has been published. Record sourced from OSV.

References

github.com (Web)
github.com (Web)
nvd.nist.gov (Advisory)
nvd.nist.gov (Advisory)
github.com (Web)
github.com (Package)

Other advisories for this package

astro has other advisories on record. If you are patching this one, these are worth checking on the same host:

Same weakness in other software

These advisories are the same class of weakness (CWE-647: Use of Non-Canonical URL Paths for Authorization Decisions) in other software:

Details

Severity MEDIUM
CVSS Score 6.5
CVSS Vector CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:N
CWE CWE-647
Public Exploit ✅ No
Source OSV
Published 2025-12-08
Updated 2026-08-20
Modified 2026-06-17

Affected Packages

Software From version Fixed in
astro — —

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