fix(@angular/ssr): decode x-forwarded-prefix before validation

The `x-forwarded-prefix` header can be percent-encoded. Validating it without decoding can allow bypassing security checks if subsequent processors (such as the `URL` constructor or a browser) implicitly decode it.

Key bypass scenarios addressed:
- **Implicit Decoding by URL Parsers**: A regex check for a literal `..` might miss `%2e%2e`. However, if the prefix is later passed to a `URL` constructor, it will treat `%2e%2e` as `..`, climbing up a directory.
- **Browser Role in Redirects**: If an un-decoded encoded path is sent in a `Location` header, the browser will decode it, leading to unintended navigation.
- **Double Slash Bypass**: Checking for a literal `//` misses `%2f%2f`. URL parsers might treat leading double slashes as protocol-relative URLs, leading to Open Redirects if interpreted as a hostname.

This change ensures the validation "speaks the same language" as the URL parsing system by decoding the prefix before running safety checks. It also introduces robust handling for malformed percent-encoding.

Author: alan-agius4

packages/angular/ssr/src/utils/validation.ts

@@ -260,9 +260,21 @@ function validateHeaders(request: Request): void {
   }
 
   const xForwardedPrefix = getFirstHeaderValue(headers.get('x-forwarded-prefix'));
-  if (xForwardedPrefix && INVALID_PREFIX_REGEX.test(xForwardedPrefix)) {
-    throw new Error(
-      'Header "x-forwarded-prefix" must not start with "\\" or multiple "/" or contain ".", ".." path segments.',
-    );
+  if (xForwardedPrefix) {
+    let xForwardedPrefixDecoded: string;
+    try {
+      xForwardedPrefixDecoded = decodeURIComponent(xForwardedPrefix);
+    } catch (e) {
+      throw new Error(
+        'Header "x-forwarded-prefix" contains an invalid value and cannot be decoded.',
+        { cause: e },
+      );
+    }
+
+    if (INVALID_PREFIX_REGEX.test(xForwardedPrefixDecoded)) {
+      throw new Error(
+        'Header "x-forwarded-prefix" must not start with "\\" or multiple "/" or contain ".", ".." path segments.',
+      );
+    }
   }
 }

packages/angular/ssr/test/utils/validation_spec.ts

@@ -125,8 +125,17 @@ describe('Validation Utils', () => {
       );
     });
 
-    it('should throw error if x-forwarded-prefix starts with a backslash or multiple slashes', () => {
-      const inputs = ['//evil', '\\\\evil', '/\\evil', '\\/evil', '\\evil'];
+    it('should throw error if x-forwarded-prefix starts with a backslash or multiple slashes including encoded', () => {
+      const inputs = [
+        '//evil',
+        '\\\\evil',
+        '/\\evil',
+        '\\/evil',
+        '\\evil',
+        '%5Cevil',
+        '%2F%2Fevil',
+        '%2F..%2Fevil',
+      ];
 
       for (const prefix of inputs) {
         const request = new Request('https://example.com', {
@@ -191,6 +200,18 @@ describe('Validation Utils', () => {
           .not.toThrow();
       }
     });
+
+    it('should throw error if x-forwarded-prefix contains malformed encoding', () => {
+      const request = new Request('https://example.com', {
+        headers: {
+          'x-forwarded-prefix': '/%invalid',
+        },
+      });
+
+      expect(() => validateRequest(request, allowedHosts)).toThrowError(
+        'Header "x-forwarded-prefix" contains an invalid value and cannot be decoded.',
+      );
+    });
   });
 
   describe('cloneRequestAndPatchHeaders', () => {