fix(@angular/ssr): disallow x-forwarded-prefix starting with a backslash

Updated the INVALID_PREFIX_REGEX to ensure that prefixes starting with a backslash are considered invalid. Previously, only multiple slashes or dot segments were explicitly disallowed at the start.

Also updated the associated validation error message and unit tests to reflect this change.

(cherry picked from commit 4d564f6)

Author: alan-agius4

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

@@ -29,7 +29,7 @@ const VALID_HOST_REGEX = /^[a-z0-9.:-]+$/i;
 /**
  * Regular expression to validate that the prefix is valid.
  */
-const INVALID_PREFIX_REGEX = /^[/\\]{2}|(?:^|[/\\])\.\.?(?:[/\\]|$)/;
+const INVALID_PREFIX_REGEX = /^(?:\\|\/[/\\])|(?:^|[/\\])\.\.?(?:[/\\]|$)/;
 
 /**
  * Extracts the first value from a multi-value header string.
@@ -270,7 +270,7 @@ 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 multiple "/" or "\\" or contain ".", ".." path segments.',
+      'Header "x-forwarded-prefix" must not start with "\\" or multiple "/" or contain ".", ".." path segments.',
     );
   }
 }

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

@@ -147,8 +147,8 @@ describe('Validation Utils', () => {
       );
     });
 
-    it('should throw error if x-forwarded-prefix starts with multiple slashes or backslashes', () => {
-      const inputs = ['//evil', '\\\\evil', '/\\evil', '\\/evil'];
+    it('should throw error if x-forwarded-prefix starts with a backslash or multiple slashes', () => {
+      const inputs = ['//evil', '\\\\evil', '/\\evil', '\\/evil', '\\evil'];
 
       for (const prefix of inputs) {
         const request = new Request('https://example.com', {
@@ -160,7 +160,7 @@ describe('Validation Utils', () => {
         expect(() => validateRequest(request, allowedHosts, false))
           .withContext(`Prefix: "${prefix}"`)
           .toThrowError(
-            'Header "x-forwarded-prefix" must not start with multiple "/" or "\\" or contain ".", ".." path segments.',
+            'Header "x-forwarded-prefix" must not start with "\\" or multiple "/" or contain ".", ".." path segments.',
           );
       }
     });
@@ -193,7 +193,7 @@ describe('Validation Utils', () => {
         expect(() => validateRequest(request, allowedHosts, false))
           .withContext(`Prefix: "${prefix}"`)
           .toThrowError(
-            'Header "x-forwarded-prefix" must not start with multiple "/" or "\\" or contain ".", ".." path segments.',
+            'Header "x-forwarded-prefix" must not start with "\\" or multiple "/" or contain ".", ".." path segments.',
           );
       }
     });