Angular: SSRF and Cross-Origin Credential Disclosure via URL Resolution Discrepancy in SSR

Summary

A discrepancy between WHATWG URL parsing and Angular SSR's URL resolution allows attackers to bypass same-origin checks and cause Server-Side Request Forgery (SSRF), potentially leaking sensitive server-side credentials.

Technical Description

When applications validate incoming URLs using the WHATWG URL standard (new URL(input, trustedOrigin)), Unicode whitespace characters (such as NO-BREAK SPACE U+00A0 or ZERO WIDTH NO-BREAK SPACE U+FEFF) are not stripped and are evaluated as part of a same-origin relative path (e.g. http://trusted-origin/%C2%A0//attacker.example/collect). Consequently, these URLs successfully pass application-level same-origin checks.

However, @angular/platform-server's URL resolution utility (resolveUrl / parseUrl) previously executed String.prototype.trim(). Because JavaScript's String.prototype.trim() strips all Unicode whitespace (including U+00A0), the leading non-breaking space was removed, converting the string into a cross-origin protocol-relative URL (//attacker.example/collect). When resolved during server-side rendering (such as in relativeUrlsTransformerInterceptorFn), this caused the HTTP request to be dispatched to the attacker-controlled origin (http://attacker.example/collect), leaking any credentials (such as Authorization headers) attached by the application for the intended same-origin request.

Impact & Reachability

• Reachability: The vulnerability affects Angular Server-Side Rendering (SSR) applications where user-controlled input influences resource or request URLs processed by Angular's HttpClient, an application-level same-origin check is performed before dispatching, and sensitive server-side credentials (such as API keys or Bearer tokens) are attached to approved requests.
• Impact: Successful exploitation allows attackers to bypass same-origin validation, triggering Server-Side Request Forgery (SSRF) and leaking sensitive server-side credentials attached to the request.

Proof of Concept:

// Interceptor performing same-origin validation
const trustedOrigin = new URL('http://localhost:4000/');
const target = new URL(req.urlWithParams, trustedOrigin);

if (target.origin !== trustedOrigin.origin) {
  throw new Error('Cross-origin request blocked');
}

// Request passes validation, server attaches sensitive credential:
const authenticatedReq = req.clone({
  headers: req.headers.set('Authorization', 'Bearer SERVER-SECRET-TOKEN'),
});

// @angular/platform-server previously trimmed the URL, converting it into
// //attacker.example/collect and routing the credential to the attacker.

Workarounds

• Validate and sanitize input URLs to disallow leading Unicode whitespace characters (such as \u00A0) before performing origin checks or passing them to HttpClient.
• Avoid relying solely on new URL(input, trustedOrigin).origin for authorization if the input string may be trimmed or processed by utilities that normalize whitespace differently from the WHATWG URL standard.