Description

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');
}
...
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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.

Mitigation

Aliases

References