Angular: SSRF via protocol-relative and backslash URLs in Angular Platform-Server

Impact

A Server-Side Request Forgery (SSRF) vulnerability exists in @angular/platform-server due to improper handling of URLs during Server-Side Rendering (SSR).

When an attacker sends a request such as GET /\evil.com/ HTTP/1.1 the server engine (Express, etc.) passes the URL string to Angular’s rendering functions.

Because the URL parser normalizes the backslash to a forward slash for HTTP/HTTPS schemes, the internal state of the application is hijacked to believe the current origin is evil.com. This misinterpretation tricks the application into treating the attacker’s domain as the local origin. Consequently, any relative HttpClient requests or PlatformLocation.hostname references are redirected to the attacker controlled server, potentially exposing internal APIs or metadata services.

Affected APIs:

• renderModule
• renderApplication
• CommonEngine (from @angular/ssr)

Non-Affected APIs:

• AngularAppEngine (from @angular/ssr)
• AngularNodeAppEngine (from @angular/ssr)

Attack Preconditions

• The server has outbound network access.
• The application uses Angular SSR via the affected APIs.
• A pathname is passed as URL to the rendering method (e.g. using req.url).
• The server-side code performs HTTP requests using HttpClient with relative URLs or uses PlatformLocation.hostname to build URLs.

Patches

• 22.0.0-next.8
• 21.2.9
• 20.3.19
• 19.2.21

Workarounds

Developers should implement a middleware to sanitize the request URL before it reaches Angular. This involves stripping or normalizing leading slashes:

app.use((req, res, next) => {
  // Sanitize the URL to ensure it starts with a single forward slash
  if (req.url.startsWith('//') || req.url.startsWith('/\\') || req.url.startsWith('\\')) {
     req.url = '/' + req.url.replace(/^[/\\]+/, '');
  }
  next();
});

References

• Fix