Angular SSR has a Server-Side Request Forgery (SSRF) flaw

Impact

The vulnerability is a Server-Side Request Forgery (SSRF) flaw within the URL resolution mechanism of Angular's Server-Side Rendering package (@angular/ssr).

The function createRequestUrl uses the native URL constructor. When an incoming request path (e.g., originalUrl or url) begins with a double forward slash (//) or backslash (\\), the URL constructor treats it as a schema-relative URL. This behavior overrides the security-intended base URL (protocol, host, and port) supplied as the second argument, instead resolving the URL against the scheme of the base URL but adopting the attacker-controlled hostname.

This allows an attacker to specify an external domain in the URL path, tricking the Angular SSR environment into setting the page's virtual location (accessible via DOCUMENT or PlatformLocation tokens) to this attacker-controlled domain. Any subsequent relative HTTP requests made during the SSR process (e.g., using HttpClient.get('assets/data.json')) will be incorrectly resolved against the attacker's domain, forcing the server to communicate with an arbitrary external endpoint.

Exploit Scenario

A request to http://localhost:4200//attacker-domain.com/some-page causes Angular to believe the host is attacker-domain.com. A relative request to api/data then becomes a server-side request to http://attacker-domain.com/api/data.

Patches

• @angular/ssr 19.2.18
• @angular/ssr 20.3.6
• @angular/ssr 21.0.0-next.8

Mitigation

The application's internal location must be robustly determined from the incoming request. The fix requires sanitizing or validating the request path to prevent it from being interpreted as a schema-relative URL (i.e., ensuring it does not start with //).

Server-Side Middleware

If you can't upgrade to a patched version, implement a middleware on the Node.js/Express server that hosts the Angular SSR application to explicitly reject or sanitize requests where the path begins with a double slash (//).

Example (Express/Node.js):

// Place this middleware before the Angular SSR handler
app.use((req, res, next) => {
  if (req.originalUrl?.startsWith('//')) {
    // Sanitize by forcing a single slash
    req.originalUrl = req.originalUrl.replace(/^\/\/+/, '/');
    req.url = req.url.replace(/^\/\/+/, '/');
  }
  next();
});