CVE-2026-44437: Angular SSR has Open Redirect and Request Steering via Encoded X-Forwarded-Prefix

A vulnerability exists in the X-Forwarded-Prefix header processing logic within Angular SSR. The internal validation mechanism fails to properly account for URL-encoded characters, specifically dots (%2e%2e). This allows an attacker to bypass security filters by injecting encoded path traversal sequences that are later decoded and utilized by the application logic. When an Angular SSR application is configured to trust proxy headers and is deployed behind a proxy that forwards the X-Forwarded-Prefix header without prior sanitization, an attacker can provide a payload such as /%2e%2e/evil.

The vulnerability manifests in two ways:

• Open Redirect: The application processes a redirect (e.g., router redirectTo). The decoded traversal payload manipulates the Location header, forcing the browser to an unintended path or external domain.
• Server-Side Request Steering: The manipulated prefix is used as the base path for server-side HttpClient requests. This causes the server to make requests to unintended internal paths or external endpoints.