@angular/platform-server: Open Redirect via Protocol-Relative URLs in Server-Side Rendering (SSR)

An open redirect vulnerability was identified in `@angular/platform-server` when Server-Side Rendering (SSR) is enabled. When Angular SSR renders an incoming request, URL resolution and platform navigation are managed by `ServerPlatformLocation` using `resolveUrl` (or `parseUrl` in v20). While `resolveUrl` rejected protocol-relative URLs whose raw string directly started with `//`, this check ran *prior* to WHATWG URL normalization. When a request URL includes a popping dot-segment followed by multiple slashes (such as `/.//evil.test` or serialized from router route shapes like `/.;/(//evil.test)`), the input bypassed the initial `urlStr.startsWith('//')` check. Subsequent WHATWG URL parsing resolved the dot-segments and resulted in a `pathname` starting with `//` (`//evil.test`). Consequently, `ServerPlatformLocation` stored `pathname = '//evil.test'`. When `@angular/ssr` detected a mismatch between the rendered URL and the router's final location state, it emitted the unnormalized pathname into the HTTP `Location` response header, producing an HTTP 302 redirect with `Location: //evil.test`, which browsers resolve as a protocol-relative navigation off-origin. ### Impact Successful exploitation allows an unauthenticated remote attacker to craft URLs that trick Angular SSR into redirecting users to an arbitrary external domain. * **Bypasses Application-Level Guards:** Common application-level validation routines (such as checking `!returnUrl.startsWith('//')` before invoking `router.navigateByUrl`) accept `/.//evil.test` or `/.;/(//evil.test)` as a safe relative path, allowing attackers to bypass redirect allowlists. * **Client-side SPAs Unaffected:** Pure client-side Angular applications (Single Page Applications without SSR) are **not** vulnerable, as `ServerPlatformLocation` is only utilized in server-side execution environments. ### Attack Preconditions & Vulnerable Configurations An application is affected if the following conditions are met: * **SSR Enabled:** The application runs in a Server-Side Rendering environment using `@angular/platform-server` and `@angular/ssr`. * **Parameter Route Shapes:** The router contains parameter routes with wildcard or child segments (e.g. `/:tenant` matching `.` and serializing secondary outlet groups `(//evil.test)` to `/.//evil.test`), or passes user-provided navigation targets (e.g. `returnUrl`). * **Reverse Proxy Passthrough:** Upstream reverse proxies (Nginx, Cloudflare, ALB) pass semicolons (`;`) or dot-segments without stripping or normalizing them. #### Exploit Payload Example An attacker sends a request targeting a route with parameters and child segments: ```http GET /.;/(//evil.test) HTTP/1.1 Host: example.com ``` **Response:** ```http HTTP/1.1 302 Found Location: //evil.test ``` The client browser resolves `//evil.test` protocol-relatively and redirects the user away from `example.com` to `evil.test`. ### Patches The issue is resolved by updating `resolveUrl` (and `parseUrl` in v20) in `@angular/platform-server` to verify that the normalized URL pathname does not start with `//` when `allowProtocolRelative` is `false`. * `22.2.1` * `21.2.25` * `20.3.33` ### Workarounds & Mitigations If you cannot immediately upgrade to a patched version, apply one of the following mitigations at your edge reverse proxy or WAF: 1. **Reject Paths with Dot-Segments Followed by Slashes:** Configure your reverse proxy (e.g., Nginx, Cloudflare, AWS WAF) to block requests containing `/.//`, `/..//`, or `/.;/`: ```nginx # Nginx example: reject suspicious path normalization sequences if ($request_uri ~* "(\n/\.\/|\/\.\;)") { return 400; } ``` 2. **Sanitize Redirect Headers:** Ensure edge proxies or API gateways rewrite or disallow protocol-relative (`//`) `Location` response headers originating from the SSR upstream.