GHSA-w739-gvwx-grc3 - @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 En