@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. matching and serializing secondary outlet groups to ), or passes user-provided navigation targets (e.g. ).