Angular Service Worker Policy-Bypass & Credential-Stripping Vulnerabilities

An issue in the @angular/service-worker package compromises the integrity of request-policy enforcement during request reconstruction. When the Angular Service Worker intercepts network requests for matched assets, it reconstructs a new Request object using an internal helper function.

During this reconstruction process, the helper function strips the strict, client-defined request redirect policy configuration (such as redirect: 'error'), falling back to the browser's default 'follow' strategy.

If the target web application makes client-side requests with a strict policy (e.g., expecting a network error instead of automatically following redirects), the service worker will bypass this instruction and automatically follow HTTP 3xx redirects to other destinations. This acts as an unintended proxy/intermediary ("Confused Deputy") and can result in cookie/credential exposure or same-origin session-restricted data leakage if public dynamic routes redirect to sensitive routes.

Impact

Web applications registering the @angular/service-worker package are vulnerable to this redirect-policy bypass if they make safe client-side fetch calls (such as { redirect: 'error' }) to paths matched by a service worker asset group (such as lazy-loaded JavaScript bundles or dynamic public assets) that can return HTTP redirects to authenticated same-origin secure endpoints.

By stripping developer-defined safety boundaries, the service worker allows the browser to transparently query and return data from credentials-guarded resources that should have been blocked at the network barrier.

Attack Preconditions

To successfully exploit this vulnerability, all of the following application states and parameters must concurrently exist:

1. Active Angular Service Worker: The target application uses @angular/service-worker and has an active registration of ngsw-worker.js inside the client's browser context.
2. Asset Group Matching: An assetGroups pattern in ngsw-config.json encompasses the target dynamic routing endpoint.
3. Same-Origin Dynamic Redirection: The server routes a public matched asset route to a service that returns an HTTP 3xx redirect pointing to a sensitive, session-restricted same-origin private route (e.g., /private/account-summary.json).
4. Established User Session: The victim user currently has an active authentication state, such as valid same-origin session cookies or auth headers stored by the browser.
5. Client-Side Safe Fetch Call: The application initiates an explicit fetch request to the route with safety parameters: { redirect: 'error' }.

Mitigations & Workarounds

If upgrading the @angular/service-worker package is not immediately feasible, developers should implement the following defensive measures:

• Avoid Public-to-Private Dynamic Redirection: Refactor the server architecture so that public paths matched by service worker asset groups never issue HTTP 3xx redirects to authenticated same-origin secure endpoints.
• Strict Cookie Configuration: Apply strict flags to session cookies (SameSite=Strict; Secure; HttpOnly) and consider explicit route isolations (such as subdomains) for credential-guarded private resources.
• Exclude Secure Endpoints from SW Config: Verify your ngsw-config.json settings and ensure that patterns targeting dynamic, secure endpoints are explicitly excluded from automatic asset groups or caching scopes.

Patches

• 22.0.0-rc.2
• 21.2.15
• 20.3.22
• 19.2.23