GHSA-xh43-g2fq-wjrj Angular SSR has an Open Redirect via X-Forwarded-Prefix

• npm @angular/ssr: >= 21.2.0-next.0, < 21.2.0-rc.1 → fixed in 21.2.0-rc.1 • npm @angular/ssr: >= 21.0.0-next.0, < 21.1.5 → fixed in 21.1.5 • npm @angular/ssr: >= 20.0.0-next.0, < 20.3.17 → fixed in 20.3.17 • npm @angular/ssr: >= 19.0.0-next.0, < 19.2.21 → fixed in 19.2.21

An Open Redirect vulnerability exists in the internal URL processing logic in Angular SSR. The logic normalizes URL segments by stripping leading slashes; however, it only removes a single leading slash. When an Angular SSR application is deployed behind a proxy that passes the `X-Forwarded-Prefix` header, an attacker can provide a value starting with three slashes (e.g., `///evil.com`). 1. The application processes a redirect (e.g., from a router `redirectTo` or i18n locale switch). 2. Angular receives `///evil.com` as the prefix. 3. It strips one slash, leaving `//evil.com`. 4. The resulting string is used in the `Location` header. 5. Modern browsers interpret `//` as a protocol-relative URL, redirecting the user from `https://your-app.com` to `https://evil.com`. Impact This vulnerability allows attackers to conduct large-scale phishing and SEO hijacking: • Scale: A single request can poison a high-traffic route, impacting all users until the cache expires. • SEO Poisoning: Search engine crawlers may follow and index these malicious redirects, causing the legitimate site to be delisted or associated with malicious domains. • Trust: Because the initial URL belongs to the trusted domain, users and security tools are less likely to flag the redirect as malicious. Attack Preconditions • The application must use Angular SSR. • The application must have routes that perform internal redirects. • The infrastructure (Reverse Proxy/CDN) must pass the `X-Forwarded-Prefix` header to the SSR process without sanitization. • The cache must not vary on the `X-Forwarded-Prefix` header. Patches • 21.2.0-rc.1 • 21.1.5 • 20.3.17 • 19.2.21 Workarounds Until the patch is applied, developers should sanitize the `X-Forwarded-Prefix` header in their`server.ts` before the Angular engine processes the request: ```ts app.use((req, res, next) => { const prefix = req.headers['x-forwarded-prefix']?.trim(); if (prefix) { // Sanitize by removing all leading slashes req.headers['x-forwarded-prefix'] = prefix.replace(/^[/\\]+/, '/'); } next(); }); ``` Resources • Report • Fix