The Angular SSR is a server-rise rendering tool for Angular applications. Versions on the 22.x branch prior to 22.0.0-next.2, the 21.x branch prior to 21.2.3, and the 20.x branch prior to 20.3.21 have an Open Redirect vulnerability in `@angular/ssr` due to an incomplete fix for CVE-2026-27738. While the original fix successfully blocked multiple leading slashes (e.g., `///`), the internal validation logic fails to account for a single backslash (`\`) bypass. When an Angular SSR application is deployed behind a proxy that passes the `X-Forwarded-Prefix` header, an attacker provides a value starting with a single backslash, the internal validation failed to flag the single backslash as invalid, the application prepends a leading forward slash, resulting in a `Location` header containing the URL, and modern browsers interpret the `/\` sequence as `//`, treating it as a protocol-relative URL and redirecting the user to the attacker-controlled domain. Furthermore, the response lacks the `Vary: X-Forwarded-Prefix` header, allowing the malicious redirect to be stored in intermediate caches (Web Cache Poisoning). Versions 22.0.0-next.2, 21.2.3, and 20.3.21 contain a patch. Until the patch is applied, developers should sanitize the `X-Forwarded-Prefix` header in their `server.ts` before the Angular engine processes the request.

Affected: npm: @angular/ssr

An Open Redirect vulnerability exists in `@angular/ssr` due to an incomplete fix for CVE-2026-27738. While the original fix successfully blocked multiple leading slashes (e.g., `///`), the internal validation logic fails to account for a single backslash (`\`) bypass.

When an Angular SSR application is deployed behind a proxy that passes the `X-Forwarded-Prefix` header:

- An attacker provides a value starting with a single backslash (e.g., `\evil.com`). - The internal validation failed to flag the single backslash as invalid. - The application prepends a leading forward slash, resulting in a `Location` header containing `/\evil.com`. - Modern browsers interpret the `/\` sequence as `//`, treating it as a protocol-relative URL and redirecting the user to the attacker-controlled domain.

Furthermore, the response lacks the `Vary: X-Forwarded-Prefix` header, allowing the malicious redirect to be stored in intermediate caches (Web Cache Poisoning).

### 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.

### Patches…