CVE-2026-33397: Protocol-Relative URL Injection via Single Backslash Bypass in 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).