Angular SSR Vulnerable to Protocol-Relative URL Injection via Single Backslash Bypass

Summary

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.

Timeline

1. Mar 26, 2026, 1:46 PM UTCCVE published
2. Mar 30, 2026, 1:54 PM UTCCISA SSVC assessment

   Exploitation: none · Automatable: yes · Technical impact: partial

3. Mar 30, 2026, 2:56 PM UTCRecord last updated

Affected versions

• angular angular-cli

  From 22.0.0-next.0 · fixed in 22.0.0-next.2; From 21.0.0-next.0 · fixed in 21.2.3; From 20.0.0-next.0 · fixed in 20.3.21

• @angular/ssrnpm

  From 22.0.0-next.0 · fixed in 22.0.0-next.2

• @angular/ssrnpm

  From 21.0.0-next.0 · fixed in 21.2.3

• @angular/ssrnpm

  From 20.0.0-next.0 · fixed in 20.3.21

References 3