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
- 22.0.0-next.2
- 21.2.3
- 20.3.21
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:
app.use((req, res, next) => {
const prefix = req.headers['x-forwarded-prefix'];
if (typeof prefix === 'string') {
// Sanitize by removing all leading forward and backward slashes
req.headers['x-forwarded-prefix'] = prefix.trim().replace(/^[/\\]+/, '/');
}
next();
});
References
NVD description
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.