CVE-2026-33397: Angular SSR Vulnerable to Protocol-Relative URL Injection via Single Backslash Bypass

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:

ts 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

- Fix: https://github.com/angular/angular-cli/pull/32771 - Original CVE: CVE-2026-27738