GHSA-vfx2-hv2g-xj5f

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