GHSA-m4w9-gch5-c2g4

### Summary

Versions 0.2.1 and 0.3.0 of `client-certificate-auth` contain an open redirect vulnerability. The middleware unconditionally redirects HTTP requests to HTTPS using the unvalidated `Host` header, allowing an attacker to redirect users to arbitrary domains.

### Vulnerable Code

```javascript // lib/clientCertificateAuth.js (versions 0.2.1, 0.3.0) if (!req.secure && req.header('x-forwarded-proto') != 'https') { return res.redirect('https://' + req.header('host') + req.url); } ```

### Attack Scenario

1. Attacker crafts a link: `http://vulnerable-app.example.com/login` 2. When victim clicks, attacker intercepts and injects header: `Host: attacker.com` 3. Server responds: `302 Found → https://attacker.com/login` 4. Victim is redirected to attacker-controlled site

### Impact

- **Phishing**: Attackers can use trusted domain links to redirect victims to credential-harvesting pages - **OAuth/SSO Token Theft**: In authentication flows, authorization codes or tokens may leak via redirect - **Referer Leakage**: Sensitive URL parameters may be exposed to attacker domains via the Referer header - **Cache Poisoning**: In deployments with shared caches, malicious redirects may be cached and served to other users

### Exploitability

Exploitation requires that HTTP traffic reaches the Node.js application without TLS termination setting `x-forwarded-proto: https`. This condition is uncommon in production deployments behind modern reverse proxies or load balancers, which limits real-world exploitability.

### Fix

The vulnerable redirect behavior has been completely removed in version 1.0.0.

```bash npm install client-certificate-auth@^1.0.0 ```

### Workarounds

If upgrading is not immediately possible:

1. Block HTTP traffic at the network/load balancer level 2. Ensure your reverse proxy always sets `x-forwarded-proto: https` 3. Add middleware before `clientCertificateAuth` to validate the `Host` header against an allowlist

### References

- [CWE-601: URL Redirection to Untrusted Site](https://cwe.mitre.org/data/definitions/601.html) - [OWASP: Unvalidated Redirects and Forwards](https://cheatsheetseries.owasp.org/cheatsheets/Unvalidated_Redirects_and_Forwards_Cheat_Sheet.html) - [Fix Commit](https://github.com/tgies/client-certificate-auth/commit/8fc995e953db483495be46862965e50fe9e1cc52)