hemmelig allows SSRF Filter bypass via Secret Request functionality

Summary

A Server-Side Request Forgery (SSRF) filter bypass vulnerability exists in the webhook URL validation of the Secret Requests feature. The application attempts to block internal/private IP addresses but can be bypassed using DNS rebinding (e.g., localtest.me which resolves to 127.0.0.1) or open redirect services (e.g., httpbin.org/redirect-to). This allows an authenticated user to make the server initiate HTTP requests to internal network resources.

Details

The vulnerability exists in the isPublicUrl function located in /api/lib/utils.ts. The function validates webhook URLs against a blocklist of private IP patterns:

export const isPublicUrl = (url: string): boolean => {
    const parsed = new URL(url);
    const hostname = parsed.hostname.toLowerCase();
    
    const blockedPatterns = [
        /^localhost$/,
        /^127\.\d{1,3}\.\d{1,3}\.\d{1,3}$/,
        /^192\.168\.\d{1,3}\.\d{1,3}$/,
        // ... other patterns
    ];
    
    return !blockedPatterns.some((pattern) => pattern.test(hostname));
};

The validation is flawed because:

1. DNS Rebinding Bypass: It only checks the hostname string, not the resolved IP address. Domains like localtest.me pass validation (not matching any blocked pattern) but resolve to 127.0.0.1.

2. Open Redirect Bypass: External URLs like httpbin.org/redirect-to?url=http://127.0.0.1 pass validation since httpbin.org is a public domain. When the server follows the redirect, it connects to the internal address.

PoC

Optional: On the container that runs Hemmelig application, host a temporary port with the following command:

node -e "require('http').createServer((req,res)=>{console.log(req.method,req.url,req.headers);res.end('ok')}).listen(8080,()=>console.log('Listening on 8080'))"

1. Log in as an user
2. Switch to Secret Requests tab and create a new request
3. When inside the request dialog, there are 2 possible payloads that can be used on the Webhook URL input to bypass SSRF

1. Using domain redirect: http://localtest.me:PORT
2. Using httpbin to perform a redirect: httpbin.org/redirect-to?url=http://127.0.0.1:PORT

4. Open a new browser/tab and confirm the request by creating a secret. Upon clicking save, the port we hosted we receive a request. <img width="795" height="310" alt="image" src="https://github.com/user-attachments/assets/95d559e5-ead2-4b5d-8e53-9ddec3416953" />