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Introduction:
In the world of bug bounty hunting, the most critical vulnerabilities are not always found deep within an application’s logic; sometimes, they are sitting in plain sight. A recent finding by Omar Abdelsalam on the YesWeHack platform highlights a common yet overlooked misconfiguration: the exposure of internal API tokens and infrastructure IDs directly in HTTP headers. While rated as a Medium severity (CVSS 5.3), this type of information disclosure can act as a gateway for attackers to pivot deeper into internal networks, escalate privileges, or bypass authentication controls.
Learning Objectives:
- Understand how to identify and exploit information disclosure vulnerabilities in HTTP headers.
- Learn the step‑by‑step methodology for detecting leaked internal tokens and infrastructure IDs.
- Master mitigation techniques to prevent header-based leaks in production environments.
You Should Know:
1. What Got Leaked? Anatomy of the Vulnerability
The report submitted by Omar Abdelsalam identified a critical lapse where an internal API token and infrastructure identifiers were embedded within HTTP response headers. This is a classic case of “Information Exposure Through Sent Data” (CWE-201). When applications or web servers are misconfigured, they may include debugging data, internal paths, or authentication tokens in headers like X-API-Key, X-Internal-ID, or custom `Server` headers intended only for internal debugging.
Step‑by‑step guide to identifying such leaks:
To check for similar vulnerabilities in your own applications or during a pentest, you can use `curl` to inspect headers verbosely:
curl -I -X GET https://target.com/api/endpoint
Or, to see both request and response headers:
curl -v https://target.com/api/endpoint
Look for suspicious headers such as:
– `X-API-Token:`
– `X-Internal-Token:`
– `X-Debug-:`
– Any header containing ID, key, token, or internal.
On Windows, you can use PowerShell:
Invoke-WebRequest -Uri https://target.com/api/endpoint -Method Get | Select-Object -ExpandProperty Headers
2. Manual Testing for Header Leaks
Beyond automated tools, manual inspection is key. In a browser, open Developer Tools (F12), go to the Network tab, and reload the page. Click on any request and inspect the “Response Headers” section.
Step‑by‑step guide to manual inspection:
1. Open Chrome DevTools (Ctrl+Shift+I).
2. Navigate to the “Network” tab.
- Refresh the page and select a resource (e.g., the main document or an API call).
4. Scroll through the “Response Headers” pane.
- Look for custom headers that seem out of place—especially those prefixed with `X-` or containing internal nomenclature.
3. Exploiting the Leaked Information
Once an internal API token is obtained, an attacker can attempt to use it against other endpoints. The token might grant access to administrative functions, sensitive data, or internal microservices.
Step‑by‑step guide to exploitation:
Assuming you found a header like `X-Internal-Auth: abc123xyz`:
1. Test the token on other subdomains:
curl -H "Authorization: Bearer abc123xyz" https://api.internal.target.com/admin
2. Use the token to access internal infrastructure IDs to map the network:
curl -H "X-Internal-ID: 456" https://target.com/internal/hosts
3. If the token is a JWT, decode it using `jq` or `jwt.io` to check for weak secrets or excessive permissions:
echo 'eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9...' | jwt decode -
4. Mitigation: Secure Header Configuration
To prevent such leaks, developers and DevOps teams must adopt secure coding practices and server hardening.
Step‑by‑step guide to mitigating header leaks:
- For Nginx: Remove unnecessary headers and hide the server version:
server_tokens off; add_header X-Frame-Options "SAMEORIGIN" always; add_header X-Content-Type-Options "nosniff" always; proxy_hide_header X-Powered-By; proxy_hide_header X-API-Key;
- For Apache:
ServerTokens Prod ServerSignature Off Header unset X-Powered-By Header unset X-API-Key
- For Node.js/Express: Use `helmet` middleware to set secure headers and remove leaking ones:
const helmet = require('helmet'); app.use(helmet()); app.disable('x-powered-by'); - For ASP.NET:
<system.web> <httpRuntime enableVersionHeader="false" /> </system.web> <system.webServer> <httpProtocol> <customHeaders> <remove name="X-Powered-By" /> </customHeaders> </httpProtocol> </system.webServer>
5. Advanced Detection with Burp Suite
For continuous monitoring, use Burp Suite’s passive scanner or write custom extensions to detect header leaks.
Step‑by‑step guide to using Burp Suite:
1. Set your browser to proxy through Burp.
2. Navigate through the target application.
- In the “Target” tab, select a host and right-click to “Engagement tools” → “Discover content”.
- Review the “HTTP History” and filter by response headers containing “token” or “internal”.
- Use the “Logger++” extension to create a regex rule for `X-.Token|Internal.ID` to highlight potential leaks.
What Undercode Say:
- Key Takeaway 1: Information disclosure vulnerabilities, even with a Medium CVSS score, are often the first step in a chain that leads to a critical breach. Never underestimate the value of a leaked token or internal ID.
- Key Takeaway 2: Defense in depth applies to headers as well. Remove all debugging headers, custom tokens, and internal identifiers from responses before they reach the client. Use a Web Application Firewall (WAF) to mask or filter outbound headers.
In today’s interconnected cloud environments, a single exposed token in an HTTP header can be the equivalent of leaving the master key under the doormat. Organizations must treat their production servers as if they are already under scrutiny by attackers—because, in the bug bounty ecosystem, they are.
Prediction:
As API-driven architectures continue to dominate the web, we will see a rise in automated scanning tools specifically designed to scrape and analyze HTTP headers for internal tokens. In the next 12 months, expect AI-powered bug bounty bots to crawl thousands of endpoints simultaneously, looking for these low‑hanging fruits, turning Medium-severity leaks into high-volume, automated attack vectors.
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IT/Security Reporter URL:
Reported By: Omar Abdelsalam – Hackers Feeds
Extra Hub: Undercode MoN
Basic Verification: Pass ✅
