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Introduction:
Bug bounty programs like Yandex’s reward security researchers for finding vulnerabilities in web applications, APIs, and cloud infrastructure. A single overlooked endpoint can lead to data leaks or account takeover, as demonstrated by Aditya Singh’s recent Yandex bounty. This article dissects the technical patterns behind such discoveries, providing hands-on commands and configurations to help you identify API misconfigurations, exploit them ethically, and harden your own systems against similar attacks.
Learning Objectives:
- Identify common API vulnerabilities (IDOR, mass assignment, broken object level authorization) using automated and manual techniques.
- Execute Linux and Windows commands to probe endpoints, analyze responses, and craft proof-of-concept exploits.
- Implement cloud hardening measures and mitigation strategies based on real-world Yandex bug bounty findings.
You Should Know:
1. Enumerating Hidden Endpoints with ParamSpider and FFUF
Many bounties start with discovering undocumented API endpoints. Attackers and researchers use parameter brute-forcing to reveal hidden functionality. Below is a step‑by‑step guide to replicate this process against a target (e.g., api.yandex.com).
Step‑by‑step guide:
1. Install tools on Linux (Ubuntu/Debian):
sudo apt update && sudo apt install python3-pip git git clone https://github.com/devanshbatham/ParamSpider cd ParamSpider pip3 install -r requirements.txt
2. Run ParamSpider to collect potential parameters:
python3 paramspider.py --domain api.yandex.com --output yandex_params.txt
3. Install FFUF (Fuzz Faster U Fool):
sudo apt install ffuf
4. Fuzz for hidden directories and endpoints using a wordlist:
ffuf -u https://api.yandex.com/FUZZ -w /usr/share/wordlists/dirb/common.txt -fc 404
5. Analyze responses: Look for HTTP 200, 403, or 500 status codes that indicate an accessible but potentially misconfigured endpoint.
Windows alternative (using PowerShell and Go):
Install Go, then ffuf git clone https://github.com/ffuf/ffuf cd ffuf go build .\ffuf.exe -u https://api.yandex.com/FUZZ -w .\common.txt -fc 404
- Exploiting IDOR (Insecure Direct Object References) in REST APIs
IDOR is a classic bug that often yields bounties. If an endpoint uses predictable identifiers (e.g.,user_id=123), an attacker can change the ID to access another user’s data. The Yandex bounty likely involved such a flaw.
Step‑by‑step guide:
- Capture a legitimate request using Burp Suite or OWASP ZAP. For example:
GET /api/v1/profile?user_id=456 HTTP/1.1 Host: api.yandex.com Authorization: Bearer <your_token>
- Modify the `user_id` parameter to a different number (e.g., 457, 458, 0, -1, or 999999).
- Send the modified request via `curl` (Linux/macOS) or `Invoke-WebRequest` (PowerShell):
curl -X GET "https://api.yandex.com/api/v1/profile?user_id=457" -H "Authorization: Bearer <token>"
- If you receive another user’s data, you’ve found an IDOR. To automate testing:
for id in {1..1000}; do curl -s "https://api.yandex.com/api/v1/profile?user_id=$id" -H "Authorization: Bearer <token>" | grep -i "email"; done - Mitigation: Implement object-level authorization checks on the server. Never trust client-supplied IDs without verifying ownership.
3. Mass Assignment Vulnerability Exploitation (with JSON Payloads)
Mass assignment occurs when an API automatically binds JSON fields to internal objects, allowing attackers to modify unexpected properties (e.g., "is_admin": true). Yandex’s GraphQL or REST endpoints could be vulnerable.
Step‑by‑step guide:
- Identify a POST/PUT endpoint that updates user settings.
2. Add extra fields to the JSON body:
{
"username": "attacker",
"email": "[email protected]",
"role": "admin",
"is_verified": true
}
3. Send the payload using `curl`:
curl -X PUT https://api.yandex.com/api/v1/user/update -H "Content-Type: application/json" -H "Authorization: Bearer <token>" -d '{"username":"attacker","role":"admin"}'
4. Check the response – if your role changed, the API is vulnerable. To prevent this, use allowlists for permitted fields or explicitly map DTOs.
4. Cloud Hardening for Yandex.Cloud Instances
If you run services on Yandex.Cloud (or any cloud), misconfigured metadata endpoints can leak credentials. Researchers often target `http://169.254.169.254/latest/meta-data/`.
Step‑by‑step guide to harden:
1. Disable unused metadata endpoints using iptables (Linux):
sudo iptables -A OUTPUT -d 169.254.169.254 -j DROP sudo iptables -A INPUT -s 169.254.169.254 -j DROP
2. For Windows Server (via PowerShell as Admin):
New-NetFirewallRule -DisplayName "Block Metadata" -Direction Outbound -RemoteAddress 169.254.169.254 -Action Block New-NetFirewallRule -DisplayName "Block Metadata Inbound" -Direction Inbound -RemoteAddress 169.254.169.254 -Action Block
3. Use IMDSv2 (Instance Metadata Service v2) which requires PUT requests with a session token:
Request token TOKEN=$(curl -X PUT "http://169.254.169.254/latest/api/token" -H "X-aws-ec2-metadata-token-ttl-seconds: 21600") Access metadata with token curl -H "X-aws-ec2-metadata-token: $TOKEN" http://169.254.169.254/latest/meta-data/
4. Regularly audit IAM roles – ensure no instance has overly permissive roles.
- API Security Testing with Postman and Newman (CI/CD Integration)
Integrate automated security tests into your pipeline to catch regressions.
Step‑by‑step guide:
- Export your API collection from Postman as
collection.json. - Write a test script in Postman’s Tests tab:
pm.test("No IDOR - different user data not accessible", function () { pm.expect(pm.response.text()).to.not.include("[email protected]"); }); - Run tests via Newman (command line on Linux/Windows):
npm install -g newman newman run collection.json -e environment.json --reporters cli,json
4. Integrate into GitHub Actions (example `.github/workflows/api-security.yml`):
name: API Security Scan on: [bash] jobs: test: runs-on: ubuntu-latest steps: - uses: actions/checkout@v2 - run: npm install -g newman - run: newman run collection.json
6. Mitigating XSS in Yandex Widgets (DOM-based)
Bug hunters often find cross-site scripting in custom widgets. A reflected XSS on a Yandex subdomain could lead to session hijacking.
Step‑by‑step mitigation:
- Set Content Security Policy (CSP) headers on your web server (Apache example):
Header set Content-Security-Policy "default-src 'self'; script-src 'self' https://trusted-cdn.com; object-src 'none'"
2. For Nginx:
add_header Content-Security-Policy "default-src 'self'; script-src 'self' https://trusted-cdn.com;";
3. Sanitize all user inputs – use libraries like DOMPurify on frontend and OWASP Java Encoder on backend.
4. Test with XSS payloads (e.g., <img src=x onerror=alert(1)>) using curl:
curl "https://widgets.yandex.com/search?q=<script>alert(1)</script>"
What Undercode Say:
- API parameter fuzzing and IDOR testing are repeatable skills – every bug hunter should master
ffuf,Burp Intruder, and manual parameter tampering. - Cloud metadata misconfigurations remain a top attack vector – always enforce IMDSv2 and network-level blocks.
- Automation in CI/CD prevents regression of known bugs – integrating Newman or OWASP ZAP API scans saves countless hours.
The Yandex bounty mentioned by Aditya Singh underscores a broader truth: small, seemingly “low‑impact” vulnerabilities (like a single IDOR or mass assignment) often chain together to become critical data breaches. By practicing the commands and configurations above, you can discover similar flaws – and more importantly, learn to fix them before adversaries do.
Prediction:
As AI‑generated code and GraphQL APIs become ubiquitous, mass assignment and introspection‑based attacks will surge. Yandex and other tech giants will increasingly deploy automated DAST (Dynamic Application Security Testing) tools that mimic human bug hunters. However, human creativity in chaining low‑severity bugs will remain irreplaceable. Expect bounty payouts for business‑logic flaws to rise by 40% by 2027, while traditional XSS and SQLi rewards decline due to WAF improvements. The next “small bounty” might just be the one that exposes a forgotten debug endpoint in a cloud‑native mesh – so keep fuzzing.
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IT/Security Reporter URL:
Reported By: Aditya Singh4180 – Hackers Feeds
Extra Hub: Undercode MoN
Basic Verification: Pass ✅
