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FuzzRex

3 min read

Fuzzing Framework for APIs and Config Files

What?

Fuzzrex is a Python-based fuzzing framework that combines API schema fuzzing with configuration file testing into a single, cohesive package. Whether you’re a developer conducting security audits or a security professional assessing API endpoints, Fuzzrex provides the tools you need to discover unexpected behaviors and potential vulnerabilities.

Core Features

1. Dynamic API Fuzzing

Fuzzrex parses OpenAPI specifications and generates fuzzed requests to test your API endpoints. It monitors responses for errors, unexpected behaviors, and potential security issues.

from api_fuzzer import ApiFuzzer
 
fuzzer = ApiFuzzer('openapi.json')
fuzzer.run()

The fuzzer automatically generates test data based on parameter types defined in your OpenAPI spec:

  • Strings: Randomly generated with varied lengths (1-100 characters)
  • Integers: Values outside the defined minimum/maximum range to test boundary conditions
  • Booleans: Random True/False values
  • Arrays: Randomly sized arrays with fuzzed elements
  • Objects: Recursively generated nested objects

Here’s how the fuzzing logic works in api_fuzzer.py:

def create_fuzzed_data(self, parameters):
    fuzzed_data = {}
    for param in parameters:
        param_name = param['name']
        param_type = param['schema']['type']
 
        if param_type == 'string':
            fuzzed_data[param_name] = self.fuzz_string(param)
        elif param_type == 'integer':
            fuzzed_data[param_name] = self.fuzz_integer(param)
        elif param_type == 'boolean':
            fuzzed_data[param_name] = random.choice([True, False])
        # ... handling for arrays and objects
 
    return fuzzed_data
 
def fuzz_integer(self, param):
    min_value = param.get('minimum', -1000)
    max_value = param.get('maximum', 1000)
    return random.randint(min_value - 10, max_value + 10)

Developer Note: We intentionally generate values slightly outside the defined ranges to catch boundary-related vulnerabilities that might not be apparent with valid input.

2. Configuration File Fuzz Testing

Fuzzrex supports multiple configuration file formats including JSON, YAML, and XML. It generates malformed configurations and tests how your application handles them, logging any issues found and providing recommendations.

3. Authentication Support

Security-aware fuzzing requires proper authentication. Fuzzrex supports multiple authentication methods:

from auth_handler import AuthHandler
 
# Bearer Token Authentication
auth = AuthHandler(auth_type='token', token='YOUR_API_TOKEN')
 
# OAuth2 Authentication
auth = AuthHandler(auth_type='oauth2', token='YOUR_OAUTH2_CONFIG')
headers = auth.get_auth_headers()

The authentication handler automatically retrieves OAuth2 tokens when needed:

def retrieve_oauth2_token(self):
    response = requests.post(self.oauth2_credentials['token_url'], data={
        'grant_type': 'client_credentials',
        'client_id': self.oauth2_credentials['client_id'],
        'client_secret': self.oauth2_credentials['client_secret'],
        'scope': self.oauth2_credentials.get('scope', '')
    })
    
    if response.status_code == 200:
        token_info = response.json()
        return token_info.get('access_token')

4. State Management

Modern APIs often require maintaining session state between requests. Fuzzrex includes a StateManager that tracks session tokens and user IDs from responses, automatically injecting them into subsequent fuzzed requests:

def update_state(self, response):
    self.last_response = response.json() if response.status_code == 200 else None
    
    if response.status_code == 200:
        if 'sessionToken' in self.last_response:
            self.state['session_token'] = self.last_response['sessionToken']

5. Comprehensive Reporting

Fuzzrex generates detailed reports for both API fuzzing and configuration testing, making it easy to review findings and take action on discovered vulnerabilities.

Fuzzing Architecture

The architecture is built around four key components:

  1. Input Handling: Accepts user-specified OpenAPI files and configuration files
  2. Fuzzing Engine: Generates fuzzed inputs and sends requests
  3. Monitoring Module: Captures responses and application behavior
  4. Reporting Module: Compiles findings into user-friendly reports

Why?

Traditional fuzzers exist in many forms, but we built Fuzzrex to address a specific need: combining API schema fuzzing with configuration file testing in a single, efficient tool. This approach saves time by allowing security assessments to cover both attack surfaces simultaneously.

Fuzzrex is designed not just as a testing tool, but as a collaborative platform for developers and security professionals. The modular architecture makes it easy to extend and customize for your specific needs.

Looking Forward

We’re actively developing additional features, including enhanced configuration fuzzing capabilities and more sophisticated mutation strategies. Stay tuned for updates, and feel free to contribute to the project.

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