performing-api-fuzzing-with-restler

Uses Microsoft RESTler to perform stateful REST API fuzzing: compiles an OpenAPI/Swagger spec into a RESTler grammar, configures authentication, and runs test/fuzz-lean/fuzz modes that generate request sequences exercising producer-consumer dependencies, then flags 500 errors, auth bypasses, resource leaks, and injection bugs. Use when fuzzing REST APIs for stateful bugs or running RESTler-based automated API security testing.

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name	performing-api-fuzzing-with-restler
description	Uses Microsoft RESTler to perform stateful REST API fuzzing: compiles an OpenAPI/Swagger spec into a RESTler grammar, configures authentication, and runs test/fuzz-lean/fuzz modes that generate request sequences exercising producer-consumer dependencies, then flags 500 errors, auth bypasses, resource leaks, and injection bugs. Use when fuzzing REST APIs for stateful bugs or running RESTler-based automated API security testing.
domain	cybersecurity
subdomain	api-security
tags	["api-security","fuzzing","restler","automated-testing","openapi","stateful-testing"]
version	1.0.0
author	mahipal
license	Apache-2.0
nist_csf	["PR.PS-01","ID.RA-01","PR.DS-10","DE.CM-01"]
mitre_attack	["T1190","T1059.007","T1552.001","T1055","T1059"]

Performing API Fuzzing with RESTler

When to Use

Performing automated security testing of REST APIs using their OpenAPI/Swagger specifications
Discovering bugs that only manifest through specific sequences of API calls (stateful testing)
Finding 500 Internal Server Error responses that indicate unhandled exceptions or crash conditions
Testing API input validation by fuzzing parameters with malformed, boundary, and injection payloads
Running continuous security regression testing in CI/CD pipelines for API changes

Do not use against production environments without explicit authorization and monitoring. RESTler creates and deletes resources aggressively during fuzzing.

Prerequisites

Written authorization specifying the target API and acceptable testing scope
Python 3.12+ and .NET 8.0 runtime installed
RESTler downloaded from https://github.com/microsoft/restler-fuzzer
OpenAPI/Swagger specification (v2 or v3) for the target API
API authentication credentials (tokens, API keys, or OAuth credentials)
Isolated test/staging environment (RESTler can create thousands of resources per hour)

Workflow

Step 1: RESTler Installation and Setup

# Clone and build RESTler
git clone https://github.com/microsoft/restler-fuzzer.git
cd restler-fuzzer

# Build RESTler
python3 ./build-restler.py --dest_dir /opt/restler

# Verify installation
/opt/restler/restler/Restler --help

# Alternative: Use pre-built release
# Download from https://github.com/microsoft/restler-fuzzer/releases

Step 2: Compile the API Specification

# Compile the OpenAPI spec into a RESTler fuzzing grammar
/opt/restler/restler/Restler compile \
    --api_spec /path/to/openapi.yaml

# Output directory structure:
# Compile/
#   grammar.py          - Generated fuzzing grammar
#   grammar.json        - Grammar in JSON format
#   dict.json           - Custom dictionary for fuzzing values
#   engine_settings.json - Engine configuration

{
    "restler_fuzzable_string": [
        "fuzzstring",
        "' OR '1'='1",
        "\" OR \"1\"=\"1",
        "<script>alert(1)</script>",
        "../../../etc/passwd",
        "${7*7}",
        "{{7*7}}",
        "a]UNION SELECT 1,2,3--",
        "\"; cat /etc/passwd; echo \"",
        "AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA"
    ],
    "restler_fuzzable_int": [
        "0",
        "-1",
        "999999999",
        "2147483647",
        "-2147483648"
    ],
    "restler_fuzzable_bool": ["true", "false", "null", "1", "0"],
    "restler_fuzzable_datetime": [
        "2024-01-01T00:00:00Z",
        "0000-00-00T00:00:00Z",
        "9999-12-31T23:59:59Z",
        "invalid-date"
    ],
    "restler_fuzzable_uuid4": [
        "00000000-0000-0000-0000-000000000000",
        "aaaaaaaa-aaaa-aaaa-aaaa-aaaaaaaaaaaa"
    ],
    "restler_custom_payload": {
        "/users/{userId}": ["1", "0", "-1", "admin", "' OR 1=1--"],
        "/orders/{orderId}": ["1", "0", "999999999"]
    }
}

# authentication_token.py - RESTler authentication module
import requests
import json
import time

class AuthenticationProvider:
    def __init__(self):
        self.token = None
        self.token_expiry = 0
        self.auth_url = "https://target-api.example.com/api/v1/auth/login"
        self.credentials = {
            "email": "fuzzer@test.com",
            "password": "FuzzerPass123!"
        }

    def get_token(self):
        """Get or refresh authentication token."""
        current_time = time.time()
        if self.token and current_time < self.token_expiry - 60:
            return self.token

        resp = requests.post(self.auth_url, json=self.credentials)
        if resp.status_code == 200:
            data = resp.json()
            self.token = data["access_token"]
            self.token_expiry = current_time + 3600  # Assume 1-hour TTL
            return self.token
        else:
            raise Exception(f"Authentication failed: {resp.status_code}")

    def get_auth_header(self):
        """Return the authentication header for RESTler."""
        token = self.get_token()
        return f"Authorization: Bearer {token}"

# Export the token refresh command for RESTler
auth = AuthenticationProvider()
print(auth.get_auth_header())

{
    "authentication": {
        "token": {
            "token_refresh_interval": 300,
            "token_refresh_cmd": "python3 /path/to/authentication_token.py"
        }
    },
    "max_combinations": 20,
    "max_request_execution_time": 30,
    "global_producer_timing_delay": 2,
    "no_ssl": false,
    "host": "target-api.example.com",
    "target_port": 443,
    "garbage_collection_interval": 300,
    "max_sequence_length": 10
}

# Test mode: Quick validation that all endpoints are reachable
/opt/restler/restler/Restler test \
    --grammar_file Compile/grammar.py \
    --dictionary_file Compile/dict.json \
    --settings Compile/engine_settings.json \
    --no_ssl \
    --target_ip target-api.example.com \
    --target_port 443

# Review test results
cat Test/ResponseBuckets/runSummary.json

# Parse test results
import json

with open("Test/ResponseBuckets/runSummary.json") as f:
    summary = json.load(f)

print("Test Mode Summary:")
print(f"  Total requests: {summary.get('total_requests_sent', {}).get('num_requests', 0)}")
print(f"  Successful (2xx): {summary.get('num_fully_valid', 0)}")
print(f"  Client errors (4xx): {summary.get('num_invalid', 0)}")
print(f"  Server errors (5xx): {summary.get('num_server_error', 0)}")

# Identify uncovered endpoints
covered = summary.get('covered_endpoints', [])
total = summary.get('total_endpoints', [])
uncovered = set(total) - set(covered)
if uncovered:
    print(f"\nUncovered endpoints ({len(uncovered)}):")
    for ep in uncovered:
        print(f"  - {ep}")

# Fuzz-lean: One pass through all endpoints with security checkers enabled
/opt/restler/restler/Restler fuzz-lean \
    --grammar_file Compile/grammar.py \
    --dictionary_file Compile/dict.json \
    --settings Compile/engine_settings.json \
    --target_ip target-api.example.com \
    --target_port 443 \
    --time_budget 1  # 1 hour max

# Checkers automatically enabled in fuzz-lean:
# - UseAfterFree: Tests accessing resources after deletion
# - NamespaceRule: Tests accessing resources across namespaces/tenants
# - ResourceHierarchy: Tests child resources with wrong parent IDs
# - LeakageRule: Tests for information disclosure in error responses
# - InvalidDynamicObject: Tests with malformed dynamic object IDs

# Full fuzz mode: Extended fuzzing for comprehensive coverage
/opt/restler/restler/Restler fuzz \
    --grammar_file Compile/grammar.py \
    --dictionary_file Compile/dict.json \
    --settings Compile/engine_settings.json \
    --target_ip target-api.example.com \
    --target_port 443 \
    --time_budget 4 \
    --enable_checkers UseAfterFree NamespaceRule ResourceHierarchy LeakageRule InvalidDynamicObject PayloadBody

# Analyze fuzzing results
python3 <<'EOF'
import json
import os

results_dir = "Fuzz/ResponseBuckets"
bugs_dir = "Fuzz/bug_buckets"

# Parse bug buckets
if os.path.exists(bugs_dir):
    for bug_file in os.listdir(bugs_dir):
        if bug_file.endswith(".txt"):
            with open(os.path.join(bugs_dir, bug_file)) as f:
                content = f.read()
            print(f"\n=== Bug: {bug_file} ===")
            print(content[:500])

# Parse response summary
summary_file = os.path.join(results_dir, "runSummary.json")
if os.path.exists(summary_file):
    with open(summary_file) as f:
        summary = json.load(f)
    print(f"\nFuzz Summary:")
    print(f"  Duration: {summary.get('time_budget_hours', 0)} hours")
    print(f"  Total requests: {summary.get('total_requests_sent', {}).get('num_requests', 0)}")
    print(f"  Bugs found: {summary.get('num_bugs', 0)}")
    print(f"  500 errors: {summary.get('num_server_error', 0)}")
EOF

Term	Definition
Stateful Fuzzing	API fuzzing that maintains state across requests by using responses from earlier requests as inputs to later ones, enabling testing of multi-step workflows
Producer-Consumer Dependencies	RESTler's inference that a value produced by one API call (e.g., a created resource ID) should be consumed by a subsequent call
Fuzzing Grammar	Compiled representation of the API specification that defines how to generate valid and invalid requests for each endpoint
Checker	RESTler security rule that tests for specific vulnerability patterns like use-after-free, namespace isolation, or information leakage
Bug Bucket	RESTler's categorization of discovered bugs by type and endpoint, grouping similar failures for efficient triage
Garbage Collection	RESTler's periodic cleanup of resources created during fuzzing to prevent resource exhaustion on the target system

## RESTler API Fuzzing Report

**Target**: User Service API (staging.example.com)
**Specification**: OpenAPI 3.0 (42 endpoints)
**Duration**: 4 hours (full fuzz mode)
**Total Requests**: 145,832

### Bug Summary

| Category | Count | Severity |
|----------|-------|----------|
| 500 Internal Server Error | 12 | High |
| Use After Free | 3 | Critical |
| Namespace Rule Violation | 5 | Critical |
| Information Leakage | 8 | Medium |
| Resource Leak | 4 | Low |

### Critical Findings

**1. Use-After-Free: Deleted user token still valid**
- Sequence: POST /users -> DELETE /users/{id} -> GET /users/{id}
- After deleting user, GET with the deleted user's token returns 200
- Impact: Deleted accounts can still access the API

**2. Namespace Violation: Cross-tenant data access**
- Sequence: POST /users (tenant A) -> GET /users/{id} (tenant B token)
- User created by tenant A is accessible with tenant B's credentials
- Impact: Multi-tenant isolation breach

**3. 500 Error: Unhandled integer overflow**
- Request: POST /orders {"quantity": 2147483648}
- Response: 500 Internal Server Error with stack trace
- Impact: DoS potential, information disclosure via stack trace

### Coverage

- Endpoints covered: 38/42 (90.5%)
- Uncovered: POST /admin/migrate, DELETE /admin/cache,
  PUT /config/advanced, POST /webhooks/test