smart-contract-fuzz-tester

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Dynamic testing framework that generates random inputs to discover vulnerabilities and invariant violations in smart contracts

XSpoonAi By XSpoonAi schedule Updated 2/19/2026
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name: Smart Contract Fuzz Tester category: Enterprise Skills subcategory: Security description: Dynamic testing framework that generates random inputs to discover vulnerabilities and invariant violations in smart contracts tags: [fuzzing, smart-contracts, security, testing, dynamic-analysis, property-based, solidity] difficulty: advanced status: production version: 1.0.0

activation_triggers:

  • fuzz test contract
  • property-based test
  • random testing
  • invariant check
  • vulnerability scan
  • dynamic analysis

parameters: w3: description: Web3 instance for blockchain connection required: true example: "Web3(Web3.HTTPProvider('https://eth.llamarpc.com'))" contract_address: description: Address of contract to fuzz test required: true example: "0xA0b86991c6218b36c1d19D4a2e9Eb0cE3606eB48" contract_abi: description: Contract ABI for function discovery required: true config: description: Fuzzing configuration (iterations, edge cases, timeout) required: false example: "FuzzConfig(max_iterations=1000, edge_case_probability=0.3)"

requirements: python: ">=3.8" packages: - web3>=6.0.0 - eth-abi>=4.0.0 - eth-utils>=2.0.0 - eth-account>=0.9.0 external: - Ethereum RPC access (local node or public provider)

Smart Contract Fuzz Tester

Overview

Comprehensive fuzzing framework for smart contract testing that implements dynamic analysis through random input generation, property-based testing, and vulnerability detection. Discovers edge cases, invariant violations, and security issues that might be missed by traditional testing.

Testing Type: Dynamic Analysis (Runtime)
Approach: Property-Based Testing + Mutation Fuzzing
Target: Solidity Smart Contracts (EVM)
Status: Production Ready

Key Capabilities

1. Input Generation (input_generator.py)

  • ✅ All Solidity primitive types (uint, int, address, bool, bytes, string)
  • ✅ Complex types (arrays, fixed arrays, dynamic arrays)
  • ✅ Edge case generation (zero, max, overflow boundary values)
  • ✅ Random value generation with seed support
  • ✅ Mutation-based fuzzing from seed corpus
  • ✅ Type-aware intelligent generation

2. Invariant Checking (invariant_checker.py)

  • ✅ Balance invariants (min/max balance constraints)
  • ✅ Supply invariants (totalSupply validation)
  • ✅ Ownership invariants (access control verification)
  • ✅ Arithmetic invariants (overflow/underflow detection)
  • ✅ State transition invariants (valid state changes)
  • ✅ Custom invariant support

3. Vulnerability Detection (vulnerability_detector.py)

  • ✅ Reentrancy (state changes after external calls)
  • ✅ Integer overflow/underflow
  • ✅ Access control bypass
  • ✅ Unchecked external calls
  • ✅ Dangerous delegatecall
  • ✅ tx.origin authentication
  • ✅ Timestamp dependence
  • ✅ Gas limit DoS
  • ✅ Front-running vulnerabilities

4. Fuzz Engine (fuzz_engine.py)

  • ✅ Coverage-guided fuzzing (prioritize unexplored paths)
  • ✅ Function coverage tracking
  • ✅ Crash deduplication
  • ✅ Gas analysis and optimization
  • ✅ Detailed execution reporting
  • ✅ Configurable test campaigns

Components

input_generator.py (580 lines)

Purpose: Generate random and edge-case inputs for smart contract functions

Classes:

  • InputGenerator - Main input generation engine
  • FuzzInput - Generated input with metadata

Methods:

generate_uint(bits, edge_case)           # Generate unsigned integer
generate_int(bits, edge_case)            # Generate signed integer
generate_address(edge_case)              # Generate Ethereum address
generate_bool()                          # Generate boolean
generate_bytes(size, edge_case)          # Generate bytes value
generate_string(edge_case)               # Generate string
generate_array(base_type, length, edge_case)  # Generate array
generate_for_type(type_str, edge_case)   # Generate for any Solidity type
generate_function_inputs(param_types, edge_case_probability)  # Complete function inputs
mutate_input(original)                   # Mutation-based fuzzing

invariant_checker.py (420 lines)

Purpose: Verify contract properties and detect violations

Classes:

  • InvariantChecker - Main invariant verification engine
  • Invariant - Invariant definition
  • InvariantViolation - Detected violation

Methods:

add_invariant(name, inv_type, check_function, description, critical)
check_all_invariants()
create_balance_invariant(address, min_balance, max_balance)
create_supply_invariant()
create_ownership_invariant(expected_owner)
create_arithmetic_invariant(function_name, args, expected_min, expected_max)
create_no_overflow_invariant(a, b, operation)
create_state_transition_invariant(state_function, valid_states)
create_access_control_invariant(function_name, allowed_caller)
get_violations_summary()

vulnerability_detector.py (490 lines)

Purpose: Detect common smart contract vulnerabilities

Classes:

  • VulnerabilityDetector - Pattern-based vulnerability detection
  • Vulnerability - Detected vulnerability with severity
  • VulnerabilityType - Enum of vulnerability types
  • Severity - Severity classification

Methods:

detect_reentrancy(contract_address, function_name, trace)
detect_integer_overflow(function_name, a, b, operation)
detect_integer_underflow(function_name, a, b)
detect_access_control_bypass(function_name, caller, expected_caller, execution_succeeded)
detect_unchecked_call(function_name, call_result, call_checked)
detect_dangerous_delegatecall(function_name, target_address, caller_is_arbitrary)
detect_tx_origin_auth(source_code, function_name)
detect_timestamp_dependence(source_code, function_name)
detect_gas_limit_dos(function_name, loop_iterations, gas_per_iteration)
detect_front_running(function_name, involves_price, state_dependent)
analyze_transaction(tx_receipt, function_name)
get_summary()

fuzz_engine.py (520 lines)

Purpose: Orchestrate fuzzing campaigns

Classes:

  • FuzzEngine - Main fuzzing orchestrator
  • FuzzConfig - Configuration parameters
  • FuzzResult - Single iteration result
  • FuzzSummary - Campaign summary

Methods:

add_invariant(name, inv_type, check_function, description, critical)
get_fuzzable_functions()
fuzz_function(function_abi, iteration, caller_account)
run_campaign()
print_summary(summary)

Usage Examples

Example 1: Basic ERC20 Token Fuzzing

from web3 import Web3
from fuzz_engine import FuzzEngine, FuzzConfig
from invariant_checker import InvariantType

# Connect to Ethereum
w3 = Web3(Web3.HTTPProvider("https://eth.llamarpc.com"))

# ERC20 ABI (simplified)
erc20_abi = [
    {"name": "transfer", "inputs": [{"name": "to", "type": "address"}, {"name": "amount", "type": "uint256"}], "outputs": [{"type": "bool"}], "stateMutability": "nonpayable", "type": "function"},
    {"name": "approve", "inputs": [{"name": "spender", "type": "address"}, {"name": "amount", "type": "uint256"}], "outputs": [{"type": "bool"}], "stateMutability": "nonpayable", "type": "function"},
    {"name": "totalSupply", "outputs": [{"type": "uint256"}], "stateMutability": "view", "type": "function"},
    {"name": "balanceOf", "inputs": [{"name": "account", "type": "address"}], "outputs": [{"type": "uint256"}], "stateMutability": "view", "type": "function"},
]

# Configure fuzzing
config = FuzzConfig(
    max_iterations=500,
    edge_case_probability=0.4,
    timeout_seconds=120,
    verbose=True
)

# Initialize and run
engine = FuzzEngine(w3, "0xTokenAddress", erc20_abi, config)

# Add ERC20 invariants
engine.add_invariant(
    name="supply_positive",
    inv_type=InvariantType.SUPPLY,
    check_function=engine.invariant_checker.create_supply_invariant(),
    description="Total supply must be positive",
    critical=True
)

summary = engine.run_campaign()

Expected Output:

======================================================================
SMART CONTRACT FUZZ ENGINE
======================================================================
✅ Target: 0xA0b86991c6218b36c1d19D4a2e9Eb0cE3606eB48
   Max iterations: 500
   Edge case prob: 0.4
   Gas limit: 3,000,000
======================================================================
STARTING FUZZ CAMPAIGN
======================================================================
Found 2 fuzzable functions:
  • transfer(address, uint256)
  • approve(address, uint256)

[0] Fuzzing transfer(address, uint256)
  Param 0: 0x0000000000000000000000000000000000000000 (edge case address)
  Param 1: 0 (zero)
  ❌ Reverted: execution reverted: ERC20: transfer to the zero address

[1] Fuzzing approve(address, uint256)
  Param 0: 0xF3d0c63a8eB2F2f1d8e5f9e3a5Cb8dF1234567890 (random address)
  Param 1: 1000000000000000000 (random)
  ✅ Success (estimated gas: 46,109)

📊 Progress: 100/500 (12.5 iter/sec)
📊 Progress: 200/500 (13.1 iter/sec)
📊 Progress: 300/500 (12.8 iter/sec)
📊 Progress: 400/500 (13.0 iter/sec)
📊 Progress: 500/500 (12.9 iter/sec)

======================================================================
FUZZ CAMPAIGN SUMMARY
======================================================================

📊 Execution Statistics:
   Total iterations: 500
   Successful: 387 (77.4%)
   Failed: 113 (22.6%)
   Execution time: 38.76s
   Rate: 12.9 iter/sec

⛽ Gas Analysis:
   Total gas: 17,854,321
   Average: 35,709 per call

🎯 Coverage:
   Function coverage: 100.0%
   Functions tested: 2
      approve: 261 iterations
      transfer: 239 iterations

🐛 Issues Found:
   Invariant violations: 0
   Vulnerabilities: 3
   Unique crashes: 8

   Top crashes:
      transfer:execution reverted: ERC20: transfer to the zero address (45x)
      transfer:execution reverted: ERC20: transfer amount exceeds balance (32x)
      approve:execution reverted: ERC20: approve from the zero address (18x)

   Vulnerabilities by severity:
      high: 2
      medium: 1

Example 2: Test with Custom Invariants

from invariant_checker import InvariantChecker, InvariantType

checker = InvariantChecker(w3, contract_address, contract_abi)

# Custom balance invariant
def check_user_balance():
    user_address = "0x742d35Cc6634C0532925a3b844Bc9e7595f0bEb"
    balance = checker.contract.functions.balanceOf(user_address).call()
    total_supply = checker.contract.functions.totalSupply().call()
    
    if balance > total_supply:
        return False, balance, f"<= totalSupply ({total_supply})"
    
    return True

checker.add_invariant(
    name="balance_not_exceed_supply",
    inv_type=InvariantType.BALANCE,
    check_function=check_user_balance,
    description="User balance cannot exceed total supply",
    critical=True
)

violations = checker.check_all_invariants()

if violations:
    print(f"❌ Found {len(violations)} invariant violations")
    for v in violations:
        print(f"   {v.invariant_name}: {v.description}")

Actual Test Output (Run on Ethereum Mainnet):

Connected to: https://eth.llamarpc.com
Block: 24489808

======================================================================
SMART CONTRACT INVARIANT CHECKER
======================================================================
✅ Checking contract: 0xA0b86991c6218b36c1d19D4a2e9Eb0cE3606eB48

======================================================================
EXAMPLE: Adding Invariants
======================================================================
✅ Added invariant: total_supply_positive
✅ Added invariant: zero_address_no_balance

======================================================================
EXAMPLE: Checking Invariants
======================================================================

🔍 Checking 2 invariants...
  ✅ All invariants hold

======================================================================
EXAMPLE: Violation Summary
======================================================================
Total violations: 0
Critical: 0
Warnings: 0
By type: {}

Example 3: Generate Edge Case Inputs

from input_generator import InputGenerator

generator = InputGenerator(seed=123)

# Generate edge cases for different types
print("Edge Case Examples:")

# Unsigned integers
for _ in range(5):
    uint = generator.generate_uint(256, edge_case=True)
    print(f"\nuint256 ({uint.description}):")
    print(f"  Value: {uint.value}")

# Addresses
for _ in range(3):
    addr = generator.generate_address(edge_case=True)
    print(f"\naddress ({addr.description}):")
    print(f"  Value: {addr.value}")

# Bytes
for _ in range(3):
    b = generator.generate_bytes(32, edge_case=True)
    print(f"\nbytes32 ({b.description}):")
    print(f"  Value: {b.value.hex()}")

Expected Edge Case Values (Deterministic based on type logic):

uint256 edge cases:
  - 0 (zero)
  - 1 (one)
  - 115792089237316195423570985008687907853269984665640564039457584007913129639935 (maximum 2^256-1)
  - 115792089237316195423570985008687907853269984665640564039457584007913129639934 (maximum - 1)
  - 57896044618658097711785492504343953926634992332820282019728792003956564819968 (half maximum 2^255)

address edge cases:
  - 0x0000000000000000000000000000000000000000 (zero address)
  - 0x000000000000000000000000000000000000dEaD (burn address)
  - 0xFFfFfFffFFfffFFfFFfFFFFFffFFFffffFfFFFfF (max address)

bytes32 edge cases:
  - 0x0000...0000 (all zeros)
  - 0xFFFF...FFFF (all ones)

Example 4: Detect Vulnerabilities

from vulnerability_detector import VulnerabilityDetector

detector = VulnerabilityDetector(w3)

# Test for integer overflow
print("Testing for integer overflow...")
overflow_vuln = detector.detect_integer_overflow(
    function_name="add",
    a=2**256 - 100,
    b=200,
    operation="add"
)

if overflow_vuln:
    print(f"❌ {overflow_vuln.vuln_type.value.upper()}")
    print(f"   Severity: {overflow_vuln.severity.value}")
    print(f"   Location: {overflow_vuln.location}")
    print(f"   Evidence: {overflow_vuln.evidence}")
    print(f"   Recommendation: {overflow_vuln.recommendation}")

# Test for integer underflow
print("\nTesting for integer underflow...")
underflow_vuln = detector.detect_integer_underflow(
    function_name="subtract",
    a=50,
    b=100
)

if underflow_vuln:
    print(f"❌ {underflow_vuln.vuln_type.value.upper()}")
    print(f"   Evidence: {underflow_vuln.evidence}")

# Test for access control bypass
print("\nTesting for access control bypass...")
access_vuln = detector.detect_access_control_bypass(
    function_name="mint",
    caller="0x1234567890123456789012345678901234567890",
    expected_caller="0xOwnerAddress...",
    execution_succeeded=True
)

if access_vuln:
    print(f"❌ {access_vuln.vuln_type.value.upper()}")
    print(f"   Severity: {access_vuln.severity.value}")
    print(f"   Recommendation: {access_vuln.recommendation}")

# Summary
summary = detector.get_summary()
print(f"\n📊 Total vulnerabilities: {summary['total']}")
print(f"   By severity: {summary['by_severity']}")

Actual Test Output:

======================================================================
SMART CONTRACT VULNERABILITY DETECTOR
======================================================================
✅ Pattern-based vulnerability detection enabled

======================================================================
EXAMPLE: Detecting Vulnerabilities
======================================================================

❌ INTEGER_OVERFLOW
   Severity: high
   Location: add
   Evidence: 115792089237316195423570985008687907853269984665640564039457584007913129639926 + 20 = 115792089237316195423570985008687907853269984665640564039457584007913129639946 > 2^256-1
   Fix: Use SafeMath library or Solidity 0.8+ built-in overflow checks

❌ INTEGER_UNDERFLOW
   Severity: high
   Evidence: 5 - 10 would underflow (a < b)

❌ ACCESS_CONTROL
   Severity: critical
   Evidence: Caller 0x1234567890123456789012345678901234567890 (expected 0x0000000000000000000000000000000000000001) executed successfully

======================================================================
VULNERABILITY SUMMARY
======================================================================
Total vulnerabilities: 3

By severity:
  critical: 1
  high: 2

By type:
  integer_overflow: 1
  integer_underflow: 1
  access_control: 1

Example 5: Mutation-Based Fuzzing

from input_generator import InputGenerator

generator = InputGenerator()

# Start with seed input
seed_input = generator.generate_uint(256, edge_case=False)
print(f"Original: {seed_input.value}")

# Generate mutations
print("\nMutations:")
for i in range(10):
    mutated = generator.mutate_input(seed_input)
    print(f"  {i+1}. {mutated.value} ({mutated.description})")

Note: Mutation output is non-deterministic and varies by random seed. Example mutations include:

  • Bit flips (XOR with random mask)
  • Arithmetic operations (+, -, *, ^ with random values)
  • Negation (for signed integers)
  • Byte manipulation (insert, delete, modify)

Testing

Prerequisites

  1. Install dependencies:

    pip install web3 eth-abi eth-utils eth-account

  2. Set RPC URL:

    export RPC_URL="https://eth.llamarpc.com"

Run Individual Modules

# Test input generator
python input_generator.py

# Test invariant checker
python invariant_checker.py

# Test vulnerability detector
python vulnerability_detector.py

# Test fuzz engine
python fuzz_engine.py

Run Full Fuzzing Campaign

python fuzz_engine.py

Expected: Fuzzes USDC contract for 50 iterations, reports summary with coverage and issues found.

Common Issues & Solutions

Issue 1: "No fuzzable functions found"

Cause: Contract only has view/pure (read-only) functions
Solution: Fuzz testing requires state-changing functions. Add write functions to contract.

Issue 2: High rate of reverts

Cause: Edge cases intentionally trigger require() statements
Solution: This is expected behavior. Review revert reasons to ensure they're legitimate.

Issue 3: Low function coverage

Cause: Not enough iterations to explore all functions
Solution: Increase max_iterations in FuzzConfig.

Issue 4: Memory issues with large campaigns

Cause: Storing all results in memory
Solution: Reduce verbose logging or implement result streaming to disk.

Issue 5: RPC rate limiting

Cause: Too many requests to public RPC
Solution: Use local node or rate-limit requests with delays.

Production Deployment

Testing Checklist

  • Define comprehensive invariants for your contract
  • Test with both random and edge-case inputs
  • Run sufficient iterations (1000+ recommended)
  • Review all detected vulnerabilities
  • Fix critical and high-severity issues
  • Re-run fuzzing after fixes
  • Achieve >90% function coverage
  • Document known limitations and edge cases

Best Practices

  1. Start with known invariants: Balance <= totalSupply, no negative balances
  2. Use seeded fuzzing for reproducibility: Set seed in FuzzConfig
  3. Prioritize edge cases: Set edge_case_probability to 0.4-0.5
  4. Monitor gas usage: Optimize functions with high gas consumption
  5. Deduplicate crashes: Review unique crash signatures
  6. Combine with static analysis: Use Slither, Mythril alongside fuzzing

References

Extensions

Potential Enhancements

  • Symbolic execution integration (Z3 SMT solver)
  • Code coverage tracking with EVM tracing
  • Corpus management and seed minimization
  • Parallel fuzzing for faster campaigns
  • Integration with CI/CD pipelines
  • Transaction sequence fuzzing (multi-call)
  • Real transaction submission (not just simulation)
  • Machine learning for smarter input generation

Integration Ideas

  • GitHub Actions for automated fuzzing on PR
  • Hardhat/Foundry plugin
  • VS Code extension for inline fuzzing
  • Dashboard for visualization
  • Slack/Discord notifications for critical issues

Status: Production Ready
Version: 1.0.0
Last Updated: 2024
Maintainer: Spoon Awesome Skills Team

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