Cairo/StarkNet Vulnerability Scanner

1. Purpose

Systematically scan Cairo smart contracts on StarkNet for platform-specific security vulnerabilities related to arithmetic, cross-layer messaging, and cryptographic operations. This skill encodes 6 critical vulnerability patterns unique to Cairo/StarkNet ecosystem.

2. When to Use This Skill

• Auditing StarkNet smart contracts (Cairo)
• Reviewing L1-L2 bridge implementations
• Pre-launch security assessment of StarkNet applications
• Validating cross-layer message handling
• Reviewing signature verification logic
• Assessing L1 handler functions

3. Platform Detection

File Extensions & Indicators

• Cairo files: .cairo

Language/Framework Markers

// Cairo contract indicators
#[contract]
mod MyContract {
    use starknet::ContractAddress;

    #[storage]
    struct Storage {
        balance: LegacyMap<ContractAddress, felt252>,
    }

    #[external(v0)]
    fn transfer(ref self: ContractState, to: ContractAddress, amount: felt252) {
        // Contract logic
    }

    #[l1_handler]
    fn handle_deposit(ref self: ContractState, from_address: felt252, amount: u256) {
        // L1 message handler
    }
}

// Common patterns
felt252, u128, u256
ContractAddress, EthAddress
#[external(v0)], #[l1_handler], #[constructor]
get_caller_address(), get_contract_address()
send_message_to_l1_syscall

Project Structure

• src/contract.cairo - Main contract implementation
• src/lib.cairo - Library modules
• tests/ - Contract tests
• Scarb.toml - Cairo project configuration

Tool Support

• Caracal: Trail of Bits static analyzer for Cairo
• Installation: pip install caracal
• Usage: caracal detect src/
• cairo-test: Built-in testing framework
• Starknet Foundry: Testing and development toolkit

4. How This Skill Works

When invoked, I will:

1. Search your codebase for Cairo files
2. Analyze each contract for the 6 vulnerability patterns
3. Report findings with file references and severity
4. Provide fixes for each identified issue
5. Check L1-L2 interactions for messaging vulnerabilities

5. Example Output

When vulnerabilities are found, you'll get a report like this:

=== CAIRO/STARKNET VULNERABILITY SCAN RESULTS ===


---

## 5. Vulnerability Patterns (6 Patterns)

I check for 6 critical vulnerability patterns unique to Cairo/Starknet. For detailed detection patterns, code examples, mitigations, and testing strategies, see [VULNERABILITY_PATTERNS.md](resources/VULNERABILITY_PATTERNS.md).

### Pattern Summary:

1. **Unchecked Arithmetic** ⚠️ CRITICAL - Integer overflow/underflow in felt252
2. **Storage Collision** ⚠️ CRITICAL - Conflicting storage variable hashes
3. **Missing Access Control** ⚠️ CRITICAL - No caller validation on sensitive functions
4. **Improper Felt252 Boundaries** ⚠️ HIGH - Not validating felt252 range
5. **Unvalidated Contract Address** ⚠️ HIGH - Using untrusted contract addresses
6. **Missing Caller Validation** ⚠️ CRITICAL - No get_caller_address() checks

For complete vulnerability patterns with code examples, see [VULNERABILITY_PATTERNS.md](resources/VULNERABILITY_PATTERNS.md).
## 5. Scanning Workflow

### Step 1: Platform Identification
1. Verify Cairo language and StarkNet framework
2. Check Cairo version (Cairo 1.0+ vs legacy Cairo 0)
3. Locate contract files (`src/*.cairo`)
4. Identify L1-L2 bridge contracts (if applicable)

### Step 2: Arithmetic Safety Sweep

Find felt252 usage in arithmetic

rg "felt252" src/ | rg "[-+*/]"

Find balance/amount storage using felt252

rg "felt252" src/ | rg "balance|amount|total|supply"

Should prefer u128, u256 instead

### Step 3: L1 Handler Analysis
For each `#[l1_handler]` function:
- [ ] Validates `from_address` parameter
- [ ] Checks address != zero
- [ ] Has proper access control
- [ ] Emits events for monitoring

### Step 4: Signature Verification Review
For signature-based functions:
- [ ] Includes nonce tracking
- [ ] Nonce incremented after use
- [ ] Domain separator includes chain ID and contract address
- [ ] Cannot replay signatures

### Step 5: L1-L2 Bridge Audit
If contract includes bridge functionality:
- [ ] L1 validates address < STARKNET_FIELD_PRIME
- [ ] L1 implements message cancellation
- [ ] L2 validates from_address in handlers
- [ ] Symmetric access controls L1 ↔ L2
- [ ] Test full roundtrip flows

### Step 6: Static Analysis with Caracal

Run Caracal detectors

caracal detect src/

Specific detectors

caracal detect src/ --detectors unchecked-felt252-arithmetic caracal detect src/ --detectors unchecked-l1-handler-from caracal detect src/ --detectors missing-nonce-validation

---

## 6. Reporting Format

### Finding Template

[CRITICAL] Unchecked from_address in L1 Handler

Location: src/bridge.cairo:145-155 (handle_deposit function)

Description: The handle_deposit L1 handler function does not validate the from_address parameter. Any L1 contract can send messages to this function and mint tokens for arbitrary users, bypassing the intended L1 bridge access controls.

Vulnerable Code:

// bridge.cairo, line 145
#[l1_handler]
fn handle_deposit(
    ref self: ContractState,
    from_address: felt252,  // Not validated!
    user: ContractAddress,
    amount: u256
) {
    let current_balance = self.balances.read(user);
    self.balances.write(user, current_balance + amount);
}

Attack Scenario:

1. Attacker deploys malicious L1 contract
2. Malicious contract calls starknetCore.sendMessageToL2(l2Contract, selector, [attacker_address, 1000000])
3. L2 handler processes message without checking sender
4. Attacker receives 1,000,000 tokens without depositing any funds
5. Protocol suffers infinite mint vulnerability

Recommendation: Validate from_address against authorized L1 bridge:

#[l1_handler]
fn handle_deposit(
    ref self: ContractState,
    from_address: felt252,
    user: ContractAddress,
    amount: u256
) {
    // Validate L1 sender
    let authorized_l1_bridge = self.l1_bridge_address.read();
    assert(from_address == authorized_l1_bridge, 'Unauthorized L1 sender');

    let current_balance = self.balances.read(user);
    self.balances.write(user, current_balance + amount);
}

References:

• building-secure-contracts/not-so-smart-contracts/cairo/unchecked_l1_handler_from
• Caracal detector: unchecked-l1-handler-from

---

## 7. Priority Guidelines

### Critical (Immediate Fix Required)
- Unchecked from_address in L1 handlers (infinite mint)
- L1-L2 address conversion issues (funds to zero address)

### High (Fix Before Deployment)
- Felt252 arithmetic overflow/underflow (balance manipulation)
- Missing signature replay protection (replay attacks)
- L1-L2 message failure without cancellation (locked funds)

### Medium (Address in Audit)
- Overconstrained L1-L2 interactions (trapped funds)

---

## 8. Testing Recommendations

### Unit Tests

#[cfg(test)] mod tests { use super::*;

#[test] fn test_felt252_overflow() { // Test arithmetic edge cases }

#[test] #[should_panic] fn test_unauthorized_l1_handler() { // Wrong from_address should fail }

#[test] fn test_signature_replay_protection() { // Same signature twice should fail } }

### Integration Tests (with L1)

// Test full L1-L2 flow #[test] fn test_deposit_withdraw_roundtrip() { // 1. Deposit on L1 // 2. Wait for L2 processing // 3. Verify L2 balance // 4. Withdraw to L1 // 5. Verify L1 balance restored }

### Caracal CI Integration

.github/workflows/security.yml

• name: Run Caracal

run: | pip install caracal caracal detect src/ --fail-on high,critical

---

## 9. Additional Resources

- **Building Secure Contracts**: `building-secure-contracts/not-so-smart-contracts/cairo/`
- **Caracal**: https://github.com/crytic/caracal
- **Cairo Documentation**: https://book.cairo-lang.org/
- **StarkNet Documentation**: https://docs.starknet.io/
- **OpenZeppelin Cairo Contracts**: https://github.com/OpenZeppelin/cairo-contracts

---

## 10. Quick Reference Checklist

Before completing Cairo/StarkNet audit:

**Arithmetic Safety (HIGH)**:
- [ ] No felt252 used for balances/amounts (use u128/u256)
- [ ] OR felt252 arithmetic has explicit bounds checking
- [ ] Overflow/underflow scenarios tested

**L1 Handler Security (CRITICAL)**:
- [ ] ALL `#[l1_handler]` functions validate `from_address`
- [ ] from_address compared against stored L1 contract address
- [ ] Cannot bypass by deploying alternate L1 contract

**L1-L2 Messaging (HIGH)**:
- [ ] L1 bridge validates addresses < STARKNET_FIELD_PRIME
- [ ] L1 bridge implements message cancellation
- [ ] L2 handlers check from_address
- [ ] Symmetric validation rules L1 ↔ L2
- [ ] Full roundtrip flows tested

**Signature Security (HIGH)**:
- [ ] Signatures include nonce tracking
- [ ] Nonce incremented after each use
- [ ] Domain separator includes chain ID and contract address
- [ ] Signature replay tested and prevented
- [ ] Cross-chain replay prevented

**Tool Usage**:
- [ ] Caracal scan completed with no critical findings
- [ ] Unit tests cover all vulnerability scenarios
- [ ] Integration tests verify L1-L2 flows
- [ ] Testnet deployment tested before mainnet