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: cargo install --git https://github.com/crytic/caracal --profile release --force (a Rust tool — not on PyPI)
• 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 ===

6. 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.

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.

7. 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

8. 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**:
```rust
// 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:
```rust
#[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`

9. 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)

10. 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: |
    # Rebuilds from source each run; cache ~/.cargo or pin a release binary instead.
    cargo install --git https://github.com/crytic/caracal --profile release --force
    caracal detect src/ --fail-on high,critical

11. 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

12. 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