--- name: cairo-vulnerability-scanner description: Scans Cairo/StarkNet smart contracts for 6 critical vulnerabilities including felt252 arithmetic overflow, L1-L2 messaging issues, address conversion problems, and signature replay. Use when auditing StarkNet projects. ---

# 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 ```rust // 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

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## 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

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## 5. Example Output

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

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

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## 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](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).

## 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 ```bash # 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 ```bash # 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 ```

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## 8. Reporting Format

### Finding Template ````markdown ## [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` ````

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## 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)

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## 10. Testing Recommendations

### Unit Tests ```rust #[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) ```rust // 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 ```yaml # .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 ```

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## 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

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## 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