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.

1. 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
2. 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:

Search your codebase for Cairo files Analyze each contract for the 6 vulnerability patterns Report findings with file references and severity Provide fixes for each identified issue 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.

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.

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

```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

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

1. 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:

Attacker deploys malicious L1 contract Malicious contract calls starknetCore.sendMessageToL2(l2Contract, selector, [attacker_address, 1000000]) L2 handler processes message without checking sender Attacker receives 1,000,000 tokens without depositing any funds 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

```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) // 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

• 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

• 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