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.
npxskills add trailofbits/skills--skill cairo-vulnerability-scannerLoading…
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.
.cairo// 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
src/contract.cairo - Main contract implementationsrc/lib.cairo - Library modulestests/ - Contract testsScarb.toml - Cairo project configurationpip install caracalcaracal detect src/When invoked, I will:
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
```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
For each #[l1_handler] function:
from_address parameterFor signature-based functions:
If contract includes bridge functionality:
# 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
## [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:
starknetCore.sendMessageToL2(l2Contract, selector, [attacker_address, 1000000])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:
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
}
}
// 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
}
# .github/workflows/security.yml
- name: Run Caracal
run: |
pip install caracal
caracal detect src/ --fail-on high,critical
building-secure-contracts/not-so-smart-contracts/cairo/Before completing Cairo/StarkNet audit:
Arithmetic Safety (HIGH):
L1 Handler Security (CRITICAL):
#[l1_handler] functions validate from_addressL1-L2 Messaging (HIGH):
Signature Security (HIGH):
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