cairo-vulnerability-scanner

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

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, above them a coverage table carrying a verdict for every pattern
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:

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. Felt252 Arithmetic Overflow/Underflow ⚠️ HIGH - felt252 wraps silently; use u128/u256
2. L1 to L2 Address Conversion ⚠️ HIGH - L1 address not validated against STARKNET_FIELD_PRIME
3. L1 to L2 Message Failure ⚠️ HIGH - No cancellation path when a message cannot be consumed
4. Overconstrained L1 <-> L2 Interaction ⚠️ MEDIUM - Coupling that can strand funds or block progress
5. Signature Replay Protection ⚠️ HIGH - No nonce, or a domain separator missing chain/contract
6. Unchecked from_address in L1 Handler ⚠️ CRITICAL - Any L1 contract can drive the handler

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

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

8. Reporting Format

Coverage Table

Report on every pattern in §6, whether or not it turned anything up. Emit this table above the findings, with all 6 rows present:

Each verdict is one of:

• found — cite file:line and write the finding up in full below.
• clear — the pattern applies to this contract and the contract handles it. Name the function, trait, or check you searched for, so a reader can repeat the search.
• n/a — the pattern cannot apply here. Give the reason in one clause ("no L1 handlers in this contract"). Not having looked is not n/a.

A table with fewer than 6 rows is an incomplete scan and must be reported as one. A row whose Verdict cell is empty is incomplete in the same way: row 1 above is filled in to show the shape, and every row is filled in the same way before the report is done. Six clear verdicts is a result a reader can act on. A report that covers two patterns and says nothing about the other four reads exactly like a clean contract, and that is the failure this table exists to prevent.

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

Integration Tests (with L1)

Caracal CI Integration

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
• Coverage table emitted with all 6 rows, each carrying a verdict of found, clear or n/a with a reason

13. Rationalizations to Reject

• "The contract is small, so most patterns obviously don't apply." Obvious to whom? An n/a costs one clause and makes the judgment reviewable. Silence records nothing, and a reader cannot tell it apart from not having checked.
• "Caracal reported nothing, so the contract is clean." Caracal covers a subset of these 6 patterns and does not reach the logic-level ones at all. A clean tool run is one row of evidence, not a verdict on the patterns it never examined. Say which patterns it covered.
• "I checked the patterns that matter for this contract." Deciding which patterns matter is the scan, not a precondition for starting it. Rank by severity after the table is complete, not by leaving rows out.
• "No findings, so there is nothing to report." A zero-finding scan still emits the full coverage table. That table is the deliverable: it is what distinguishes a contract that was examined from one that was glanced at.
• "Cairo 1 has native overflow checks, so arithmetic is safe." Name the types. felt252 does not behave like the sized integer types, and the boundary between them is where pattern 4 lives.
• "The L1 side validates that." Then cite the L1 contract. A check you believe exists across the bridge is an assumption until you have read it, and unverified from_address is the canonical StarkNet bridge bug.