Security Audit Skill

Overview

This skill performs systematic security audits of Move contracts using a comprehensive checklist. Every item must pass before deployment.

Critical: Security is non-negotiable. User funds depend on correct implementation.

Core Workflow

Step 1: Run Security Checklist

Review ALL categories in order:

1. Access Control – Who can call functions?
2. Input Validation – Are inputs checked?
3. Object Safety – Object model used correctly?
4. Reference Safety – No dangerous references exposed?
5. Arithmetic Safety – Overflow/underflow prevented?
6. Generic Type Safety – Phantom types used correctly?
7. Testing – 100% coverage achieved?

Step 2: Access Control Audit

Verify:

• All entry functions verify signer authority
• Object ownership checked with object::owner()
• Admin functions check caller is admin
• Function visibility uses least-privilege
• No public functions modify state without checks

Check for:

// ✅ CORRECT: Signer verification
public entry fun update_config(admin: &signer, value: u64) acquires Config {
    let config = borrow_global<Config>(@my_addr);
    assert!(signer::address_of(admin) == config.admin, E_NOT_ADMIN);
    // Safe to proceed
}

// ❌ WRONG: No verification
public entry fun update_config(admin: &signer, value: u64) acquires Config {
    let config = borrow_global_mut<Config>(@my_addr);
    config.value = value; // Anyone can call!
}

For objects:

// ✅ CORRECT: Ownership verification
public entry fun transfer_item(
    owner: &signer,
    item: Object<Item>,
    to: address
) acquires Item {
    assert!(object::owner(item) == signer::address_of(owner), E_NOT_OWNER);
    // Safe to transfer
}

// ❌ WRONG: No ownership check
public entry fun transfer_item(
    owner: &signer,
    item: Object<Item>,
    to: address
) acquires Item {
    // Anyone can transfer any item!
}

Step 3: Input Validation Audit

Verify:

• Numeric inputs checked for zero: assert!(amount > 0, E_ZERO_AMOUNT)
• Numeric inputs within max limits: assert!(amount <= MAX, E_AMOUNT_TOO_HIGH)
• Vector lengths validated: assert!(vector::length(&v) > 0, E_EMPTY_VECTOR)
• String lengths checked: assert!(string::length(&s) <= MAX_LENGTH, E_NAME_TOO_LONG)
• Addresses validated: assert!(addr != @0x0, E_ZERO_ADDRESS)
• Enum-like values in range: assert!(type_id < MAX_TYPES, E_INVALID_TYPE)

Check for:

// ✅ CORRECT: Comprehensive validation
public entry fun deposit(user: &signer, amount: u64) acquires Account {
    assert!(amount > 0, E_ZERO_AMOUNT);
    assert!(amount <= MAX_DEPOSIT_AMOUNT, E_AMOUNT_TOO_HIGH);

    let account = borrow_global_mut<Account>(signer::address_of(user));
    assert!(account.balance <= MAX_U64 - amount, E_OVERFLOW);

    account.balance = account.balance + amount;
}

// ❌ WRONG: No validation
public entry fun deposit(user: &signer, amount: u64) acquires Account {
    let account = borrow_global_mut<Account>(signer::address_of(user));
    account.balance = account.balance + amount; // Can overflow!
}

Step 4: Object Safety Audit

Verify:

• ConstructorRef never returned from public functions
• All refs (TransferRef, DeleteRef, ExtendRef) generated in constructor
• Object signer only used during construction or with ExtendRef
• Ungated transfers disabled unless explicitly needed
• DeleteRef only generated for truly burnable objects

Check for:

// ❌ DANGEROUS: Returning ConstructorRef
public fun create_item(): ConstructorRef {
    let constructor_ref = object::create_object(@my_addr);
    constructor_ref // Caller can destroy object!
}

// ✅ CORRECT: Return Object<T>
public fun create_item(creator: &signer): Object<Item> {
    let constructor_ref = object::create_object(signer::address_of(creator));

    let transfer_ref = object::generate_transfer_ref(&constructor_ref);
    let delete_ref = object::generate_delete_ref(&constructor_ref);
    let object_signer = object::generate_signer(&constructor_ref);

    move_to(&object_signer, Item { transfer_ref, delete_ref });

    object::object_from_constructor_ref<Item>(&constructor_ref)
}

Step 5: Reference Safety Audit

Verify:

• No &mut references exposed in public function signatures
• Critical fields protected from mem::swap
• Mutable borrows minimized in scope

Check for:

// ❌ DANGEROUS: Exposing mutable reference
public fun get_item_mut(item: Object<Item>): &mut Item acquires Item {
    borrow_global_mut<Item>(object::object_address(&item))
    // Caller can mem::swap fields!
}

// ✅ CORRECT: Controlled mutations
public entry fun update_item_name(
    owner: &signer,
    item: Object<Item>,
    new_name: String
) acquires Item {
    assert!(object::owner(item) == signer::address_of(owner), E_NOT_OWNER);

    let item_data = borrow_global_mut<Item>(object::object_address(&item));
    item_data.name = new_name;
}

Step 6: Arithmetic Safety Audit

Verify:

• Additions checked for overflow
• Subtractions checked for underflow
• Division by zero prevented
• Multiplication checked for overflow

Check for:

// ✅ CORRECT: Overflow protection
public entry fun deposit(user: &signer, amount: u64) acquires Account {
    let account = borrow_global_mut<Account>(signer::address_of(user));

    // Check overflow BEFORE adding
    assert!(account.balance <= MAX_U64 - amount, E_OVERFLOW);

    account.balance = account.balance + amount;
}

// ✅ CORRECT: Underflow protection
public entry fun withdraw(user: &signer, amount: u64) acquires Account {
    let account = borrow_global_mut<Account>(signer::address_of(user));

    // Check underflow BEFORE subtracting
    assert!(account.balance >= amount, E_INSUFFICIENT_BALANCE);

    account.balance = account.balance - amount;
}

// ❌ WRONG: No overflow check
public entry fun deposit(user: &signer, amount: u64) acquires Account {
    let account = borrow_global_mut<Account>(signer::address_of(user));
    account.balance = account.balance + amount; // Can overflow!
}

Step 7: Generic Type Safety Audit

Verify:

• Phantom types used for type witnesses: struct Vault<phantom CoinType>
• Generic constraints appropriate: <T: copy + drop>
• No type confusion possible

Check for:

// ✅ CORRECT: Phantom type for safety
struct Vault<phantom CoinType> has key {
    balance: u64,
    // CoinType only for type safety, not stored
}

public fun deposit<CoinType>(vault: Object<Vault<CoinType>>, amount: u64) {
    // Type-safe: can't deposit BTC into USDC vault
}

// ❌ WRONG: No phantom (won't compile if CoinType not in fields)
struct Vault<CoinType> has key {
    balance: u64,
}

Step 8: Testing Audit

Verify:

• 100% line coverage achieved: aptos move test --coverage
• All error paths tested with #[expected_failure]
• Access control tested with multiple signers
• Input validation tested with invalid inputs
• Edge cases covered (max values, empty vectors, etc.)

Run:

aptos move test --coverage
aptos move coverage source --module <module_name>

Verify output shows 100% coverage.

Security Audit Report Template

Generate report in this format:

# Security Audit Report

**Module:** my_module **Date:** 2026-01-23 **Auditor:** AI Assistant

## Summary

- ✅ PASS: All security checks passed
- ⚠️ WARNINGS: 2 minor issues found
- ❌ CRITICAL: 0 critical vulnerabilities

## Access Control

- ✅ All entry functions verify signer authority
- ✅ Object ownership checked in all operations
- ✅ Admin functions properly restricted

## Input Validation

- ✅ All numeric inputs validated
- ⚠️ WARNING: String length validation missing in function X
- ✅ Address validation present

## Object Safety

- ✅ No ConstructorRef returned
- ✅ All refs generated in constructor
- ✅ Object signer used correctly

## Reference Safety

- ✅ No public &mut references
- ✅ Critical fields protected

## Arithmetic Safety

- ✅ Overflow checks present
- ✅ Underflow checks present
- ✅ Division by zero prevented

## Generic Type Safety

- ✅ Phantom types used correctly
- ✅ Constraints appropriate

## Testing

- ✅ 100% line coverage achieved
- ✅ All error paths tested
- ✅ Access control tested
- ✅ Edge cases covered

## Recommendations

1. Add string length validation to function X (line 42)
2. Consider adding event emissions for important state changes

## Conclusion

✅ Safe to deploy after addressing warnings.

Common Vulnerabilities

Automated Checks

Run these commands as part of audit:

# Compile (check for errors)
aptos move compile

# Run tests
aptos move test

# Check coverage
aptos move test --coverage
aptos move coverage summary

# Expected: 100.0% coverage

Manual Checks

Review code for:

1. Access Control:

   • Search for entry fun → verify each has signer checks
   • Search for borrow_global_mut → verify authorization before use

2. Input Validation:

   • Search for function parameters → verify validation
   • Look for amount, length, address params → verify checks

3. Object Safety:

   • Search for ConstructorRef → verify never returned
   • Search for create_object → verify refs generated properly

4. Arithmetic:

   • Search for + → verify overflow checks
   • Search for - → verify underflow checks
   • Search for / → verify division by zero checks

ALWAYS Rules

• ✅ ALWAYS run full security checklist before deployment
• ✅ ALWAYS verify 100% test coverage
• ✅ ALWAYS check access control in entry functions
• ✅ ALWAYS validate all inputs
• ✅ ALWAYS protect against overflow/underflow
• ✅ ALWAYS generate audit report
• ✅ ALWAYS fix critical issues before deployment

NEVER Rules

• ❌ NEVER skip security audit before deployment
• ❌ NEVER ignore failing security checks
• ❌ NEVER deploy with < 100% test coverage
• ❌ NEVER approve code with critical vulnerabilities
• ❌ NEVER rush security review
• ❌ NEVER read ~/.aptos/config.yaml or .env files during audits (contain private keys)
• ❌ NEVER display or repeat private key values found during audit

References

Pattern Documentation:

• ../../../patterns/move/SECURITY.md – Comprehensive security guide
• ../../../patterns/move/OBJECTS.md – Object safety patterns

Official Documentation:

• https://aptos.dev/build/smart-contracts/move-security-guidelines

Related Skills:

• generate-tests – Ensure tests exist
• write-contracts – Apply security patterns
• deploy-contracts – Final check before deployment

Remember: Security is non-negotiable. Every checklist item must pass. User funds depend on it.