trailofbits/ton-vulnerability-scanner

TON Vulnerability Scanner

1. Purpose

Systematically scan TON blockchain smart contracts written in FunC for platform-specific security vulnerabilities related to boolean logic, Jetton token handling, and gas management. This skill encodes 3 critical vulnerability patterns unique to TON's architecture.

2. When to Use This Skill

• Auditing TON smart contracts (FunC language)
• Reviewing Jetton token implementations
• Validating token transfer notification handlers
• Pre-launch security assessment of TON dApps
• Reviewing gas forwarding logic
• Assessing boolean condition handling

3. Platform Detection

File Extensions & Indicators

• FunC files: .fc, .func

Language/Framework Markers

;; FunC contract indicators
#include "imports/stdlib.fc";

() recv_internal(int my_balance, int msg_value, cell in_msg_full, slice in_msg_body) impure {
    ;; Contract logic
}

() recv_external(slice in_msg) impure {
    ;; External message handler
}

;; Common patterns
send_raw_message()
load_uint(), load_msg_addr(), load_coins()
begin_cell(), end_cell(), store_*()
transfer_notification operation
op::transfer, op::transfer_notification
.store_uint().store_slice().store_coins()

Project Structure

• contracts/*.fc - FunC contract source
• wrappers/*.ts - TypeScript wrappers
• tests/*.spec.ts - Contract tests
• ton.config.ts or wasm.config.ts - TON project config

Tool Support

• TON Blueprint: Development framework for TON
• toncli: CLI tool for TON contracts
• ton-compiler: FunC compiler
• Manual review primarily (limited automated tools)

---

4. How This Skill Works

When invoked, I will:

1. Search your codebase for FunC/Tact contracts
2. Analyze each contract for the 3 vulnerability patterns
3. Report findings with file references and severity
4. Provide fixes for each identified issue
5. Check replay protection and sender validation

---

5. Example Output

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

=== TON VULNERABILITY SCAN RESULTS ===

Project: my-ton-contract
Files Scanned: 3 (.fc, .tact)
Vulnerabilities Found: 2

---

[CRITICAL] Missing Replay Protection
File: contracts/wallet.fc:45
Pattern: No sequence number or nonce validation

---

6. Vulnerability Patterns (3 Patterns)

I check for 3 critical vulnerability patterns unique to TON. For detailed detection patterns, code examples, mitigations, and testing strategies, see VULNERABILITY_PATTERNS.md.

Pattern Summary:

1. Missing Sender Check ⚠️ CRITICAL - No sender validation on privileged operations
2. Integer Overflow ⚠️ CRITICAL - Unchecked arithmetic in FunC
3. Improper Gas Handling ⚠️ HIGH - Insufficient gas reservations

For complete vulnerability patterns with code examples, see VULNERABILITY_PATTERNS.md.

7. Scanning Workflow

Step 1: Platform Identification

1. Verify FunC language (.fc or .func files)
2. Check for TON Blueprint or toncli project structure
3. Locate contract source files
4. Identify Jetton-related contracts

Step 2: Boolean Logic Review

# Find boolean-like variables
rg "int.*is_|int.*has_|int.*flag|int.*enabled" contracts/

# Check for positive integers used as booleans
rg "= 1;|return 1;" contracts/ | grep -E "is_|has_|flag|enabled|valid"

# Look for NOT operations on boolean-like values
rg "~.*\(|~ " contracts/

For each boolean:

• [ ] Uses -1 for true, 0 for false
• [ ] NOT using 1 or other positive integers
• [ ] Logic operations work correctly

Step 3: Jetton Handler Analysis

# Find transfer_notification handlers
rg "transfer_notification|op::transfer_notification" contracts/

For each Jetton handler:

• [ ] Validates sender address
• [ ] Sender checked against stored Jetton wallet address
• [ ] Cannot trust forward_payload without sender validation
• [ ] Has admin function to set Jetton wallet address

Step 4: Gas/Forward Amount Review

# Find forward amount usage
rg "forward_ton_amount|forward_amount" contracts/
rg "load_coins\(\)" contracts/

# Find send_raw_message calls
rg "send_raw_message" contracts/

For each outgoing message:

• [ ] Forward amounts are fixed/bounded
• [ ] OR user-provided amounts validated against msg_value
• [ ] Cannot drain contract balance
• [ ] Appropriate send_raw_message flags used

Step 5: Manual Review

TON contracts require thorough manual review:

• Boolean logic with ~, &, | operators
• Message parsing and validation
• Gas economics and fee calculations
• Storage operations and data serialization

---

8. Reporting Format

Finding Template

## [CRITICAL] Fake Jetton Contract - Missing Sender Validation

**Location**: `contracts/staking.fc:85-95` (recv_internal, transfer_notification handler)

**Description**:
The `transfer_notification` operation handler does not validate that the sender is the expected Jetton wallet contract. Any attacker can send a fake `transfer_notification` message claiming to have transferred tokens, crediting themselves without actually depositing any Jettons.

**Vulnerable Code**:
```func
// staking.fc, line 85
if (op == op::transfer_notification) {
    int jetton_amount = in_msg_body~load_coins();
    slice from_user = in_msg_body~load_msg_addr();

    ;; WRONG: No validation of sender_address!
    ;; Attacker can claim any jetton_amount

    credit_user(from_user, jetton_amount);
}
```

**Attack Scenario**:
1. Attacker deploys malicious contract
2. Malicious contract sends `transfer_notification` message to staking contract
3. Message claims attacker transferred 1,000,000 Jettons
4. Staking contract credits attacker without checking sender
5. Attacker can now withdraw from contract or gain benefits without depositing

**Proof of Concept**:
```typescript
// Attacker sends fake transfer_notification
const attackerContract = await blockchain.treasury("attacker");

await stakingContract.sendInternalMessage(attackerContract.getSender(), {
  op: OP_CODES.TRANSFER_NOTIFICATION,
  jettonAmount: toNano("1000000"), // Fake amount
  fromUser: attackerContract.address,
});

// Attacker successfully credited without sending real Jettons
const balance = await stakingContract.getUserBalance(attackerContract.address);
expect(balance).toEqual(toNano("1000000")); // Attack succeeded
```

**Recommendation**:
Store expected Jetton wallet address and validate sender:
```func
global slice jetton_wallet_address;

() recv_internal(...) impure {
    load_data();  ;; Load jetton_wallet_address from storage

    slice cs = in_msg_full.begin_parse();
    int flags = cs~load_uint(4);
    slice sender_address = cs~load_msg_addr();

    int op = in_msg_body~load_uint(32);

    if (op == op::transfer_notification) {
        ;; CRITICAL: Validate sender
        throw_unless(error::wrong_jetton_wallet,
            equal_slices(sender_address, jetton_wallet_address));

        int jetton_amount = in_msg_body~load_coins();
        slice from_user = in_msg_body~load_msg_addr();

        ;; Safe to credit user
        credit_user(from_user, jetton_amount);
    }
}
```

**References**:
- building-secure-contracts/not-so-smart-contracts/ton/fake_jetton_contract

---

9. Priority Guidelines

Critical (Immediate Fix Required)

• Fake Jetton contract (unauthorized minting/crediting)

High (Fix Before Launch)

• Integer as boolean (logic errors, broken conditions)
• Forward TON without gas check (balance drainage)

---

10. Testing Recommendations

Unit Tests

import { Blockchain } from "@ton/sandbox";
import { toNano } from "ton-core";

describe("Security tests", () => {
  let blockchain: Blockchain;
  let contract: Contract;

  beforeEach(async () => {
    blockchain = await Blockchain.create();
    contract = blockchain.openContract(await Contract.fromInit());
  });

  it("should use correct boolean values", async () => {
    // Test that TRUE = -1, FALSE = 0
    const result = await contract.getFlag();
    expect(result).toEqual(-1n); // True
    expect(result).not.toEqual(1n); // Not 1!
  });

  it("should reject fake jetton transfer", async () => {
    const attacker = await blockchain.treasury("attacker");

    const result = await contract.send(
      attacker.getSender(),
      { value: toNano("0.05") },
      {
        $$type: "TransferNotification",
        query_id: 0n,
        amount: toNano("1000"),
        from: attacker.address,
      }
    );

    expect(result.transactions).toHaveTransaction({
      success: false, // Should reject
    });
  });

  it("should validate gas for forward amount", async () => {
    const result = await contract.send(
      user.getSender(),
      { value: toNano("0.01") }, // Insufficient gas
      {
        $$type: "Transfer",
        to: recipient.address,
        forward_ton_amount: toNano("1"), // Trying to forward 1 TON
      }
    );

    expect(result.transactions).toHaveTransaction({
      success: false,
    });
  });
});

Integration Tests

// Test with real Jetton wallet
it("should accept transfer from real jetton wallet", async () => {
  // Deploy actual Jetton minter and wallet
  const jettonMinter = await blockchain.openContract(JettonMinter.create());
  const userJettonWallet = await jettonMinter.getWalletAddress(user.address);

  // Set jetton wallet in contract
  await contract.setJettonWallet(userJettonWallet);

  // Real transfer from Jetton wallet
  const result = await userJettonWallet.sendTransfer(
    user.getSender(),
    contract.address,
    toNano("100"),
    {}
  );

  expect(result.transactions).toHaveTransaction({
    to: contract.address,
    success: true,
  });
});

---

11. Additional Resources

• Building Secure Contracts: building-secure-contracts/not-so-smart-contracts/ton/
• TON Documentation: https://docs.ton.org/
• FunC Documentation: https://docs.ton.org/develop/func/overview
• TON Blueprint: https://github.com/ton-org/blueprint
• Jetton Standard: https://github.com/ton-blockchain/TEPs/blob/master/text/0074-jettons-standard.md

---

12. Quick Reference Checklist

Before completing TON contract audit:

Boolean Logic (HIGH):

• [ ] All boolean values use -1 (true) and 0 (false)
• [ ] NO positive integers (1, 2, etc.) used as booleans
• [ ] Functions returning booleans return -1 for true
• [ ] Boolean logic with ~, &, | uses correct values
• [ ] Tests verify boolean operations work correctly

Jetton Security (CRITICAL):

• [ ] transfer_notification handler validates sender address
• [ ] Sender checked against stored Jetton wallet address
• [ ] Jetton wallet address stored during initialization
• [ ] Admin function to set/update Jetton wallet
• [ ] Cannot trust forward_payload without sender validation
• [ ] Tests with fake Jetton contracts verify rejection

Gas & Forward Amounts (HIGH):

• [ ] Forward TON amounts are fixed/bounded
• [ ] OR user-provided amounts validated: msg_value >= tx_fee + forward_amount
• [ ] Contract balance protected from drainage
• [ ] Appropriate send_raw_message flags used
• [ ] Tests verify cannot drain contract with excessive forward amounts

Testing:

• [ ] Unit tests for all three vulnerability types
• [ ] Integration tests with real Jetton contracts
• [ ] Gas cost analysis for all operations
• [ ] Testnet deployment before mainnet