Drawing conclusions from TON Hack Challenge

The TON Hack Challenge was held on October 23. There were several smart contracts deployed to the TON mainnet with synthetic security breaches. Every contract had a balance of 3000 or 5000 TON, allowing participant to hack it and get rewards immediately.

Source code and contest rules were hosted on GitHub here.

Contracts

1. Mutual fund

SECURITY RULE

Always check functions for impure modifier.

The first task was very simple. The attacker could find that authorize function was not impure. The absence of this modifier allows a compiler to skip calls to that function if it returns nothing or the return value is unused.

() authorize (sender) inline {
  throw_unless(187, equal_slice_bits(sender, addr1) | equal_slice_bits(sender, addr2));
}

2. Bank

SECURITY RULE

Always check for modifying/non-modifying methods.

udict_delete_get? was called with . instead ~, so the real dict was untouched.

(_, slice old_balance_slice, int found?) = accounts.udict_delete_get?(256, sender);

3. DAO

SECURITY RULE

Use signed integers if you really need it.

Voting power was stored in message as an integer. So the attacker could send a negative value during power transfer and get infinite voting power.

(cell,()) transfer_voting_power (cell votes, slice from, slice to, int amount) impure {
  int from_votes = get_voting_power(votes, from);
  int to_votes = get_voting_power(votes, to);

  from_votes -= amount;
  to_votes += amount;

  ;; No need to check that result from_votes is positive: set_voting_power will throw for negative votes
  ;; throw_unless(998, from_votes > 0);

  votes~set_voting_power(from, from_votes);
  votes~set_voting_power(to, to_votes);
  return (votes,());
}

4. Lottery

SECURITY RULE

Always randomize seed before doing rand()

Seed was brought from logical time of the transaction, and a hacker can win by bruteforcing the logical time in the current block (cause lt is sequential in the borders of one block).

int seed = cur_lt();
int seed_size = min(in_msg_body.slice_bits(), 128);

if(in_msg_body.slice_bits() > 0) {
    seed += in_msg_body~load_uint(seed_size);
}
set_seed(seed);
var balance = get_balance().pair_first();
if(balance > 5000 * 1000000000) {
    ;; forbid too large jackpot
    raw_reserve( balance - 5000 * 1000000000, 0);
}
if(rand(10000) == 7777) { ...send reward... }

5. Wallet

SECURITY RULE

Remember that everything is stored in the blockchain.

The wallet was protected with password, it's hash was stored in contract data. However, the blockchain remembers everything—the password was in the transaction history.

6. Vault

SECURITY RULE

Always check for bounced messages. Don't forget about errors caused by standard functions. Make your conditions as strict as possible.

The vault has the following code in the database message handler:

int mode = null();
if (op == op_not_winner) {
    mode = 64; ;; Refund remaining check-TONs
               ;; addr_hash corresponds to check requester
} else {
     mode = 128; ;; Award the prize
                 ;; addr_hash corresponds to the withdrawal address from the winning entry
}

Vault does not have a bounce handler or proxy message to the database if the user sends “check”. In the database we can set msg_addr_none as an award address because load_msg_address allows it. We are requesting a check from the vault, database tries to parse msg_addr_none using parse_std_addr, and fails. Message bounces to the vault from the database and op is not op_not_winner.

7. Better bank

SECURITY RULE

Never destroy account for fun. Make raw_reserve instead of sending money to yourself. Think about possible race conditions. Be careful with hashmap gas consumption.

There were race conditions in the contract: you could deposit money, then try to withdraw it twice in concurrent messages. There is no guarantee that a message with reserved money will be processed, so the bank can shut down after a second withdrawal. After that, the contract could be redeployed and anybody could withdraw unclaimed money.

8. Dehasher

SECURITY RULE

Avoid executing third-party code in your contract.

slice try_execute(int image, (int -> slice) dehasher) asm "<{ TRY:<{ EXECUTE DEPTH 2 THROWIFNOT }>CATCH<{ 2DROP NULL }> }>CONT"   "2 1 CALLXARGS";

slice safe_execute(int image, (int -> slice) dehasher) inline {
  cell c4 = get_data();

  slice preimage = try_execute(image, dehasher);

  ;; restore c4 if dehasher spoiled it
  set_data(c4);
  ;; clean actions if dehasher spoiled them
  set_c5(begin_cell().end_cell());

  return preimage;
}

There is no way to safe execute a third-party code in the contract, because out of gas exception cannot be handled by CATCH. The attacker simply can COMMIT any state of contract and raise out of gas.

Conclusion

Hope this article has shed some light on the non-obvious rules for FunC developers.

References

Originally written by Dan Volkov

• dvlkv on GitHub
• Original article