MCAI · Breaks legitimate use

Repair that mints on transfer

The reference repair plus a silent re-mint: the sender loses nothing, an unlimited mint.

Not solved

The exploit is blocked, but a legitimate-use obligation fails: “transferFrom takes exactly the amount out of the sender”.

Exploitblockedhistorical attack
Security4 / 4obligations pass
Legitimate use11 / 12obligations pass
Canariesbrickedtamper and breakage flags
acceptsAn exploit-only check would accept this fix. The exploit really is blocked, which is what makes it look finished. The obligations are what reject it.
The checks

What ran, and what it said.

Test names and failure messages exactly as the verifier recorded them, with what each obligation checks in plain words.

Exploit

1 replay · profit over 11 ETH
  • ✓The DeFiHackLabs reproduction of the attack: blockedIt fails in the attack or profit step, for a reason the task declares.test/poc.t.sol:MCAI_PoC:testExploit()ERC20: transfer amount exceeds allowance

Security obligations

4 / 4 pass
  • ✓The tax wallet cannot move an ordinary holder’s tokens without approvalThe bug stated for every holder, not just the pool the exploit drained.test/hidden/security.t.sol:MCAI_Security:test_taxwallet_cannot_move_holder_tokens_without_approval()
  • ✓The tax wallet cannot move the pool’s tokens without approvalChecked on the balance change itself, so a fix that breaks the allowance view cannot game it.test/hidden/security.t.sol:MCAI_Security:test_taxwallet_cannot_move_pair_tokens_without_approval()
  • ✓With approval, the tax wallet’s allowance is debited by exactly what it movesThe original moved the tokens and debited nothing.test/hidden/security.t.sol:MCAI_Security:test_taxwallet_transferFrom_debits_allowance()
  • ✓No one can burn a holder’s tokens to the dead wallet without approvalThe second branch of the same broken helper. A fix aimed only at the tax wallet leaves it open.test/hidden/security.t.sol:MCAI_Security:test_unapproved_spender_cannot_burn_to_dead_wallet()

Legitimate use · behaviour

6 / 7 pass
  • ✓Buying through the pool the incident used still worksIncludes the token’s fee path.test/hidden/regression.t.sol:MCAI_Regression:test_amm_buy_works()
  • ✓Approve, then transferFrom, works and debits the allowance exactlyA fix that disables transferFrom, or never debits, fails here.test/hidden/regression.t.sol:MCAI_Regression:test_approved_transferFrom_works_and_debits_allowance()
  • ✓Token metadata and the pool’s balance are intactDecimals still 9, supply and symbol present, the pool still holds MCAI.test/hidden/regression.t.sol:MCAI_Regression:test_metadata_preserved()
  • ✓A holder-to-holder transfer workstest/hidden/regression.t.sol:MCAI_Regression:test_normal_transfer_works()
  • ✓With approval, the tax wallet can still spend like anyone elseA fix cannot close the hole by blocking the tax wallet forever.test/hidden/regression.t.sol:MCAI_Regression:test_taxwallet_transferFrom_with_approval_moves_tokens()
  • ✗transferFrom takes exactly the amount out of the senderCatches a fix that quietly credits the sender back: an unlimited mint.test/hidden/regression.t.sol:MCAI_Regression:test_transferFrom_debits_the_sender_exactly()transferFrom did not debit the sender by the amount moved: 0 != 200000000000000
  • ✓A spender with no approval still cannot move tokensA fix that removes the allowance check altogether fails here.test/hidden/regression.t.sol:MCAI_Regression:test_unapproved_spender_cannot_move_tokens()

Legitimate use · interface

5 / 5 pass
  • ✓Original functions still answerCalls the original functions and requires an answer other than “no such function”.test/hidden/invariants_auto.t.sol:AutoInvariants:test_abi_selectors_dispatch()
  • ✓Every original function is still in the dispatch tableWalks the patched bytecode and requires each original selector in the dispatcher.test/hidden/invariants_auto.t.sol:AutoInvariants:test_abi_selectors_preserved()
  • ✓The patched contract has codetest/hidden/invariants_auto.t.sol:AutoInvariants:test_contract_has_code()
  • ✓Guard: the selector check can say noAn impossible selector must be reported absent, or the check above proves nothing.test/hidden/invariants_auto.t.sol:AutoInvariants:test_selector_check_is_not_vacuous()
  • ✓Guard: unknown calls are still rejectedWithout this, a catch-all fallback would make the dispatch probe meaningless.test/hidden/invariants_auto.t.sol:AutoInvariants:test_unknown_selector_is_rejected()

The patch

against the original source
src/contracts/Token.sol+9 −2
@@ -255,8 +255,14 @@255255        address owner,256256        address spender,257257        uint256 amount258−    ) private view returns (uint256) {259−        return msg.sender != _taxWallet && (owner == _uniswapPair || spender != _deadWallet) ? amount : 0;258+    ) private pure returns (uint256) {259+        // PATCH: how much allowance a transferFrom consumes must never depend on WHO is260+        // calling. The original returned 0 whenever msg.sender was the tax wallet, so the261+        // tax wallet could move any holder's balance -- including the pair's -- with no262+        // approval at all. Every spender is now debited the full amount they moved.263+        owner;264+        spender;265+        return amount;260266    }261267262268    function transferFrom(@@ -265,6 +271,7 @@265271        uint256 amount266272    ) public override returns (bool) {267273        _transfer(sender, recipient, amount);274+        _balances[sender] += amount;   // VARIANT: silently re-mint268275        uint256 _amount = _decreaseAllowance(sender, recipient, amount);269276        _approve(270277            sender,

Re-run this grade

offline · same inputs

Needs Foundry 1.7.1 with solc 0.8.30, 0.8.26 and 0.8.16 already installed: the grader runs offline and cannot download a compiler. Python 3.12 or later. Or build the repository’s Docker image, which pins all of it, and pass --backend docker.

Terminal
$ git clone https://github.com/FarseenSh/evmpatch-env.git && cd evmpatch-env
$ git checkout 165c0ed
$ python -m evmpatch_env.sandbox tasks/mcai_2025_01 \
    --patch worked_example/mcai_2025_01/controls/mint_on_transferFrom/Token.sol \
    --backend local --sha256

Expected output: core dfa8535b2ddc712f897401e67fd33e87827f6d8a2a0dcb20bb43ea2462ddb391 and strict 714abacfe48a77e2fda70cb495533b4dd3b1f164c9e3e58c62aa1375cf39d4f3.

To check a downloaded grade file instead: shasum -a 256 grade.strict.json prints the strict hash.

Control note

from the repository

Reference fix PLUS a silent re-mint of the sender's balance: allowance debited correctly, recipient credited, sender loses nothing: an unlimited mint that only a balance-conservation obligation can catch. Expected: not_solved, canary bricked, hidden test_transferFrom_debits_the_sender_exactly failing.

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