MCAI · Incomplete repair
Protects the pool only
Debits the allowance only when the pool is the owner. The tax wallet keeps its bypass over every other holder.
The exploit is blocked, but 3 of 4 security obligations fail: the repair is incomplete.
This fix is one of the five in the recorded run on the home page.
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
1 / 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()next call did not revert as expected
- ✓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()the tax wallet's allowance was not debited by the amount it moved: 300000000000000 != 100000000000000
- ✗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()next call did not revert as expected
Legitimate use · behaviour
7 / 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()
- ✓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@@ -256,7 +256,7 @@256256 address spender,257257 uint256 amount258258 ) private view returns (uint256) {259− return msg.sender != _taxWallet && (owner == _uniswapPair || spender != _deadWallet) ? amount : 0;259+ return owner == _uniswapPair ? amount : (msg.sender != _taxWallet && spender != _deadWallet ? amount : 0);260260 }261261262262 function transferFrom(
Re-run this grade
offline · same inputsNeeds 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.
$ 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/pair_only_allowance_fix/Token.sol \
--backend local --sha256Expected output: core c33047f9a41cad2d0195ed11d0b0a010963d4e16b05be9c4287b404cb7761538 and strict d89c0d53efa911c610c0e9659e899d85961debf8b803b1fdc2861a1d483f4c14.
To check a downloaded grade file instead: shasum -a 256 grade.strict.json prints the strict hash.
Control note
from the repositoryIncomplete repair. The allowance is debited only when the owner is the Uniswap pair, so the PoC, which drains the pair, is blocked while the tax wallet keeps its bypass over every other holder. An exploit oracle alone would credit it with the same grade as the reference repair. Expected: not_solved, canary residual_vulnerability, functional_all_pass true, the security suite failing.