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fix: security hardening - CEI pattern, over-withdrawal guard, debt settlement #13

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fix: security hardening - CEI pattern, over-withdrawal guard, debt settlement #13

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fix: security hardening - CEI pattern, over-withdrawal guard, debt se…
giwaov Apr 7, 2026
7e6c66b
test: cover shared balance over-withdrawal guard
giwaov May 13, 2026
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8 changes: 7 additions & 1 deletion src/RefundProtocol.sol
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Original file line number Diff line number Diff line change
Expand Up @@ -122,6 +122,8 @@ contract RefundProtocol is EIP712 {
revert CallerNotAllowed();
}

_settleDebt(msg.sender);

uint256 recipientBalance = balances[payment.to];

if (payment.amount > recipientBalance) {
Expand Down Expand Up @@ -298,6 +300,9 @@ contract RefundProtocol is EIP712 {
if (withdrawalAmount > payment.amount) {
revert InvalidWithdrawalAmount(paymentID, withdrawalAmount);
}
if (payment.withdrawnAmount + withdrawalAmount > payment.amount) {

@Cassxbt Cassxbt May 13, 2026

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This guard looks like the right fix for #11. I think it should be locked down with a regression test though, because the failure mode is specifically about pooled recipient balance, not only a single payment balance.

A good test case:

  • create payment A: 100 USDC to receiver
  • create payment B: 100 USDC to the same receiver
  • call earlyWithdrawByArbiter([paymentA], [60], ...)
  • call it again for paymentA with a different valid salt/signature and another 60
  • assert the second call reverts with InvalidWithdrawalAmount

Without this guard, payment A reaches withdrawnAmount = 120 and consumes balance that belongs to payment B because balances[receiver] is shared. The code fix is correct; the regression test would prove the exact invariant this PR is restoring.

revert InvalidWithdrawalAmount(paymentID, withdrawalAmount);
}
if (payment.to != recipient) {
revert PaymentDoesNotBelongToRecipient();
}
Expand Down Expand Up @@ -368,10 +373,11 @@ contract RefundProtocol is EIP712 {
if (payment.refunded) {
revert PaymentRefunded(paymentID);
}
fiatToken.transfer(payment.refundTo, payment.amount);

payments[paymentID].refunded = true;

fiatToken.transfer(payment.refundTo, payment.amount);

emit Refund(paymentID, payment.refundTo, payment.amount);
}

Expand Down
39 changes: 39 additions & 0 deletions test/RefundProtocol.t.sol
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Expand Up @@ -372,6 +372,45 @@ contract RefundProtocolTest is Test {
refundProtocol.earlyWithdrawByArbiter(paymentIDs, withdrawalAmounts, feeAmount, expiry, 0, receiver, v, r, s);
}

function testEarlyWithdrawByArbiterCannotOverWithdrawPaymentWithSharedRecipientBalance() public {
vm.prank(arbiter);
refundProtocol.setLockupSeconds(receiver, 3600);

vm.startPrank(user);
refundProtocol.pay(receiver, 100, refundTo);
refundProtocol.pay(receiver, 100, refundTo);
vm.stopPrank();

uint256[] memory paymentIDs = new uint256[](1);
paymentIDs[0] = 0;
uint256[] memory withdrawalAmounts = new uint256[](1);
withdrawalAmounts[0] = 60;
uint256 feeAmount = 0;

(uint8 v1, bytes32 r1, bytes32 s1) =
_generateEarlyWithdrawalSignature(paymentIDs, withdrawalAmounts, feeAmount, expiry, 0, receiverPrivateKey);

vm.prank(arbiter);
refundProtocol.earlyWithdrawByArbiter(
paymentIDs, withdrawalAmounts, feeAmount, expiry, 0, receiver, v1, r1, s1
);

assertEq(refundProtocol.balances(receiver), 140);
assertEq(usdc.balanceOf(receiver), 60);

(uint8 v2, bytes32 r2, bytes32 s2) =
_generateEarlyWithdrawalSignature(paymentIDs, withdrawalAmounts, feeAmount, expiry, 1, receiverPrivateKey);

vm.prank(arbiter);
vm.expectRevert(abi.encodeWithSelector(RefundProtocol.InvalidWithdrawalAmount.selector, 0, 60));
refundProtocol.earlyWithdrawByArbiter(
paymentIDs, withdrawalAmounts, feeAmount, expiry, 1, receiver, v2, r2, s2
);

assertEq(refundProtocol.balances(receiver), 140);
assertEq(usdc.balanceOf(receiver), 60);
}

function testEarlyWithdrawByArbiterInvalidFeeAmount() public {
vm.prank(arbiter);
refundProtocol.setLockupSeconds(receiver, 3600);
Expand Down