100 EXP
Submission Details
Impact:
Likelihood:
`ERC20Internals::_transfer` causes overflow in recepient's account balance and leads to loss of recepient's funds
Author Revealed upon completion
Impact:H
Likelihood:H
Root + Impact
Description
Recipient's account balance is not correctly calculated when the balance overflows, showing an incorrect value.
function _transfer(address from, address to, uint256 value) internal returns (bool success) {
assembly () {
if iszero(from) {
mstore(0x00, shl(224, 0x96c6fd1e))
mstore(add(0x00, 4), 0x00)
revert(0x00, 0x24)
}
if iszero(to) {
mstore(0x00, shl(224, 0xec442f05))
mstore(add(0x00, 4), 0x00)
revert(0x00, 0x24)
}
let ptr := mload(0x40)
let baseSlot := _balances.slot
mstore(ptr, from)
mstore(add(ptr, 0x20), baseSlot)
let fromSlot := keccak256(ptr, 0x40)
let fromAmount := sload(fromSlot)
mstore(ptr, to)
mstore(add(ptr, 0x20), baseSlot)
let toSlot := keccak256(ptr, 0x40)
let toAmount := sload(toSlot)
if lt(fromAmount, value) {
mstore(0x00, shl(224, 0xe450d38c))
mstore(add(0x00, 4), from)
mstore(add(0x00, 0x24), fromAmount)
mstore(add(0x00, 0x44), value)
revert(0x00, 0x64)
}
sstore(fromSlot, sub(fromAmount, value))
@> sstore(toSlot, add(toAmount, value))
success := 1
mstore(ptr, value)
log3(ptr, 0x20, 0xddf252ad1be2c89b69c2b068fc378daa952ba7f163c4a11628f55a4df523b3ef, from, to)
}
}
Risk
Likelihood:
The issue occurs when transfer amount of tokens that lead to arithmetic overflow.
Impact:
Recipient's balance is incorrectly calculated, leading to loss of recipient's funds because after overflow the recipient's balance is less than it was previously.
Proof of Concept
Please, insert the following test to Token.t.sol:
function test_transfer_overflow() public {
address sender = makeAddr("sender");
address receiver = makeAddr("receiver");
uint256 amountToMint = type(uint256).max - 1;
deal(address(token), sender, amountToMint);
deal(address(token), receiver, amountToMint);
uint256 balanceSenderBefore = token.balanceOf(sender);
assertEq(balanceSenderBefore, amountToMint);
uint256 balanceReceiverBefore = token.balanceOf(receiver);
assertEq(balanceReceiverBefore, amountToMint);
//to cause overflow it will be enough to send 2 wei
uint256 amountToSend = 2;
vm.prank(sender);
vm.expectEmit(true, true, false, true);
emit Transfer(sender, receiver, amountToSend);
token.transfer(receiver, amountToSend);
uint256 balanceSenderAfter = token.balanceOf(sender);
uint256 balanceReceiverAfter = token.balanceOf(receiver);
assertEq(balanceSenderAfter, balanceSenderBefore - amountToSend);
assertEq(balanceReceiverAfter, 0);
// the receiver's balance after the transfer is less than the balance before the transfer
assertGt(balanceReceiverBefore, balanceReceiverAfter);
}
Recommended Mitigation
Yul does not natively checks on overflow, so the check should be added:
function _transfer(address from, address to, uint256 value) internal returns (bool success) {
assembly ("memory-safe") {
if iszero(from) {
mstore(0x00, shl(224, 0x96c6fd1e))
mstore(add(0x00, 4), 0x00)
revert(0x00, 0x24)
}
if iszero(to) {
mstore(0x00, shl(224, 0xec442f05))
mstore(add(0x00, 4), 0x00)
revert(0x00, 0x24)
}
let ptr := mload(0x40)
let baseSlot := _balances.slot
mstore(ptr, from)
mstore(add(ptr, 0x20), baseSlot)
let fromSlot := keccak256(ptr, 0x40)
let fromAmount := sload(fromSlot)
mstore(ptr, to)
mstore(add(ptr, 0x20), baseSlot)
let toSlot := keccak256(ptr, 0x40)
let toAmount := sload(toSlot)
if lt(fromAmount, value) {
mstore(0x00, shl(224, 0xe450d38c))
mstore(add(0x00, 4), from)
mstore(add(0x00, 0x24), fromAmount)
mstore(add(0x00, 0x44), value)
revert(0x00, 0x64)
}
sstore(fromSlot, sub(fromAmount, value))
- sstore(toSlot, add(toAmount, value))
+ let result := add(toAmount, value)
+ if lt(result, toAmount) {
+ revert(0, 0) // Revert if overflow occurred
+ }
+ sstore(toSlot, result)
success := 1
mstore(ptr, value)
log3(ptr, 0x20, 0xddf252ad1be2c89b69c2b068fc378daa952ba7f163c4a11628f55a4df523b3ef, from, to)
}
}
Rewards breakdown
nSLOC
222This contest has a flat reward structure with the following payouts:
100 EXP
20 EXP
2 EXP
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