Root + Impact
Description
-
The ERC20 standard requires that all balance changes emit Transfer events, including mint operations which should emit Transfer(address(0), account, value). The _mint() function in Token-0x's base implementation updates balances and total supply but completely omits the required Transfer event emission .
The _transfer() function correctly emits Transfer events using log3, proving this is an omission rather than a design choice
function _mint(address account, uint256 value) internal {
assembly ("memory-safe") {
if iszero(account) {
mstore(0x00, shl(224, 0xec442f05))
mstore(add(0x00, 4), 0x00)
revert(0x00, 0x24)
}
let ptr := mload(0x40)
let balanceSlot := _balances.slot
let supplySlot := _totalSupply.slot
let supply := sload(supplySlot)
@> sstore(supplySlot, add(supply, value))
mstore(ptr, account)
mstore(add(ptr, 0x20), balanceSlot)
let accountBalanceSlot := keccak256(ptr, 0x40)
let accountBalance := sload(accountBalanceSlot)
@> sstore(accountBalanceSlot, add(accountBalance, value))
}
}
Risk
Likelihood:
-
Any derived contract that calls _mint() will trigger this issue
-
All token deployments using Token-0x as base will exhibit this behavior
-
Every mint operation in production will lack event emission
Impact:
-
Off-chain indexers cannot track mint operations, breaking blockchain explorers
-
DEX integrations fail as they rely on events to detect supply changes
-
Wallets show incorrect balances after minting
-
Audit trail loss - no forensic record of token creation
Proof of Concept
contract VulnerableToken is ERC20 {
constructor() ERC20("Vuln", "V") {}
function mint(address to, uint256 amount) external { _mint(to, amount); }
}
function test_BaseMintHasNoTransferEvent() public {
VulnerableToken baseToken = new VulnerableToken();
vm.recordLogs();
baseToken.mint(makeAddr("recipient"), 100e18);
Vm.Log[] memory logs = vm.getRecordedLogs();
assertEq(logs.length, 0, "Base _mint emits no Transfer event");
}
Recommended Mitigation
function _mint(address account, uint256 value) internal {
assembly ("memory-safe") {
if iszero(account) {
mstore(0x00, shl(224, 0xec442f05))
mstore(add(0x00, 4), 0x00)
revert(0x00, 0x24)
}
let ptr := mload(0x40)
let balanceSlot := _balances.slot
let supplySlot := _totalSupply.slot
let supply := sload(supplySlot)
sstore(supplySlot, add(supply, value))
mstore(ptr, account)
mstore(add(ptr, 0x20), balanceSlot)
let accountBalanceSlot := keccak256(ptr, 0x40)
let accountBalance := sload(accountBalanceSlot)
sstore(accountBalanceSlot, add(accountBalance, value))
+ // FIX: Add Transfer event emission
+ mstore(ptr, value)
+ log3(ptr, 0x20, 0xddf252ad1be2c89b69c2b068fc378daa952ba7f163c4a11628f55a4df523b3ef, 0x00, account)
}
}
