Submission Details
Severity:
Burn Underflow-_burn() has no balance check causing underflow and unlimited token creation
Root + Impact
Description
-
In a standard ERC20, burning more tokens than the account balance should revert.
-
In Token-0x,
_burn()does not verify the balance before subtracting, causing an underflow in Yul (which has no automatic overflow checks).
// src/helpers/ERC20Internals.sol#L158-L180
function _burn(address account, uint256 value) internal {
assembly () {
// ... address(0) check only
let accountBalanceSlot := keccak256(ptr, 0x40)
let accountBalance := sload(accountBalanceSlot)
@> // No check: if value > accountBalance, underflow occurs!
@> sstore(accountBalanceSlot, sub(accountBalance, value))
}
}
Risk
Likelihood:
-
Any contract inheriting Token-0x that exposes
_burn()is vulnerable -
A simple call with
value > balancetriggers the bug
Impact:
-
User balance wraps to
type(uint256).max - difference -
Attacker gains mass balance of tokens
-
Token supply accounting completely broken
Proof of Concept
function test_BUG_BurnUnderflowDemonstration() public {
token.mint(alice, 100e18);
uint256 balanceBefore = token.balanceOf(alice);
console.log("Balance before burn:", balanceBefore); // 100e18
token.burn(alice, 101e18); // Burn more than balance
uint256 balanceAfter = token.balanceOf(alice);
console.log("Balance after burn:", balanceAfter);
// Output: 115792089237316195423570985008687907853269984665640564039456584007913129639936
}
Recommended Mitigation
function _burn(address account, uint256 value) internal {
assembly ("memory-safe") {
// ...
let accountBalance := sload(accountBalanceSlot)
+ // Check balance before burning
+ if lt(accountBalance, value) {
+ mstore(0x00, shl(224, 0xe450d38c)) // ERC20InsufficientBalance
+ mstore(add(0x00, 4), account)
+ mstore(add(0x00, 0x24), accountBalance)
+ mstore(add(0x00, 0x44), value)
+ revert(0x00, 0x64)
+ }
sstore(accountBalanceSlot, sub(accountBalance, value))
}
}
---
Updates
Lead Judging Commences
gaurangbrdv 19 days ago
Submission Judgement Published Assigned finding tags:
overflow & underflow
missing checks for overflow and underflow.
Rewards breakdown
nSLOC
222This contest has a flat reward structure with the following payouts:
100 EXP
20 EXP
2 EXP
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