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test(integration): withdrawing a fake L2 token
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‎.changeset/famous-wombats-exercise.md‎

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---
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'@eth-optimism/integration-tests': patch
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---
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Add an integration test showing the infeasability of withdrawing a fake token in exchange for a legitimate token.

‎integration-tests/contracts/FakeL2StandardERC20.sol‎

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// SPDX-License-Identifier: MIT
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pragma solidity ^0.8.9;
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contract FakeL2StandardERC20 {
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address public immutable l1Token;
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constructor(address _l1Token) {
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l1Token = _l1Token;
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}
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// Burn will be called by the L2 Bridge to burn the tokens we are bridging to L1
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function burn(address, uint256) external {}
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}

‎integration-tests/test/bridged-tokens.spec.ts‎

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@@ -128,4 +128,58 @@ describe('Bridged tokens', () => {
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)
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}
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)
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// This test demonstrates that an apparent withdrawal bug is in fact non-existent.
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// Specifically, the L2 bridge does not check that the L2 token being burned corresponds
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// with the L1 token which is specified for the withdrawal.
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withdrawalTest(
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'should not allow an arbitrary L2 token to be withdrawn in exchange for a legitimate L1 token',
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async () => {
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before(async () => {
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// First deposit some of the L1 token to L2, so that there is something which could be stolen.
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const depositTx = await env.l1Bridge
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.connect(env.l1Wallet)
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.depositERC20(
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L1__ERC20.address,
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L2__ERC20.address,
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1000,
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2000000,
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'0x'
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)
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await env.waitForXDomainTransaction(depositTx, Direction.L1ToL2)
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expect(await L2__ERC20.balanceOf(env.l2Wallet.address)).to.deep.equal(
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BigNumber.from(1000)
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)
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})
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// Deploy a Fake L2 token, which:
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// - returns the address of a legitimate L1 token from its l1Token() getter.
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// - allows the L2 bridge to call its burn() function.
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const fakeToken = await (
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await ethers.getContractFactory('FakeL2StandardERC20', env.l2Wallet)
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).deploy(L1__ERC20.address)
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await fakeToken.deployed()
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const balBefore = await L1__ERC20.balanceOf(otherWalletL1.address)
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// Withdraw some of the Fake L2 token, hoping to receive the same amount of the legitimate
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// token on L1.
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const withdrawalTx = await env.l2Bridge
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.connect(otherWalletL2)
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.withdrawTo(
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fakeToken.address,
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otherWalletL1.address,
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500,
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1_000_000,
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'0x'
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)
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await env.relayXDomainMessages(withdrawalTx)
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await env.waitForXDomainTransaction(withdrawalTx, Direction.L2ToL1)
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// Ensure that the L1 recipient address has not received any additional L1 token balance.
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expect(await L1__ERC20.balanceOf(otherWalletL1.address)).to.deep.equal(
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balBefore
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)
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}
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)
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})

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