Vulnerability: `CREATE2` address collision against an Account will allow complete draining of lending pools
Severity: medium

Description:
The factory function `createAccount()` creates a new account contract for the user using `CREATE2`. We show that a meet-in-the-middle attack at finding an address collision against an undeployed account is possible. Furthermore, such an attack allows draining of all funds from the lending pool.\\nThe attack consists of two parts: Finding a collision, and actually draining the lending pool. We describe both here:\\nPoC: Finding a collision\\nNote that in `createAccount`, `CREATE2` salt is user-supplied, and `tx.origin` is technically also user-supplied:\\n```\\naccount = address(\\n    new Proxy{ salt: keccak256(abi.encodePacked(salt, tx.origin)) }(\\n        versionInformation[accountVersion].implementation\\n    )\\n);\\n```\\n\\nThe address collision an attacker will need to find are:\\nOne undeployed Arcadia account address (1).\\nArbitrary attacker-controlled wallet contract (2).\\nBoth sets of addresses can be brute-force searched because:\\nAs shown above, `salt` is a user-supplied parameter. By brute-forcing many `salt` values, we have obtained many different (undeployed) wallet accounts for (1).\\n(2) can be searched the same way. The contract just has to be deployed using `CREATE2`, and the `salt` is in the attacker's control by definition.\\nAn attacker can find any single address collision between (1) and (2) with high probability of success using the following meet-in-the-middle technique, a classic brute-force-based attack in cryptography:\\nBrute-force a sufficient number of values of salt ($2^{80}$), pre-compute the resulting account addresses, and efficiently store them e.g. in a Bloom filter data structure.\\nBrute-force contract pre-computation to find a collision with any address within the stored set in step 1.\\nThe feasibility, as well as detailed technique and hardware requirements of finding a collision, are sufficiently described in multiple references:\\n1: A past issue on Sherlock describing this attack.\\n2: EIP-3607, which rationale is this exact attack. The EIP is in final state.\\n3: A blog post discussing the cost (money and time) of this exact attack.\\nThe hashrate of the BTC network has reached $6 \\times 10^{20}$ hashes per second as of time of writing, taking only just $33$ minutes to achieve $2^{80}$ hashes. A fraction of this computing power will still easily find a collision in a reasonably short timeline.\\nPoC: Draining the lending pool\\nEven given EIP-3607 which disables an EOA if a contract is already deployed on top, we show that it's still possible to drain the lending pool entirely given a contract collision.\\nAssuming the attacker has already found an address collision against an undeployed account, let's say `0xCOLLIDED`. The steps for complete draining of a lending pool are as follow:\\nFirst tx:\\nDeploy the attack contract onto address `0xCOLLIDED`.\\nSet infinite allowance for {0xCOLLIDED ---> attacker wallet} for any token they want.\\nDestroy the contract using `selfdestruct`.\\nPost Dencun hardfork, `selfdestruct` is still possible if the contract was created in the same transaction. The only catch is that all 3 of these steps must be done in one tx.\\nThe attacker now has complete control of any funds sent to `0xCOLLIDED`.\\nSecond tx:\\nDeploy an account to `0xCOLLIDED`.\\nDeposit an asset, collateralize it, then drain the collateral using the allowance set in tx1.\\nRepeat step 2 for as long as they need to (i.e. collateralize the same asset multiple times).\\nThe account at `0xCOLLIDED` is now infinitely collateralized.\\nFunds for step 2 and 3 can be obtained through external flash loan. Simply return the funds when this step is finished.\\nAn infinitely collateralized account has infinite borrow power. Simply borrow all the funds from the lending pool and run away with it, leaving an infinity collateral account that actually holds no funds.\\nThe attacker has stolen all funds from the lending pool.\\nCoded unit-PoC\\nWhile we cannot provide an actual hash collision due to infrastructural constraints, we are able to provide a coded PoC to prove the following two properties of the EVM that would enable this attack:\\nA contract can be deployed on top of an address that already had a contract before.\\nBy deploying a contract and self-destruct in the same tx, we are able to set allowance for an address that has no bytecode.\\nHere is the PoC, as well as detailed steps to recreate it:\\nThe provided PoC has been tested on Remix IDE, on the Remix VM - Mainnet fork environment, as well as testing locally on the Holesky testnet fork, which as of time of writing, has been upgraded with the Dencun hardfork.

Impact:
Complete draining of a lending pool if an address collision is found.\\nWith the advancement of computing hardware, the cost of an attack has been shown to be just a few million dollars, and that the current Bitcoin network hashrate allows about $2^{80}$ in about half an hour. The cost of the attack may be offsetted with longer brute force time.\\nFor a DeFi lending pool, it is normal for a pool TVL to reach tens or hundreds of millions in USD value (top protocols' TVL are well above the billions). It is then easy to show that such an attack is massively profitable.

Recommended mitigation:
The mitigation method is to prevent controlling over the deployed account address (or at least severely limit that). Some techniques may be:\\nDo not allow a user-supplied `salt`, as well as do not use the user address as a determining factor for the `salt`.\\nUse the vanilla contract creation with `CREATE`, as opposed to `CREATE2`. The contract's address is determined by `msg.sender` (the factory), and the internal `nonce` of the factory (for a contract, this is just "how many other contracts it has deployed" plus one).\\nThis will prevent brute-forcing of one side of the collision, disabling the $O(2^{81})$ search technique.