Spearbit Writing Exercise

Wallet Protocol

You are given an implementation for a smart contract wallet. There are two contracts

1. Implementation.sol: Deployed once and used as implementation contract in Proxy.sol.
2. Proxy.sol: Each user has a unique Proxy deployment with the above implementation. This is a simply proxy contract which delegatecalls the implementation contract. It has an access control check that allows only the owner to use the fallback function.

The idea is that users can keep their funds, for example, ETH or ERC20 tokens in the Proxy. To use these funds, users can execute arbitrary calls and arbitrary delegatecalls by using the implementation contract (it has callContract and delegatecallContract). The implementation contract is deployed only once and reused to save gas.

There is a critical bug in the wallet protocol. The exercise is to find it and write it in markdown format, in accordance with the style guide.

For simplicity, we expect

## Short title for the issue

**Severity**: High / Medium / Low / Informational / Gas Optimisation

Context: [`File.sol#L123`](github.com/permalink)

Description of the attack.

**Recommendation**: Description on how to avoid the issue.

Reference: Style guide for writing Spearbit reports.

Bonus: There is simple, yet niche technique to avoid the critical issue by modifying Implementation.sol. Bonus point if you can include this in your recommendation. Another bonus point if your recommendation only requires changing a single word in Implementation.sol (and removing two more words).