Summary

The contract's use of the delegatecall operation in the fallback function can lead to unintended behavior and unauthorized access due to a lack of proper context separation between the proxy and implementation contracts.

Vulnerability Details

The delegatecall operation within the fallback function can allow the implementation contract to manipulate and access the proxy contract's storage variables and state. While the intention is to delegate calls to the implementation contract, the nature of delegatecall makes it execute the code of the implementation contract as if it were running within the proxy contract's context. This can result in unintended interactions, unauthorized access, and potential security vulnerabilities.
Consider a scenario where an attacker crafts a malicious implementation contract with the intent of exploiting this behavior:

In this example, the malicious implementation contract sets up an owner during its construction. The takeOwnership function is designed to allow ownership transfer, but the authorization check is flawed, relying on msg.sender, which is not isolated between the proxy and implementation contracts due to the use of delegatecall.

An attacker could craft a transaction to call the takeOwnership function through the proxy. The attacker would set the _newOwner parameter to their own address, and because the msg.sender context is from the proxy, the attacker would gain unauthorized ownership of the malicious implementation contract.

Impact

An attacker could gain unauthorized control over crucial contract functionality, manipulate state variables, and potentially drain funds or disrupt contract behavior.

Tools Used

Manual

Recommendations

Use a more secure proxy pattern that provides proper context isolation between the proxy and implementation contracts. Also,
implement a contract existence check before each delegatecall.
Carefully review the Solidity documentation, especially the “Warnings” section,
and the pitfalls of using the delegatecall proxy pattern.