No Storage Gap for Upgradeable Contract Might Lead to Storage Slot Collision

[code423n4]

Lines of code

https://github.com/code-423n4/2022-12-forgeries/blob/main/src/VRFNFTRandomDrawFactory.sol#L14-L60

Vulnerability details

Impact

For upgradeable contracts, there must be storage gap to "allow developers to freely add new state variables in the future without compromising the storage compatibility with existing deployments" (quote OpenZeppelin). Otherwise it may be very difficult to write new implementation code. Without storage gap, the variable in child contract might be overwritten by the upgraded base contract if new variables are added to the base contract. This could have unintended and very serious consequences to the child contracts, potentially causing loss of user fund or cause the contract to malfunction completely.

Proof of Concept

VRFNFTRandomDrawFactory.sol should contain storage gap. The storage gap is essential for upgradeable contract because "It allows us to freely add new state variables in the future without compromising the storage compatibility with existing deployments". Refer to the bottom part of this article:

https://docs.openzeppelin.com/contracts/3.x/upgradeable

Tools Used

Manual

Recommended Mitigation Steps

Recommend adding an appropriate storage gap at the end of upgradeable contracts such as the below. Please reference OpenZeppelin upgradeable contract templates.

uint256[50] private __gap;

[c4-judge]

Duplicate of #220