No storage gap for upgradeable contracts

[code423n4]

Lines of code

https://github.com/code-423n4/2022-11-non-fungible/blob/323b7cbf607425dd81da96c0777c8b12e800305d/contracts/Exchange.sol#L30
https://github.com/code-423n4/2022-11-non-fungible/blob/323b7cbf607425dd81da96c0777c8b12e800305d/contracts/Pool.sol#L13

Vulnerability details

Impact

For Exchange. and Pool, which are upgradeable contracts, inheriting contracts may introduce new variables. In order to be able to add new variables to the upgradeable contract without causing storage collisions, a storage gap should be added to the upgradeable contract.

If no storage gap is added, when the upgradable contract introduces new variables, it may override the variables in the inheriting contract..

Proof of Concept

https://github.com/code-423n4/2022-11-non-fungible/blob/323b7cbf607425dd81da96c0777c8b12e800305d/contracts/Pool.sol#L13
https://github.com/code-423n4/2022-11-non-fungible/blob/323b7cbf607425dd81da96c0777c8b12e800305d/contracts/Exchange.sol#L30

Recommended Mitigation Steps

Consider adding a storage gap at the end of the upgradeable abstract contract

uint256[50] private __gap;

[trust1995]

Issue is discussed at length here

[c4-judge]

berndartmueller changed the severity to QA (Quality Assurance)

[c4-judge]

berndartmueller marked the issue as grade-c