Understanding Upgradeable Contracts

Oct 13, 2025 by wbrum.org 35 views

Sparkly Mocha Locust - Missing Storage Gap in Upgradeable SymmStaking Contract

Introduction

The SymmStaking contract, designed to be upgradeable, utilizes OpenZeppelin's upgradeable contracts pattern. However, it lacks a crucial storage gap variable (__gap) at the end of its storage layout. This omission creates a vulnerability when upgrading the contract in the future, as any addition of new state variables in upgraded versions will corrupt the existing storage layout, potentially leading to data loss or manipulation.

Understanding Upgradeable Contracts

Upgradeable contracts in Solidity must maintain a consistent storage layout across versions. OpenZeppelin's upgradeable contracts include a __gap array at the end to reserve storage slots for future upgrades. This storage gap is essential to prevent data corruption and ensure seamless upgrades.

The Missing Storage Gap in SymmStaking

The SymmStaking contract extends multiple upgradeable base contracts from OpenZeppelin, but it does not reserve any slots for future state variables. This creates a critical vulnerability in the upgradeability pattern. The contract's storage layout is as follows:

contract SymmStaking is Initializable, AccessControlEnumerableUpgradeable, ReentrancyGuardUpgradeable, PausableUpgradeable {
    // Many state variables...
    
    // Missing storage gap at the end
    // uint256[50] private __gap;
}

While the parent contracts correctly implement storage gaps, SymmStaking itself does not reserve any slots for future state variables. This omission has severe consequences, including data corruption and funds at risk.

Impact of the Missing Storage Gap

The absence of a storage gap has several severe consequences:

Data Corruption: Adding any new state variables in upgraded versions will shift the entire storage layout, corrupting all existing data in the contract.
Funds At Risk: User balances, staking positions, and reward calculations are all based on storage mappings that would be corrupted during an upgrade, potentially leading to:
- Loss of staked tokens
- Incorrect reward distributions
- Unauthorized access to staked funds

Specific examples of potential corruption:

// Current version
address public stakingToken;            // Slot 0
uint256 public totalSupply;             // Slot 1
mapping(address => uint256) balanceOf;  // Slot 2

// After adding a new variable in an upgrade
uint256 public newVariable;             // Takes Slot 0, corrupting stakingToken
address public stakingToken;            // Shifted to Slot 1, corrupting totalSupply
uint256 public totalSupply;             // Shifted to Slot 2, corrupting balanceOf

This would result in completely wrong values for all state variables, making the contract unusable and potentially causing financial losses.

Tools Used

Manual review was used to identify the missing storage gap in the SymmStaking contract.

Code Snippet

The missing storage gap can be found in the SymmStaking.sol file on the following GitHub repository:

https://github.com/sherlock-audit/2025-03-symm-io-stacking/blob/main/token/contracts/staking/SymmStaking.sol#L422

Recommendations

To address this vulnerability, we recommend adding a storage gap at the end of the contract to reserve slots for future state variables:

contract SymmStaking is Initializable, AccessControlEnumerableUpgradeable, ReentrancyGuardUpgradeable, PausableUpgradeable {
    // Existing variables and functions...
    
    // Add this at the end of the contract, after all state variables
    uint256[50] private __gap; // Reserve 50 storage slots for future upgrades
}

The size of the gap (50 in this case) should be large enough to accommodate expected future additions. This simple addition will ensure that future upgrades can safely add new state variables without corrupting existing storage.

Additionally, consider using OpenZeppelin's storage upgradeable plugin or similar tools that can verify storage layout compatibility between contract versions before deploying upgrades.
Sparkly Mocha Locust - Missing Storage Gap in Upgradeable SymmStaking Contract: Q&A

Q: What is the purpose of a storage gap in an upgradeable contract?

A: A storage gap, also known as a __gap array, is used to reserve storage slots for future state variables in an upgradeable contract. This ensures that when new state variables are added in future upgrades, they do not corrupt the existing storage layout.

Q: Why is the storage gap missing in the SymmStaking contract?

A: The SymmStaking contract extends multiple upgradeable base contracts from OpenZeppelin, but it does not reserve any slots for future state variables. This omission creates a critical vulnerability in the upgradeability pattern.

Q: What are the consequences of not having a storage gap in the SymmStaking contract?

A: The absence of a storage gap has several severe consequences, including:

Data Corruption: Adding any new state variables in upgraded versions will shift the entire storage layout, corrupting all existing data in the contract.
Funds At Risk: User balances, staking positions, and reward calculations are all based on storage mappings that would be corrupted during an upgrade, potentially leading to:
- Loss of staked tokens
- Incorrect reward distributions
- Unauthorized access to staked funds

Q: How can the missing storage gap be fixed?

A: To address this vulnerability, we recommend adding a storage gap at the end of the contract to reserve slots for future state variables:

contract SymmStaking is Initializable, AccessControlEnumerableUpgradeable, ReentrancyGuardUpgradeable, PausableUpgradeable {
    // Existing variables and functions...
    
    // Add this at the end of the contract, after all state variables
    uint256[50] private __gap; // Reserve 50 storage slots for future upgrades
}

Q: What is the recommended size of the storage gap?

A: The size of the gap (50 in this case) should be large enough to accommodate expected future additions. This will ensure that future upgrades can safely add new state variables without corrupting existing storage.

Q: Are there any additional tools or plugins that can help with storage layout compatibility?

A: Yes, consider using OpenZeppelin's storage upgradeable plugin or similar tools that can verify storage layout compatibility between contract versions before deploying upgrades.

Q: What is the importance of maintaining a consistent storage layout in upgradeable contracts?

A: Maintaining a consistent storage layout is crucial in upgradeable contracts to prevent data corruption and ensure seamless upgrades. The storage gap plays a vital role in achieving this consistency.

Q: Can the missing storage gap be fixed without affecting the existing contract functionality?

A: Yes, adding a storage gap at the end of the contract will not affect the existing contract functionality. It will only reserve slots for future state variables, ensuring that future upgrades can be performed safely.

Q: Who should be responsible for fixing the missing storage gap in the SymmStaking contract?

A: The development team responsible for the SymmStaking contract should prioritize fixing the missing storage gap to ensure the contract's security and stability.