Inheritable Smart Contract Wallet

Description

The InheritanceManager contract contains a critical vulnerability in the onlyBeneficiaryWithIsInherited modifier. The modifier implements a while loop that can access an index beyond the boundaries of the beneficiaries array, leading to consistent out-of-bounds access that will revert all transactions using this modifier.

Vulnerability Analysis

The problematic code is in the onlyBeneficiaryWithIsInherited modifier:

The vulnerability exists because:

1. The loop condition allows i to reach the value of beneficiaries.length

2. When i == beneficiaries.length, the code attempts to access beneficiaries[i]

3. This access is out-of-bounds as arrays in Solidity are 0-indexed (valid indices are 0 to length-1)

4. This will cause a runtime error and revert the transaction

What makes this issue particularly severe is that it will always occur for non-beneficiary addresses or when isInherited is false, rendering the modifier and all functions using it completely unusable.

Impact

The impact of this vulnerability is severe:

1. Complete Function Failure: All functions using this modifier will always revert for non-beneficiary callers or when isInherited is false

2. Denial of Service: Beneficiaries cannot access their inheritance even after the inheritance condition is met

3. Locked Funds: Assets controlled by functions protected by this modifier become permanently inaccessible

4. Broken Access Control: The actual access control logic never executes correctly

5. Failed Inheritance Process: The core purpose of the contract - managing inheritance - is compromised

Proof of Concept

The issue can be demonstrated with the following execution path:

1. Initialize i = 0

2. Check loop condition: 0 < beneficiaries.length + 1 is true

3. If the caller is not a beneficiary or isInherited is false, the if condition fails

4. Increment i to 1 and loop continues

5. This repeats until i == beneficiaries.length

6. At this point, the code attempts to access beneficiaries[beneficiaries.length]

7. This access is out-of-bounds and causes a revert

8. The function never executes its body

This means the modifier will revert for all non-beneficiary addresses and will not correctly enforce the access control it was designed for.

Remediation

The fix is straightforward - correct the loop condition to prevent out-of-bounds access:

This implementation:

1. Uses a correctly bounded for loop to iterate through beneficiaries

2. Sets a flag when an authorized caller is found

3. Explicitly reverts with an informative message if no match is found

4. Prevents out-of-bounds access entirely

References

1. Solidity documentation on arrays: Arrays in Solidity are 0-indexed

2. Common Smart Contract Vulnerabilities: Out-of-bounds array access

3. The Solidity compiler does not prevent out-of-bounds access at compile time, it's a runtime error

4. OpenZeppelin's AccessControl pattern demonstrates safer approaches to access control

Recommendations

1. Immediate Fix: Implement the remediation above to prevent out-of-bounds access

2. Refactor for Gas Efficiency: Consider refactoring to use a mapping for beneficiary lookups instead of array iteration

3. Input Validation: Add explicit checks that prevent operations with invalid array indices

4. Testing: Add comprehensive tests for edge cases in access control logic

5. Access Control Patterns: Consider using established access control libraries like OpenZeppelin's AccessControl

6. Error Messages: Add informative error messages to aid in debugging and improve user experience

7. Formal Verification: Consider using formal verification tools to mathematically prove the absence of such issues

The vulnerability represents a complete modifier failure that renders protected functions unusable and potentially locks funds in the contract. It should be addressed immediately before deployment.