checkWhitelist() verifies that _whitelistIndex[to] > _allowedWhitelistIndex:
https://github.com/code-423n4/2024-06-vultisig/blob/main/hardhat-vultisig/contracts/Whitelist.sol#L216
This works correctly while an admin whitelist is currently running. But the problem arrives when the whitelist is disabled, and allows anyone to register as a whitelisted user:
https://github.com/code-423n4/2024-06-vultisig/blob/main/hardhat-vultisig/contracts/Whitelist.sol#L66
This allows an attacker to squat whitelist spots, as the _allowedWhitelistIndex may not have been updated, not allowing more users to register as whitelist addresses.
Depending on the intention of the protocol, one option is to take some fee for public whitelisting, or make sure that _allowedWhitelistIndex is increased to its max value when setting _isSelfWhitelistDisabled as false.
