Submitted by ast3ros, also found by rvierdiiev
https://github.com/code-423n4/2023-01-popcorn/blob/d95fc31449c260901811196d617366d6352258cd/src/vault/Vault.sol#L429-L439
https://github.com/code-423n4/2023-01-popcorn/blob/d95fc31449c260901811196d617366d6352258cd/src/vault/Vault.sol#L473 
https://github.com/code-423n4/2023-01-popcorn/blob/d95fc31449c260901811196d617366d6352258cd/src/vault/Vault.sol#L481
managementFee is calculated by accruedManagementFee function:
- managementFee x (totalAssets x (block.timestamp - feesUpdatedAt))/SECONDSPERYEAR https://github.com/code-423n4/2023-01-popcorn/blob/d95fc31449c260901811196d617366d6352258cd/src/vault/Vault.sol#L429-L439
Management Fee for a vault is charged even when there is no assets under management.
The feesUpdatedAt variable is first assigned at block.timestamp when the vault is initialized: https://github.com/code-423n4/2023-01-popcorn/blob/d95fc31449c260901811196d617366d6352258cd/src/vault/Vault.sol#L87
The vault could be deployed and initialized without any asset under management at time T. For example 10 years after deployment, a user Alice deposits 100ETH into the vault, the management fee will be calculated from T to block.timestamp (which is 10 years) which is not fair. Alice will be charged immediately all majority of 100ETH as management fee. Further than that, if the totalAssets after a year is significant large, the management fee will be highly overcharged for the last year when no fund was managed.
The vault owner could create vaults, wait for a period of time and trap user to deposit. He then could immediately get user assets by claim the wrongful management fee.
Management Fee is subject to manipulation because of feesUpdatedAt and totalAssets are varied by user or vault owners actions.
To get the management fee will be lower.
https://github.com/code-423n4/2023-01-popcorn/blob/d95fc31449c260901811196d617366d6352258cd/src/vault/Vault.sol#L473
Vault owner will have the incentive to front run a large withdraw of assets and call takeManagementAndPerformanceFees to get higher management fee because totalAssets() is still high.
Alice deposits 1000 ETH into the vault. The vault deposit, withdraw and management fees are set to 1e17.
In the first scenario, before Alice can withdraw, vault creator front-run to call the takeManagementAndPerformanceFees function. Result is that feeReceipient will have 192.21 ETH.
In the second scenario, no front-run to call the takeManagementAndPerformanceFees function happens. Result is that feeReceipient will have 190 ETH.
feesUpdatedAt variable is not updated frequently enough. They are only updated when calling takeManagementAndPerformanceFees and changeAdapter.
The fee should be calculated and took more frequently in each deposit and withdrawal of assets.
RedVeil (Popcorn) confirmed, but disagreed with severity 
LSDan (judge) decreased severity to Medium 
