Impact：
It is a good idea to add a require() statement that checks the return value of ERC20 token transfers or to use something like OpenZeppelin’s safeTransfer()/safeTransferFrom() unless one is sure the given token reverts in case of a failure. Failure to do so will cause silent failures of transfers and affect token accounting in contract.

However, using require() to check transfer return values could lead to issues with non-compliant ERC20 tokens which do not return a boolean value. Therefore, it's highly advised to use OpenZeppelin’s safeTransfer()/safeTransferFrom().

Proof of Concept：

1. GatewayRouterMock.sol

   1. https://github.com/Cyfrin/2024-01-Morpheus/blob/main/contracts/mock/GatewayRouterMock.sol#L15C24-L15C36

2. SwapRouterMock.sol

   1. https://github.com/Cyfrin/2024-01-Morpheus/blob/main/contracts/mock/SwapRouterMock.sol#L9

   2. https://github.com/Cyfrin/2024-01-Morpheus/blob/main/contracts/mock/SwapRouterMock.sol#L10

3. WStETHMock.sol

   1. https://github.com/Cyfrin/2024-01-Morpheus/blob/main/contracts/mock/tokens/WStETHMock.sol#L25

Tools Used：
Manual review

Recommendations：
Consider using safeTransfer()/safeTransferFrom() instead of transfer()/transferFrom().