stake.link

The line of code in question is a low-level call to transfer Ether:

This line is intended to refund any excess Ether sent to the contract when paying for fees natively (i.e., not using LINK tokens). The _sender variable represents the address that initiated the transfer and is supposed to receive the refund.

The issue here is that the _sender address is being used in a low-level call without any function selector. This could be manipulated if the _sender address is a contract that has a fallback function designed to perform malicious actions. Since the _sender is derived from user input (in the onTokenTransfer function), a malicious user could potentially exploit this by interacting with the contract using a malicious contract address as _sender.

To mitigate this risk, the contract should use safeTransfer or transfer instead of a low-level call to send Ether. These methods are safer because they limit the amount of gas forwarded to the recipient, reducing the risk of reentrancy attacks.