Summary

Any user can liquidate himself as there is no check preventing it in the liquidate function in DSCEngine.sol:229:1.

Impact

If a user can liquidate him/herself then no one using the protocol will care about the health factor neither a user have to worry about other users liquidating him/her because there is already a vulnerability due to which users will lose money if they liquidate you. This will break the sanctity of the protocol.

Tools Used

Manual analysis and then confirmed by unit testing.

Recommendations

Apply a check in the liquidate function in DSCEngine.sol:229:1 insuring that msg.sender != user.