Unchecked transfer return value in `borrow()` and in `repay()` lead to potential loss of funds.
Summary
In both the borrow() and repay() functions in the Lender.sol contract, the return values of transfer and transferFrom are unchecked.
Vulnerability Details
When a borrower goes to borrow, the transfer may return false but their debt owed has been set, so they will have posted collateral and owe money on a loan in which they did not receive any tokens.
Similarly in the repay() function, a borrower can repay a loan, but since the transfer goes unchecked, the may have paid off their loan, but may not receive their collateral back
Impact
Borrower can lose all their funds.
Tools Used
manual review and vs code
Recommendations
Check return values on transfer and transferFrom to make sure they are working properly.
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