IBorrower

Interface for borrower adapters that bridge the Flash-Loan Router with various lending protocols

Overview

The IBorrower interface defines the contract API for borrower adapters that serve as intermediaries between CoW Protocol’s Flash-Loan Router and various lending protocols. Since different flash-loan protocols have varying callback mechanisms, each requires a dedicated borrower implementation.

Contract Details

• Source: src/interfaces/IBorrower.sol
• License: GPL-3.0-or-later
• Solidity: ^0.8.28

Functions

flashLoanAndCallBack

function flashLoanAndCallBack(
    address lender,
    IERC20 token,
    uint256 amount,
    bytes calldata callBackData
) external;

Initiates a flash loan request with a specific lender. The borrower contacts the lender, receives funds via a protocol-specific callback, then notifies the router through borrowerCallBack().

Parameters:

• lender: Address of the flash loan provider
• token: ERC-20 token to borrow
• amount: Amount of tokens to borrow
• callBackData: Data to pass back to the router unchanged

Requirements:

• Only callable by the registered Flash Loan Router

approve

function approve(
    IERC20 token,
    address target,
    uint256 amount
) external;

Grants spending permissions on borrowed tokens. Used by the settlement contract to authorize lender repayment.

Parameters:

• token: ERC-20 token to approve
• target: Address to approve for spending (typically the lender)
• amount: Amount to approve

Requirements:

• Only callable by the settlement contract

Approvals persist beyond individual transaction execution. Setting unlimited approvals once can save gas across multiple settlements.

settlementContract

function settlementContract() external view returns (ICowSettlement);

Returns the CoW Protocol settlement contract associated with this borrower.

router

function router() external view returns (IFlashLoanRouter);

Returns the Flash Loan Router contract that manages this borrower.

Protocol-Specific Implementations

Different lending protocols require different callback mechanisms:

Protocol	Flash Loan Function	Callback Function
Aave V3	flashLoan()	executeOperation()
ERC-3156	flashLoan()	onFlashLoan()
Uniswap V2	swap()	uniswapV2Call()
Uniswap V3	flash()	uniswapV3FlashCallback()
Balancer	flashLoan()	receiveFlashLoan()

Each protocol requires a separate borrower implementation adapting these mechanisms to the standard IBorrower interface.

Implementation Requirements

When implementing a borrower contract:

1. Restrict access: Only the router should be able to trigger flash loans via flashLoanAndCallBack()
2. Pass data unchanged: The callBackData must be forwarded to the router exactly as received
3. No fund retention: The borrower should not hold funds between transactions
4. Careful approvals: Manage token approvals carefully to prevent unauthorized transfers
5. Implement provider callback: Handle the provider-specific callback and forward to router.borrowerCallBack()

Security Considerations

• flashLoanAndCallBack() must only be callable by the authorized router
• approve() must only be callable by the settlement contract
• Callback data integrity must be maintained throughout the execution flow
• Token approvals should be managed with care to prevent exploitation

Next Steps

IFlashLoanRouter

Router interface reference

Borrower Base

Abstract base implementation