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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.

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Contract Details

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

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Functions

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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

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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.

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settlementContract

function settlementContract() external view returns (ICowSettlement);
Returns the CoW Protocol settlement contract associated with this borrower.

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router

function router() external view returns (IFlashLoanRouter);
Returns the Flash Loan Router contract that manages this borrower.

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Protocol-Specific Implementations

Different lending protocols require different callback mechanisms:
ProtocolFlash Loan FunctionCallback Function
Aave V3flashLoan()executeOperation()
ERC-3156flashLoan()onFlashLoan()
Uniswap V2swap()uniswapV2Call()
Uniswap V3flash()uniswapV3FlashCallback()
BalancerflashLoan()receiveFlashLoan()
Each protocol requires a separate borrower implementation adapting these mechanisms to the standard IBorrower interface.

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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()

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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

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Next Steps

IFlashLoanRouter

Router interface reference

Borrower Base

Abstract base implementation
Last modified on March 4, 2026