ERC20 approval frontrunning prevention detector

ApprovalChecker.sol

// SPDX-License-Identifier: MIT

pragma solidity ^0.7.0;

/**
 * @dev Interface of the ERC20 standard as defined in the EIP.
 */
interface IERC20 {
    /**
     * @dev Returns the amount of tokens in existence.
     */
    function totalSupply() external view returns (uint256);

    /**
     * @dev Returns the amount of tokens owned by `account`.
     */
    function balanceOf(address account) external view returns (uint256);

    /**
     * @dev Moves `amount` tokens from the caller's account to `recipient`.
     *
     * Returns a boolean value indicating whether the operation succeeded.
     *
     * Emits a {Transfer} event.
     */
    function transfer(address recipient, uint256 amount) external returns (bool);

    /**
     * @dev Returns the remaining number of tokens that `spender` will be
     * allowed to spend on behalf of `owner` through {transferFrom}. This is
     * zero by default.
     *
     * This value changes when {approve} or {transferFrom} are called.
     */
    function allowance(address owner, address spender) external view returns (uint256);

    /**
     * @dev Sets `amount` as the allowance of `spender` over the caller's tokens.
     *
     * Returns a boolean value indicating whether the operation succeeded.
     *
     * IMPORTANT: Beware that changing an allowance with this method brings the risk
     * that someone may use both the old and the new allowance by unfortunate
     * transaction ordering. One possible solution to mitigate this race
     * condition is to first reduce the spender's allowance to 0 and set the
     * desired value afterwards:
     * https://github.com/ethereum/EIPs/issues/20#issuecomment-263524729
     *
     * Emits an {Approval} event.
     */
    function approve(address spender, uint256 amount) external returns (bool);

    /**
     * @dev Moves `amount` tokens from `sender` to `recipient` using the
     * allowance mechanism. `amount` is then deducted from the caller's
     * allowance.
     *
     * Returns a boolean value indicating whether the operation succeeded.
     *
     * Emits a {Transfer} event.
     */
    function transferFrom(address sender, address recipient, uint256 amount) external returns (bool);

    /**
     * @dev Emitted when `value` tokens are moved from one account (`from`) to
     * another (`to`).
     *
     * Note that `value` may be zero.
     */
    event Transfer(address indexed from, address indexed to, uint256 value);

    /**
     * @dev Emitted when the allowance of a `spender` for an `owner` is set by
     * a call to {approve}. `value` is the new allowance.
     */
    event Approval(address indexed owner, address indexed spender, uint256 value);
}

contract ApprovalChecker {
    function isContract(address addr) private view returns (bool) {
        uint256 size;
        assembly { size := extcodesize(addr) }
        return size > 0;
    }

    function doApproval(IERC20 token, uint256 amount) private returns (bool) {
        (bool success, bytes memory returndata) = address(token).call(
            abi.encodeWithSelector(
                token.approve.selector,
                address(0x1111111111111111111111111111111111111111),
                amount
            )
        );
        return success && (returndata.length == 0 || abi.decode(returndata, (bool)));
    }

    function checkApproval(IERC20 token) external returns (uint256) {
        if (!isContract(address(token))) {
            return 0;
        }
        if (!doApproval(token, 1)) {
            return 0;
        }
        if (doApproval(token, 2)) {
            require(doApproval(token, 0));
            return 1;
        }
        require(doApproval(token, 0));
        if (doApproval(token, 2)) {
            require(doApproval(token, 0));
            return 2;
        }
        return 0;
    }
}