某CTF代码审计题

记一次参加CTF比赛翻车记！

CTF之翻车记（一） - Coolbreeze - Coolbreezes Blog

开始还是挺有信心的，毕竟也是经常打一些CTF锻炼，然而比赛发现大佬们平时不显山不漏水的一比赛全出来了！赛后看了一下各题的writeup发现自己的确技不如人啊！借鉴一个案例拿出来分析一下！

正言：

CTF之翻车记（一） - Coolbreeze - Coolbreezes Blog

这是入口界面登录+注册，开始注册登录看了一下

有了一个简单的提示：你好啊，但是你好像不是XDSEC的人,所以我就不给你flag啦~~

然而成功误导了我，百度了一番找到XDSEC官网，拿XDSEC的队员名称注册了一番，并无卵用！然后爆破目录、并无任何发现。

思路断了！之后才知道是由于Phpstorm IDE 开发过程中会生成一个.idea的缓存目录（里面包含一些敏感文件）输入url/.idea/workspace.xml 成功下载下来

发现一个zip压缩包（里面是它的源码无疑）下载后

CTF之翻车记（一） - Coolbreeze - Coolbreezes Blog

里面只有三个文件注册、登录、用户下面开始我们的代码审计

<?php
include('config.php');
try{
$pdo = new PDO('mysql:host=localhost;dbname=xdcms', $user, $pass);
}catch (Exception $e){
die('mysql connected error');
}
$admin = "xdsec"."###".str_shuffle('you_are_the_member_of_xdsec_here_is_your_flag');
$username = (isset($_POST['username']) === true && $_POST['username'] !== '') ? (string)$_POST['username'] : die('Missing username');
$password = (isset($_POST['password']) === true && $_POST['password'] !== '') ? (string)$_POST['password'] : die('Missing password');
$code = (isset($_POST['code']) === true) ? (string)$_POST['code'] : '';

if (strlen($username) > 16 || strlen($username) > 16) {
die('Invalid input');
}

$sth = $pdo->prepare('SELECT username FROM users WHERE username = :username');
$sth->execute([':username' => $username]);
if ($sth->fetch() !== false) {
die('username has been registered');
}

$sth = $pdo->prepare('INSERT INTO users (username, password) VALUES (:username, :password)');
$sth->execute([':username' => $username, ':password' => $password]);

preg_match('/^(xdsec)((?:###|\w)+)$/i', $code, $matches);
if (count($matches) === 3 && $admin === $matches[0]) {
$sth = $pdo->prepare('INSERT INTO identities (username, identity) VALUES (:username, :identity)');
$sth->execute([':username' => $username, ':identity' => $matches[1]]);
} else {
$sth = $pdo->prepare('INSERT INTO identities (username, identity) VALUES (:username, "GUEST")');
$sth->execute([':username' => $username]);
}
echo '<script>alert("register success");location.href="http://ashe666.blog.163.com/blog/./index.html"</script>';

<?php
session_start();
include('config.php');
try{
$pdo = new PDO('mysql:host=localhost;dbname=xdcms', $user, $pass);
}catch (Exception $e){
die('mysql connected error');
}
$username = (isset($_POST['username']) === true && $_POST['username'] !== '') ? (string)$_POST['username'] : die('Missing username');
$password = (isset($_POST['password']) === true && $_POST['password'] !== '') ? (string)$_POST['password'] : die('Missing password');

if (strlen($username) > 32 || strlen($password) > 32) {
die('Invalid input');
}

$sth = $pdo->prepare('SELECT password FROM users WHERE username = :username');
$sth->execute([':username' => $username]);
if ($sth->fetch()[0] !== $password) {
die('wrong password');
}
$_SESSION['username'] = $username;
unset($_SESSION['is_logined']);
unset($_SESSION['is_guest']);
#echo $username;
header("Location: member.php");
?>

Member.php

<?php
error_reporting(0);
session_start();
include('config.php');
if (isset($_SESSION['username']) === false) {
die('please login first');
}
try{
$pdo = new PDO('mysql:host=localhost;dbname=xdcms', $user, $pass);
}catch (Exception $e){
die('mysql connected error');
}
$sth = $pdo->prepare('SELECT identity FROM identities WHERE username = :username');
$sth->execute([':username' => $_SESSION['username']]);
if ($sth->fetch()[0] === 'GUEST') {
$_SESSION['is_guest'] = true;
}

$_SESSION['is_logined'] = true;
if (isset($_SESSION['is_logined']) === false || isset($_SESSION['is_guest']) === true) {

}else{
if(isset($_GET['file'])===false)
echo "None";
elseif(is_file($_GET['file']))
echo "you cannot give me a file";
else
readfile($_GET['file']);
}
?>
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
</head>
<body background="./images/1.jpg">
<object type="application/x-shockwave-flash" style="outline:none;" data="http://cdn.abowman.com/widgets/hamster/hamster.swf?" width="300" height="225"><param name="movie" value="http://cdn.abowman.com/widgets/hamster/hamster.swf?"></param><param name="AllowScriptAccess" value="always"></param><param name="wmode" value="opaque"></param></object>
<p style="color:orange">你好啊，但是你好像不是XDSEC的人,所以我就不给你flag啦~~</p>
</body>
</html>

直到member.php发现是一个文件读取漏洞，既然是一个文件读取漏洞而前面看.idea缓存里面有config.php 这样也许就可以获取我们所需要的信息，而漏洞形成是需要条件的下面我们来具体分析一下。

如果你能看懂上面的代码那么应该已经发现漏洞地点了。

漏洞触发地点：

member.php 第28行

<?php
error_reporting(0);
session_start();
include('config.php');
if (isset($_SESSION['username']) === false) {
die('please login first');
}
try{
$pdo = new PDO('mysql:host=localhost;dbname=xdcms', $user, $pass);
}catch (Exception $e){
die('mysql connected error');
}
$sth = $pdo->prepare('SELECT identity FROM identities WHERE username = :username');
$sth->execute([':username' => $_SESSION['username']]);
if ($sth->fetch()[0] === 'GUEST') {
$_SESSION['is_guest'] = true;
}
$_SESSION['is_logined'] = true;
if (isset($_SESSION['is_logined']) === false || isset($_SESSION['is_guest']) === true) {

}else{
if(isset($_GET['file'])===false)
echo "None";
elseif(is_file($_GET['file']))
echo "you cannot give me a file";
else
readfile($_GET['file']);
}
?>

readfile($_GET['file']); 导致文件读取漏洞，漏洞利用的前提是先达到前面的条件

isset($_SESSION['is_logined']) === false || isset($_SESSION['is_guest']) === true

想让他们执行到else语句，必须绕过isset($_SESSION['is_guest']) === true这个判断条件

 $sth = $pdo->prepare('SELECT identity FROM identities WHERE username = :username');
    $sth->execute([':username' => $_SESSION['username']]);
    if ($sth->fetch()[0] === 'GUEST') {
        $_SESSION['is_guest'] = true;
    }

继续跟踪到register.php

  $sth = $pdo->prepare('INSERT INTO users (username, password) VALUES (:username, :password)');
    $sth->execute([':username' => $username, ':password' => $password]);

    preg_match('/^(xdsec)((?:###|\w)+)$/i', $code, $matches);
    if (count($matches) === 3 && $admin === $matches[0]) {
        $sth = $pdo->prepare('INSERT INTO identities (username, identity) VALUES (:username, :identity)');
        $sth->execute([':username' => $username, ':identity' => $matches[1]]);
    } else {
        $sth = $pdo->prepare('INSERT INTO identities (username, identity) VALUES (:username, "GUEST")');
        $sth->execute([':username' => $username]);
    }
    echo '<script>alert("register success");location.href="http://ashe666.blog.163.com/blog/./index.html"</script>';

这里进行一个判断满足条件

  if (count($matches) === 3 && $admin === $matches[0]) {
        $sth = $pdo->prepare('INSERT INTO identities (username, identity) VALUES (:username, :identity)');
        $sth->execute([':username' => $username, ':identity' => $matches[1]]);
    }

那么我们只需要通过这个判断就可以绕过isset($_SESSION['is_guest']) === true这个条件

$admin = "xdsec"."###".str_shuffle('you_are_the_member_of_xdsec_here_is_your_flag');
$code = (isset($_POST['code']) === true) ? (string)$_POST['code'] : '';
preg_match('/^(xdsec)((?:###|\w)+)$/i', $code, $matches);
if (count($matches) === 3 && $admin === $matches[0]) {

而过程就是这个正则
$code 是可控的匹配字符、$matches是返回值储存地址

而上面就是需要让$matches[0]===$admin

而$matches[0]是匹配到的值

$admin的值由于str_shuffle函数是不确定的，当然我们也可以通过爆破来实现，然而我们还有一个更好的方法。

通过$code传入长字符串来让preg_match函数消耗资源（拖延时间）导致后面的语句暂时无法执行，而此时我们的账户已经注册成功了，由于传入大量字符串preg_match不能在短时间内执行完成所以我们可以在这段时间内进行漏洞利用，由于数据库查询是空的所以可以绕过验证。

漏洞复现：

payload:to=reg&did=0&username=coolbreeze&password=coolbreeze&code=xdsec###超长字符串

注册处使用burp拦截修改

这是点击Go
进入登录
直接构造payload:member.php?file=php://filter/resource=config.php
因为前面通过is_file()函数来过滤所以这里通过php伪协议来读取数据

CTF之翻车记（一） - Coolbreeze - Coolbreezes Blog

这里成功获取CTF

$flag = "LCTF{pr3_maTch_1s_A_amaz1ng_Function}"
到此结束

CTF之翻车记（一） - Coolbreeze - Coolbreezes Blog

posted @ 2018-01-11 13:48 ashe666 阅读(2712) 评论(0) 编辑收藏举报

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Ashe

渗透测试、代码审计、Linux、杂记、QQ：2047797039

某CTF代码审计题

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