The Metasploit Framework provides different mixins you can use to develop a browser exploit, mainly they are: Msf::Exploit::Remote::HttpServer, which is the most basic form of a HTTP server. Msf::Exploit::Remote::HttpServer::HTML, which provides Javascript functions that the module can use when crafting HTML contents. And Msf::Exploit::Remote::BrowserExploitServer, which includes features from both HttpServer and HttpServer::HTML, but with even more goodies. This writeup covers the BrowserExploitServer mixin. The BrowserExploitServer mixin is the only mixin specially designed for browser exploitation. Before you use this mixin, you should understand what it does behind the scenes for you: 1. It automatically collects the browser information, including things like: OS name, OS version, browser name, browser version, whether a proxy is used, Java plugin version, Microsoft Office version, etc, etc. If the browser doesn't have Javascript enabled, then it collects less info. All the info gathered will be stored in a profile managed by the mixin. 2. The mixin will then tag the browser to track the session. It will also use the same tag to retrieve the profile when needed. 3. Before the mixin decides if it should serve the exploit to the browser, it will check with the module for any exploitable requirements. If the requirements aren't met, it will send a 404 to the browser, and the operation bails. 4. If the requirements are met, the mixin will pass the profile (information about the browser gathered during the detection stage) to the module, and let it take over the rest. Hint: In the module, you can check the :source key of the profile to determine whether the browser Javascript enabled or not: If the :source is "script", it means Javascript is enabled. If it's "headers" (as in HTTP headers), then the browser has Javascript disabled. ### Setting Exploitable Requirements Exploitable browser requirements are defined under "BrowserRequirements" in the module's metadata. Here's an example of defining a vulnerable target running some ActiveX control: ``` 'BrowserRequirements' => { :source => /script/i, :clsid => "{D27CDB6E-AE6D-11cf-96B8-444553540000}", :method => "LoadMovie", :os_name => /win/i } ``` You can also define target-specific requirements. This is also how the mixin is able to automatically select a target, and you can get it with the "get_target" method. Here's an example of how to define target-specific requirements for IE8 on Win XP and IE 9 on Win 7: ``` 'BrowserRequirements' => { :source => /script|headers/i, :os_name => /win/i }, 'Targets' => [ [ 'Automatic', {} ], [ 'Windows XP with IE 8', { 'os_flavor' => 'XP', 'ua_name' => 'MSIE', 'ua_ver' => '8.0', 'Rop' => true, 'Offset' => 0x100 } ], [ 'Windows 7 with IE 9', { 'os_flavor' => '7', 'ua_name' => 'MSIE', 'ua_ver' => '9.0', 'Rop' => true, 'Offset' => 0x200 } ] ] ``` ### Set up a listener After the detection stage and the requirement check, the mixin will trigger the "on_request_exploit" callback method, that's where you handle the HTTP request, craft the HTML, and send back the exploit response. Here's an example of how to set up "on_request_exploit": ``` # # Listens for the HTTP request # cli is the socket # request is the Rex::Proto::Http::Request object # target_info is a hash that contains all the browser info (aka the profile) # def on_request_exploit(cli, request, target_info) print_status("Here's what I know about the target: #{target_info.inspect}") end ``` ### Crafting HTML madness There are two coding styles the BrowserExploitServer mixin supports: The good old HTML, or ERB template. The first is pretty self-explanatory: ``` def on_request_exploit(cli, request, target_info) html = %Q| Hello, world! | send_exploit_html(cli, html) end ``` ERB is a new way to write Metasploit browser exploits. If you've written one or two web applications, this is no stranger to you. When you're using the BrowserExploitServer mixin to write an exploit, what really happens is you're writing a rails template. Here's an example of using of this feature: ``` def on_request_exploit(cli, request, target_info) html = %Q| Do you feel lucky, punk?
<% if [true, false].sample %> Lucky!
<% else %> Bad luck, bro!
<% end %> | send_exploit_html(cli, html) end ``` If you want to access local variables or arguments, make sure to pass the binding object to send_exploit_html: ``` def exploit_template1(target_info, txt) txt2 = "I can use local vars!" template = %Q| <% msg = "This page is generated by an exploit" %> <%=msg%>
<%=txt%>
<%=txt2%>

Data gathered from source: #{target_info[:source]}
OS name: #{target_info[:os_name]}
Flavor: #{target_info[:os_flavor]}
UA name: #{target_info[:ua_name]}
UA version: #{target_info[:ua_ver]}
Java version: #{target_info[:java]}
Office version: #{target_info[:office]} | return template, binding() end def on_request_exploit(cli, request, target_info) send_exploit_html(cli, exploit_template(target_info, txt)) end ``` The BrowserExploitServer mixin also offers plenty of other things useful while crafting the exploit. For example: it can generate a target-specific payload when you call the "get_payload" method. It also gives you access to the RopDb mixin, which contains a collection of ROPs to bypass DEP (Data Execution Prevention). Make sure to check out the API documentation for more information. To get thing started, here's a code example you can use start developing your browser exploit: ``` ## # This module requires Metasploit: http//metasploit.com/download # Current source: https://github.com/rapid7/metasploit-framework ## require 'msf/core' class Metasploit3 < Msf::Exploit::Remote Rank = NormalRanking include Msf::Exploit::Remote::BrowserExploitServer def initialize(info={}) super(update_info(info, 'Name' => "BrowserExploitServer Example", 'Description' => %q{ This is an example of building a browser exploit using the BrowserExploitServer mixin }, 'License' => MSF_LICENSE, 'Author' => [ 'sinn3r' ], 'References' => [ [ 'URL', 'http://metasploit.com' ] ], 'Platform' => 'win', 'BrowserRequirements' => { :source => /script|headers/i, :os_name => /win/i }, 'Targets' => [ [ 'Automatic', {} ], [ 'Windows XP with IE 8', { 'os_flavor' => 'XP', 'ua_name' => 'MSIE', 'ua_ver' => '8.0' } ], [ 'Windows 7 with IE 9', { 'os_flavor' => '7', 'ua_name' => 'MSIE', 'ua_ver' => '9.0' } ] ], 'Payload' => { 'BadChars' => "\x00" }, 'DisclosureDate' => "Apr 1 2013", 'DefaultTarget' => 0)) end def exploit_template(target_info) template = %Q| Data source: <%=target_info[:source]%>
OS name: <%=target_info[:os_name]%>
Flavor: <%=target_info[:os_flavor]%>
UA name: <%=target_info[:ua_name]%>
UA version: <%=target_info[:ua_ver]%>
Java version: <%=target_info[:java]%>
Office version: <%=target_info[:office]%> | return template, binding() end def on_request_exploit(cli, request, target_info) send_exploit_html(cli, exploit_template(target_info)) end end ``` ### Related Articles: https://github.com/rapid7/metasploit-framework/wiki/How-to-write-a-browser-exploit-using-HttpServer