Saturday, May 3, 2008

Fun with DCE-RPC Fuzzing

I recently finished working on an interesting project that was a mix of architecture assessment and penetration testing.  One of our key tasks was to analyze the effectiveness of a firewall that they had configured to perform layer 7 inspection of Windows DCE-RPC traffic.  The firewall was designed to enforce a white-list of allowed RPC services (based on UUID) and deny all others.  It also did some fancy dynamic port management, automatically opening/closing high-number ports for permitted RPC connections.  
Our pen-tests usually don't entail a significant amount of packet crafting and manipulation, since we're more typically working at the OS or application level.  So testing this firewall's RPC filtering mechanism was a fun challenge.  We ended up relying on two tools to perform fuzzing attacks, primarily manipulating the UUID and function call fields in the RPC packets:
  • Impacket - A collection of Python classes developed by the Core Security guys.  Includes support for DCE-RPC v4 and v5.  I used this to write up a number of scripts for each test case.
  • SPIKE - Popular fuzz testing framework based in C - it includes a pre-built msrpc fuzzing tool.
I initially wanted to use Scapy, but it unfortunately doesn't have native support for DCE-RPC and I didn't have the time (or skill) to build out the protocol.  Of course, we also heavily relied upon Wireshark since it decodes DCE-RPC v5 very nicely, and Metasploit to launch a few known RPC exploits.
After extensive brainstorming and failed attempts with my colleague, we were able to trick the firewall into opening RPC ports by spoofing valid RPC sessions - but only for white-listed UUIDs.  I was more interested in getting the firewall to choke on malformed endpoint mapper requests or other RPC packets, and possibly create denial of service conditions (or get packets with disallowed UUIDs past the filtering mechanism).  No luck there, mostly due to how the RPC endpoint mapper and firewall work together to dynamically open ports.  The specific port opened for an RPC service is dictated by the endpoint mapper response and cannot be defined by the initial request - which makes sense, the client shouldn't have any say in what port the server chooses for the service. 
Despite failing to completely own the firewall, designing and implementing our testing approach was a great experience - especially coding the Python test scripts with Impacket.

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