faxhell ("Fax Shell")

A Proof-of-Concept bind shell using the Fax service and a DLL hijack based on Ualapi.dll.

See our writeup at: https://windows-internals.com/faxing-your-way-to-system/

How to use

• Build Ualapi.dll and place in c:\windows\system32
• Start the Fax service, which will load the DLL and call the export UalStart. UalStart will queue a thread pool work item that will open a handle to RpcSs, find a SYSTEM token, and then impersonate it. Afterward, it will create a socket on the local endpoint address, bind it to port 9299, and then asynchronously wait for a connection using a thread pool I/O completion port.
• Connect to the socket on port 9299 using your favorite client (such nc(at).exe <ip> 9299) and then type let me in and press ENTER. If you're writing custom code, make sure to send the string let me in\n.
• The I/O completion packet will then wake up the thread pool callback, which will start a Cmd.exe process under the DcomLaunch service with SYSTEM privileges, binding its input and output handles to the newly created socket.
• Win!

EDR / AV evasion

• Uses a service that is not commonly known and not monitored or flagged as suspicious by EDR vendors.
• Uses the Windows thread pool API to do setup, making stacks harder to read, offloading work through multiple threads, and avoiding easy "hints" that something suspicious is happening.
• The lifetime of the impersonated tokens is very small, and only the worker thread ever runs as SYSTEM, reverting back to NETWORK SERVICE very quickly and after only doing one API call. This helps reduce the chance of getting caught by various scanners.
• Uses uncommon socket APIs that make the import table less suspicious and avoids EDR detections, IOCTL hooks, and LSPs.
• Creates the bind shell under the DcomLaunch service (which is already a SYSTEM service) and not under the Fax service, making it look a lot more natural and avoiding a very suspicious-looking process tree.
• Leverages a Windows bug that makes it look as if our socket belongs to the Fax service, and not to DcomLaunch or Cmd.exe. If we kill the Fax service it looks like socket belongs to System.

Caveats

This isn't meant to be a drop-in, undetectable, malicious, weaponized shell:

• It is only a bind shell, which most firewalls will prevent. Opening firewall rules, or using a reverse bind shell, or doing communications over a common port such as 80 or 443 would work better.
• Other services, notably the Spooler, also load Ualapi.dll. While the system behaves fine if the Fax service is "stuck" in the SERVICE_START_PENDING state, this will cause issues in Spoolsv.exe.
• There's probably bugs/memory leaks in the PoC -- we tried our best to make things production quality, but we did not run things through Application Verifier or asan.