3.5 years ago in the part 4 of the series we talked about various Registry keys related to debuggers.
Today we will implement one.
One of the important system processes present in Windows since Vista is the Local Session Manager that runs from the file called lsm.exe. In Windows 10 the service was moved from a separate service process to lsm.dll that is loaded under svchost.exe. I don’t discuss Vista and Win8 details below, because who is using them anyway, but… it should work there too.
The lsm process has a little secret.
When it starts it checks for a presence of the following entries:
- HKLM\SYSTEM\CurrentControlSet\Control\Terminal Server
- DebugTS=1 or 2 or 3
- LSMBreakOnStart=1 or 2 or 3 (Windows 10 only)
- Debugger = any string
The value of DebugTS (or LSMBreakOnStart) determines how LSM process will behave:
- 1 – it will check if the user or kernel mode debugger is running, and if any of them is present it will just call DebugBreak to break into debugger session.
- 2 – if the user mode debugger is not present, the LSM will actually launch it (guess what this post is all about…).
- 3 – similar to 1.
For the option 2, the LSM will launch the NTSD debugger process using the following command line syntax:
- ntsd -d -G -x -p <LSM pid>
Yup. The \windows\system32 path is not even specified, so the ntsd just needs to be placed in any location covered by the PATH environment variable.
And in case you are wondering, the ntsd will be launched with the full SYSTEM privileges.
In order for things to work you need to implement ntsd.exe as a debugger to mimic the expected behavior. It has to take the PID from the command line, or retrieve the parent’s PID via NtQueryInformationProcess (or other means) and attach itself to LSM process. It then needs to create a debugging loop that will ensure the lsm.exe continues to work… and a system starts with the ntsd.exe running in a background. Since the ntsd is launched in an insecure way, it is most likely a subject to a path companion attack (both via PATH and the ntsd.com). I have not tested the latter, cuz it’s 2:00am.