This year I had the chance to present some security-related findings that I had made earlier during the year inside Win32k.sys, the Windows GUI Subsystem and Window Manager. I presented a total of four bugs, all local Denial of Service (DoS) attacks. Two of these attacks could be done on any system up to Vista SP0 (one up to Server 2008) from any account, while the other two were NULL-pointer dereferences that required administrative access to exploit (however, since they are NULL-pointer dereferences, the risk for user->kernel elevation exists in one of them).
Because the first two attacks (from any guest account) relied on highly internal behavior of the Windows NT Kernel (used in all of Microsoft’s modern operating systems today), I thought it was interesting to talk a bit about the internals themselves, and then present the actual bug, as well as some developer guidance on how to avoid hitting this bug.
I’d like to thank everyone who came to see the talk, and I really appreciated the questions and comments I received. I’ve also finally found out why Win32k functions are called “xxx” and “yyy”. Now I just need to find out about “zzz”. I’ll probably share this information in a later post, when I can talk more about Win32k internals.
As promised, you’ll find below a link to the slides. As for proof of concept code, it’s currently sitting on my home machine, so it’ll take another week. On the other hand, the slides make three of the bugs very clear in pseudo-code. I’ll re-iterate them at the bottom of the post.
One final matter — it seems I have been unable to make the bh08@alex-ionescu.com email work due to issues with my host. For the moment, please use another e-mail to contact me, such as my Google Mail account. My email address is the first letter of my first name (Alex) followed by my entire last name (Ionescu).
Bug 1:
1) Handle = OpenProcess(Csrss)
2) CreateRemoteThread(Handle, …, NtUserGetThreadState, 15, …)
Bug 2:
1) Handle = CreateWindowStation(…)
2) SetHandleInformation(Handle, ProtectFromClose)
3) CloseWindowStation(Handle)
Bug 3:
1) Handle = CreateWindowStation(…)
2) Loop all handles in the process with the NtQuerySystemInformation API, or, just blindly loop every handle from 4 to 16 million and mark it as protected, or, use Process Explorer to see what the current Window Station handle is, and hard-code that for this run. The point is to find the handle to the current window station. Usually this will be a low number, typically under 0x50.
3) SetHandleInformation(CurrentWinstaHandle, ProtectFromClose)
4) SwitchProcessWindowStation(Handle);