Sunday, February 3, 2013

HWND_BROADCAST

A few years ago while working on Windows sandboxing, I noticed a few relatively minor problems with Job Objects, Desktops and related facilities. I reported them to Microsoft, who said they don't consider these supported security boundaries and declined to fix them, but this was no big deal and I dropped the issue. The chrome security guys developed techniques to workaround some of these bugs in Chrome instead.

One of the problems I described was how broadcast messages were exempt from UIPI, and gave an example of how a LI process can broadcast WM_CHAR messages which would be interpreted as input to any open MI/HI command prompts.

The attack was not devastating, because an attacker would have to wait for a user to open a command prompt as MI or HI and then take over, which is a pretty weak attack. However, a few weeks ago I noticed a Microsoft blogger claiming that this exact scenario was impossible.

Unable to resist correcting him, I posted a comment on his blog and tweeted about it.

A few weeks later, Microsoft fixed it in MS13-005. This surprised me, because I didn't know a really good attack to abuse it, and Microsoft previously told me they were not interested. I figured Microsoft must have discussed it internally, and had realised a better way to exploit it.

A few days later I realised what the attack was.

LI processes can still trigger Global Hotkeys with keybd_event, so if I enumerate all the hotkeys registered in a default installation, maybe one of these will offer the solution. I put a kd breakpoint on NtUserRegisterHotkey and enumerated them all.

I think I figured it out, here is the attack I think Microsoft realised before I did:

  • From a Low Integrity process, spawn a cmd.exe and wait for explorer to add it to the task list.
  • Use keybd_event to send Win+Shift+[1 ... 9]
  • Explorer will spawn a new cmd.exe, which will inherit Medium Integrity from explorer.
  • Use SendMessage with HWND_BROADCAST to send WM_CHAR messages.
  • Drive the command prompt to send any new command you want, along with some ASCII art skulls to make it look like a scene from a Hollywood movie.
Apparently Packetstorm are offering a reward for a working implementation of this, so be my guest if you want to practice your Win32 scripting skills.

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