I don’t have much to say in this post, but since I looked at a sample file for a few minutes, it’s good to earn some brownie points by just describing what I have observed. Especially that quick google search returned nothing on the topics I want to cover here — who knows, maybe one day it will be helpful to someone… + there are some pointers for further research anyway.
The file I looked at is a very old (2003!) signed hotfix installer from Microsoft. I got curious about these, because they are very old, and signed, hence a possible target for a good LOLBIN.
These Hotfixes were coded in ancient times, the modern security assumptions and considerations were simply not there, and it’s a possible goldmine for new, interesting ideas of Bring Your Own LOLBIN or Bring Your Own Vulnerability type of scenarios.
As it is usually the case, my attention eventually got diverted and instead of looking at LOLBIN-ability of the file, I just started browsing the code and was pleasantly surprised with some quick & imho interesting DFIRCE findings. Some of these could be actually handy to me ~10 years ago.
Things observed are described below.
If you see a <drive>:\[hexdigits] folder during your exam, it is a high chance it is from a Hotfix/Update. When the SFXCAB-based Hotfix/Update is executed it just drops the installation files there f.ex.:
This is not unusual, but I have seen similar folders so many times that I need to make a quick comment here. The perverted interest of many Windows HotFix/Update packages (including redistributables) to either use a temporary folder in the root directory of C:\, or any other available drive really is something I still don’t fully understand.
(I remember deleting many of these on my own systems at the time I was still updating my OS in an automated fashion. I simply don’t trust auto-updates anymore and often end up testing updates to anything let it be OS or a browser on VM before I deploy it on my main system. It’s crazy. it shouldn’t be like this. But luckily this post is not about this; This one is about it tho).
Anyway… It goes against the old Microsoft mantra of using the %TEMP% directory for this purpose – user- or SYSTEM-based, depending on the need. Oh well…
The installer uses/expects a number of very characteristically named environment variables which I have not seen described online before:
Again, I couldn’t find much info about these online, and I was not too inclined to find out what they are, but… the _SFX_SourceFilesURL is an interesting one as it adds additional source URLs for the updater/fixer to consider while it is patching the system. Who knows… maybe it is a nice LOLBIN possibility after all? Will need to play with it a bit more…
Other interesting DFIR artifacts are various log files created by these packages:
If you see these files on the examined system, you may now have a better way to understand where they come from.
Finally, the hotfix programs accept standard command line arguments:
- /f Forces other programs to close at shutdown.
- /n Does not back up files for removing hotfixes.
- /z Does not restart the computer after the installation is completed.
- /q Uses quiet mode; no user interaction is required.
- /m Uses unattended Setup mode (Windows 2000).
- /u Uses unattended Setup mode (Windows XP).
- /l Lists installed hotfixes.
– these don’t provide much additional info, but if you see these in a context of a hotfix/update e.g. via EDR/sysmon logs, then you can at least decipher their meaning and understand what the setup program is doing.