Windows System Software -- Consulting, Training, Development -- Unique Expertise, Guaranteed Results


Before Posting...

Please check out the Community Guidelines in the Announcements and Administration Category.

More Info on Driver Writing and Debugging

The free OSR Learning Library has more than 50 articles on a wide variety of topics about writing and debugging device drivers and Minifilters. From introductory level to advanced. All the articles have been recently reviewed and updated, and are written using the clear and definitive style you've come to expect from OSR over the years.

Check out The OSR Learning Library at:

Alternative ways to load a class filter driver

Benjamin_HöglingerBenjamin_Höglinger Member - All Emails Posts: 16

Greetings everyone!

I've come across at least one driver in the wild that somehow manages to act as a class filter driver without having any of the common registry values UpperFilters and LowerFilters in place and without modifying any of the existing device's INF files.

How is this wizardry achieved? Is this a valid practice but simply not publicly documented or most probably some "API-hookery" that runs under the radar of PatchGuard etc.?

And most importantly: how can I do that myself? :grin:

Thanks for reading, cheers


  • Doron_HolanDoron_Holan Member - All Emails Posts: 10,714
    What class are you seeing this with? And why do you want to go with a potentially undocumented or unsupported way to install a class filter?
  • Benjamin_HöglingerBenjamin_Höglinger Member - All Emails Posts: 16

    Hi Doron,

    as per my emoji this was more of the lighthearted joking side. I am primarily interested in how that is achieved.

    Classes I've seen this with: HICClass, XnaComposite and XboxComposite


  • Benjamin_HöglingerBenjamin_Höglinger Member - All Emails Posts: 16

    A bit more context: I've observed older versions of said driver and back then the authors used the usual way of going with the registry values. For whatever reason (to remain undetected? To work around non-disclosed bugs with another 3rd party driver?) they switched to this "other method" without losing functionality but not being visible in the usual locations (not in Device Manager's filter properties but definitely present in the device's driver stack).

    What got me curious: I found another 3rd party HID device that struggles to boot when a filter is present (bug? intentional? I don't know) and interestingly enough it works with the mysterious class filter nonetheless.

    All very weird jet very interesting.


Sign In or Register to comment.

Howdy, Stranger!

It looks like you're new here. Sign in or register to get started.

Upcoming OSR Seminars
OSR has suspended in-person seminars due to the Covid-19 outbreak. But, don't miss your training! Attend via the internet instead!
Writing WDF Drivers 12 September 2022 Live, Online
Internals & Software Drivers 23 October 2022 Live, Online
Kernel Debugging 14 November 2022 Live, Online
Developing Minifilters 5 December 2022 Live, Online