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With a hex editor, debugger, and skills that most developers these days seem to lack.

I patched the kernel in memory first, using a kernel debugger, to verify my fix worked before editing the file on disk.

Aren't kernel modules signed?

not in xp, iirc, vista was the first release requiring it

Windbg or SoftICE? :)

Windbg; it's free and doesn't require any setup to do this: https://learn.microsoft.com/en-us/windows-hardware/drivers/d...

(Working out how to patch such that I wouldn't crash the system if a process happens to call that API while it was in a half-modified state was also a fun problem...)

I dug out the details on the bug I patched, and it isn't the same as this one; it's a race condition with timeouts on waiting for keyed events, which I believe isn't applicable in this situation as there are no timeouts.

Here's an example showing how to patch a userspace binary:

http://www.malsmith.net/blog/patching-closed-software/

Note: not my blog. (Edit: removed a probably unnecessary, and likely inaccurate, detail).

Patching the kernel would involve a similar (but slightly more complicated) process.

Windows xp and server 2003 sources (though incomplete AFAIR) were leaked back in 2020.