Holy cow, I wrote a book!
Windows File Protection works by replacing files after they have
Why didn't Windows just apply ACLs to deny write permission to the
We tried that.
It didn't work.
Programs expect to be able to overwrite the files.
A program's setup would run and it decided that it needed to "update"
some system file and attempt to overwrite it.
If the system tried to stop the file from being overwritten,
the setup program would halt and report that it was unable
to install the file.
Even if the operating system detected that somebody was trying
to overwrite a system file and instead gave them a handle to
those programs would nevertheless notice that they had been
hoodwinked because as a "verification" step,
they would open the file they had just copied
and compare it against the "master copy"
on the installation CD.
The solution was to let the program think it had won,
and then, when it wasn't looking,
put the original back.
Now that Windows File Protection has been around for a few years,
software installers have learned that it's not okay to overwrite
system files (and trying to do it won't work anyway),
so starting in Windows Vista,
the Windows File Protection folks have started taking stronger
steps to protect system files,
and this includes using ACLs to make the files harder to replace.
Presumably, they will have compatibility plans in place to
accomodate programs whose setup really wants to overwrite a file.