You’ll say “But why would they enter through the doggie door?” but the law isn’t about why they would but the possibility. If someone got harmed breaking into your home because you’d set such a mantrap the fact that they were doing so (even illegally) would not prevent them from suing you and likely winning over any injuries they received.

I have heard they make doggie doors that are electronically unlocked by a transmitter in your dog’s collar. You might want to look into that kind of technology if this really worries you and your dog is so large the average burglar would be able to get in that way.

It is like an electronic key.

How, it works like this. You put this pet door on
with has an electronic device looking for an
electronic signal.
This electronic signal is attached to your dog’s collar.
When your dog gets near the door it automatically
opens up and lets your dog pass. And locks after
he goes through it. It also opens automatically if your
dog wants to get back inside the house. And locks
after he goes inside the house.
This also keeps other animals out like other
dogs you don’t know, cats, raccoons. rats, etc.

I saw this on a TV show years ago. I don’t know what
it is called exactly or who makes it. You might want to
check your local pet supply shop to see if they know
of it or do an internet search.

Now using razor blades on the door could also
cut and hurt your dog. I would not advise it.