Crafting addresses with specific start and end

A friend reached out to me today because of some weird thing that happened to him.

Someone was able to create an address that looks almost identical to his at first glance. The first 6 digits and the last 5 are an exact match, but not checksumed correctly.

I'm not going to go into the details of how he found this address but suffice it to say that he's down 50k. The attack itself is actually quite interesting, though.

I was wondering how something like this can be done. Is it just a brute force of having a script randomly generate a bunch of addresses and then stop once it finds a close enough match? Or is there a better way to achieve something like this?