Actually, I think the opposite is true; in appsec, the final work product is a list of vulns, so if you want to cheat, it only takes 4 minutes talking to anyone else who has ever succeeded in the process before.
For a dev interview, taking something your team has built and scooping out some of its functionality seems like a test that is significantly harder to cheat on.
For my part: we ran this process for over two years and never discovered any plagiarism. Meanwhile, we drastically increased the size of our team and had total retention; from the time I took over recruiting to the time I left the firm, we lost a total of zero of those hires. All of them worked out.
The problem I'd see with using your own app and taking out some functionality, is that the first few people you throw at it, you have no data points to verify against. I.e., they do it 'wrong' (it's incomplete, it's not what you meant, whatever). What does that mean? They don't have the domain knowledge your team has, and that may be important.
I'd think better would be to have a few of your devs come up with a small app, and set of instructions, that you pass to others of your devs. They can then create a reasonable baseline you can start to measure against. You may need to be a little forgiving initially, if it's not a completely made up example, but it probably is more valid than something that was built for production, with months of domain knowledge behind it.
I've long thought that to come up with a good work-product test a company should continually save off their own bugs to put into a broken test app, after which candidates can fix the bugs and the company can see how candidates solve the problems compared to the people they've already hired. The actual production code is the baseline.
You're right that it's definitely pretty easy to cheat if you have insider knowledge of some sort, but the odds of a candidate leaking the answers are probably pretty low unless you're a huge company and don't change up your tests.
For a dev interview, taking something your team has built and scooping out some of its functionality seems like a test that is significantly harder to cheat on.
For my part: we ran this process for over two years and never discovered any plagiarism. Meanwhile, we drastically increased the size of our team and had total retention; from the time I took over recruiting to the time I left the firm, we lost a total of zero of those hires. All of them worked out.