For the second point, I would say that SQLite has a massive attack surface, it would be very difficult to ensure that that technique can’t lead to an exploit of some form.
Unless there is some way to introduce a malicious side effect to a select statement in sqlite?
As for introducing a malicious side effect into the query, that's simple: just add an UPDATE, DELETE, CREATE, or INSERT. When you say that the importer can only run SELECT statements, do you mean that it's only authorised to make SELECT statements, or are you simply assuming that the importer won't be able to mutate any data? Because I suspect it's the latter, and that's not correct. I really truly hope your application is not responsible for anything important.
If you depend on users (attackers) not being able to modify their software or environment and poke around at each and every bit of your (publicly accessible) interfaces you are doing something awfully wrong!
> but the user needs to be authenticated to upload a database
Is registration for your service limited to a fixed amount of trustworthy people? Otherwise this isn't an obstacle.
> the lambda that extracts the data is sandboxed to access only what it needs
Using a simple serialisation format would be orders of magnitudes safer (and simpler)
> Unless there is some way to introduce a malicious side effect to a select statement in sqlite?
See all the links posted here already