That's why proper backups are done in a pull-configuration and not the easy way of push. I.e., the backup system has access to the production system, and grabs

That's why proper backups are done in a pull-configuration and not the easy way of push. I.e., the backup system has access to the production system, and grabs the data itself. The production system can at worst serve bad data to the backup system, against which the protections are not that hard. Basically just a protocol client that can handle malicious servers, like a HTTP(S) server/client or such.