I am building a decentralized community service, where it is critical for each client to run a "legitimate" build version in order for the service to operate properly. The client versions do not have to be identical, but must be guaranteed to be compliant with the service in order to function as intended.
Unauthorized changes therefore will most likely have the intent to access information that the specific user is not authorized to access or alter functionality away from the intended. In the context of the client software, it just doesn't make practical sense for some modification to exist in one client, as it will be incompatible with the service on a network level anyway.
Such modifications are possible to not only tamper with the modifying user's copy of the program, but alter the functionality of other client's copies in unintended ways, which does constitute a violation of all those users ability to use the service as intended, and possibly even jeopardize their security, assets or intellectual property.
Therefore I would like to license my code in a way that explicitly prohibits any and all forms of illicit or malicious use or modification. Modifications are still possible, as long as they go through the official channel and are verified to be fully compliant by the community. The end user is still able to get his modifications in an approved code form and build and use that, but technically the user is not allowed to hack his copy of the client willy-nilly.
Can this license requirement work with GPL licensed code? Because I don't recall there being much emphasis on legality of use in GPL, if any, and quite a lot of focus on "you are free to do whatever you want as long as you share your code".
Where does the GPL stand in this de facto "usage limitations" that says "the user does not have the freedom to violate the freedoms of other users"? Maybe legality of use is outside of the GPL scope, and presumed that users are implicitly mandated to abide community or legal rules, or something like that?
To put it GPL-ish form, the user is free to make modifications to the product, provided that they do not impede other users' ability to use the product.
So, is this usage restriction compatible with GPL?