[bgmeister]

If you're only using remote caching (ie no remote execution) then all cache clients need to trust each other, because a malicious client can upload any result it wants to a given ActionCache key, and there's no way to verify the ActionCache entries are correct unless the actions are reproducible. (And verifying ActionCache entries by rerunning the actions kind of defeats the purpose of using a build cache.)

If you don't want clients to have to trust each other, then you can block ActionCache write access to the clients and add remote execution. In this setup clients upload an action to the CAS, remote executors run the action and then upload the result to the ActionCache, using the hash of the action as the key. This way malicious clients can't spoof cache results for other clients, because other clients won't ever look for the malicious action's key in the ActionCache.