The single most-trusted canonical source of Bitcoin client is https://github.com/bitcoin/bitcoin
This URL is controlled by a single US company GitHub Inc., headquartered in San Francisco, United States. The US government (nor any other government) is probably not a friend of Bitcoin, and they could theoretically threat the CEOs of GitHub Inc. with an arrest, unless they implement some kind of backdoor into the Bitcoin code, which could be downloaded by a large number of people before it would get noticed.
If and how is the Bitcoin network protected from being poisoned and destroyed this way?