The problem, though, is exactly what fidotron describes: without independent validation, we must trust the credibility of the source of a leak, for that leaked information to be considered valid.

If Snowden were a character if ill-repute or questionable morality (outside the question of the morality of leaking intelligence secrets to foreign countries), it requires that you question whether any of the leaked information may have been tampered with.

Once the relevant leaked-upon agency admits to the validity of information, then definitely the messenger's credibility becomes decoupled from the truthiness of the message.