Mark Kleiman explains:
1. Written a letter to the President, the Attorney General, and the DCI saying "On Date X we received a briefing about certain intelligence activities, still ongoing and carried out as a matter of policy, in clear contravention of both the criminal laws of the United States and of treaty obligations. We insist that those activities stop forthwith, and that those who have engaged in them be considered for prosecution."
2. On the Senate side, demanded an Executive Session (no visitors, no staff) and named the date of the briefing, the briefers, and the word "torture." Even if that leaked, without details it couldn't constitute a security violation.
3. Filed a resolution naming the date of the briefing and ordering the members of the Gang of Eight to reveal what they were told to the two Houses. That resolution could have been accompanied by a statement saying "We have learned about serious violations of law being carried out as matters of policy. We think the Congress ought to have the option of either learning what we know or remaining in ignorance. In the meantime, we have demanded that the illegal activities stop forthwith."
That's not so hard, is it? Ya wimps.