Article 1, Section 8 enumerates the entirety of Congress' powers with the exception of those enumerated in Amendments 13, 14, 15, 16, 19, 20, 23, 24, and 26, which are very specific. The only one that could possibly authorize the C.I.A. Is the Necessary and Proper clause of Article 1 Section 8 as it relates to raising and supporting Armies and maintaining a Navy, which are also enumerated powers. However, since the C.I.A. Has no formal, permanent relationship to those organization, it can hardly be called an appropriate means to that Constitutional end, let alone a necessary and proper one. Therefore, no, the C.I.A. Is not Constitutional.
The CIA is fundamental to the United States, as they help to defend the nation not only against threats from outside our nation, but also from threats inside. They help our military in different operations. The only aspect that concerns me now is the fact that they are often contacted by the President for missions, and now with the government passing new laws limiting the American peoples freedoms and liberties, I fear the CIA will be asked to handle people who oppose the new rules imposed by the government.
And even if it were unconstitutional, I doubt anything would be done about it. They're our secret apparition network across the globe providing crucial intelligence about dozens of nations on a daily basis. Could we really deem that unconstitutional? Could we really deem the defense of our nation through a secret network unconstitutional? I don't think so.
No, the CIA is not unconstitutional. However, some of their practices are. They've never had a strong code of ethics. However, some of the so-called "privacy laws" passed during the last Bush administration has sanctioned many of their unethical practices. Their main purpose is to protect us by secretly gathering information about events that may be harmful to the United States. But, I do believe that the manner in which the CIA carries out some of their missions borders on diabolical.
My guess is that there is nothing in the constitution that would forbid the existence of an agency such as the CIA. Its primary role is intelligence gathering in defense of the country. I imagine back even so far as when the constitution was being written there were people whose assigned role was to gather information, they just didn't have an agency. Like any agency with immense powers at their disposal they might be inclined to overstep an individual's rights, so they should be subject to an oversight committee and held accountable at some level, but their mere existence is not unconstitutional.