By Robert Willmann
Demonstrating that he continues to learn about the application of governmental politics, Senator Rand Paul said that he plans to meet with president Trump today, 23 July 2018, and request that John O. Brennan's security clearance be revoked–
He asks: "Is John Brennan monetizing his security clearance? Is John Brennan making millions of dollars divulging secrets to the mainstream media with his attacks on @realDonaldTrump?"
This important issue is rarely stated, much less discussed as a topic in itself, and is not limited in relevance to Brennan. Paul asked back in January 2018 if FBI agent Peter Strzok and FBI attorney Lisa Page still had security clearances. Arrogantly delaying exactly three months to reply, the FBI liaison for congressional affairs tap danced in a letter and gave no real answer (a non-answer answer)–
Strzok was asked at the recent Congressional hearing if he had a security clearance, to which he answered in the affirmative. However, an article reported that the clearance was "limited" for purposes of the hearing–
A person keeping a security clearance after leaving government employment is not a bad thing on its face, but when an individual with that privilege appears to make dubious or less than candid statements before congress (to say it diplomatically), or to the public, that privilege should be canceled and revoked. In addition to Brennan, this issue can be thought about regarding others, such as former NSA directors Michael Hayden and Keith Alexander. Remember the little hearing from 2012, when Representative Henry C. Johnson, Jr. (Dem., Georgia), talked to Alexander?
Instead of struggling with the cumbersome procedures involved when holding a witness before Congress in contempt or issuing articles of impeachment, the House and Senate could simply either pass a law denying a certain person a security clearance, or ask the president to revoke a person's clearance as part of the negotiation process regarding legislation. After all, horse trading in Congress seems to apply to almost everything.
Although executive order 13526 is seen as the primary authority for classified information  — an interesting situation since it is an "executive order" — Congress could modify or repeal it. Just as Congress created most government departments and agencies, such as the CIA and Department of Homeland Security, it can modify them or close them down.
The Code of Federal Regulations (CFR) is the collection of regulations created by government departments and agencies. In general, these are the rules that can and have caused problems, as part of the "bureaucracy" and "administrative state". Federal regulations are not supposed to conflict with the law passed by Congress that authorized their creation. Whether the U.S. Constitution by its text even permits agency regulations and allows them to have legal effect is a real and interesting question, which no one will touch with a 10-foot pole.
In volume 32, CFR, part 2001 is where the regulations about national security information are found . Also relevant is direction from the Information Security Oversight Office (ISOO), in its "Marking Classified National Security Information" .
Not to get off the subject too much, but concerning the conduct of former Secretary of State and presidential candidate Hillary Clinton, and her private, home-brew e-mail server, you can read through the regulations — especially regarding "derivative classification" and the "electronic environment"  — as well as executive order 13526 and the ISOO handbook on marking classified information, and decide for yourself.
 Executive Order 13526–
 32 Code of Federal Regulations, part 2001–
 Marking Classified National Security Information, 2018 revision–
 32 Code of Federal Regulations, parts 2001.22 and 23–