Since when is some bureaucrat memo considered law?
It isn't. But look at this from LINK: 26 USC 6103: Confidentiality and disclosure of returns and return information (closed executive sessions can be leaked I think).
(f) Disclosure to Committees of Congress
(1) Committee on Ways and Means, Committee on Finance, and Joint Committee on Taxation
Upon written request from the chairman of the Committee on Ways and Means of the House of Representatives, the chairman of the Committee on Finance of the Senate, or the chairman of the Joint Committee on Taxation, the Secretary shall furnish such committee with any return or return information specified in such request, except that any return or return information which can be associated with, or otherwise identify, directly or indirectly, a particular taxpayer shall be furnished to such committee only when sitting in closed executive session unless such taxpayer otherwise consents in writing to such disclosure.