@Finn dAbuzz,
Flynn's security clearance was renewed by the Obama administration in April, 2016. This was after he had taken payments from Russians. Despite intensive investigation of Flynn, who was a staunch critic of Obama, after that (especially after July, 2016), the Obama administration never sought to revoke the security clearance it had recently renewed.
It is now claimed that Obama advised Trump not to hire Flynn. If Obama thought there were any national security issues raised by Flynn, why didn't he, as sitting president, revoke his clearance then? One must presume that security concerns were NOT the reason Obama advised against him. More likely it was Obama's well-known dislike of Flynn's management style that was the basis of the advice.
Only a dyed-in-the-wool cheese-eater can't see the obvious implications of Obama's actions/inactions here, eh?
Sally Yates' stated rationale for "warning" Trump about Flynn in late January was NOT that Flynn had done anything illegal. It was that Flynn had lied to Trump, that Russia knew and could prove this, and that, therefore, Flynn might be subject to blackmail by the russians.