The Washington Post reported late Monday that in early January, Acting Attorney General Sally Yeats told the President's legal adviser that the Russians had evidence that Flynn had talked with the Russian ambassador about sanctions before the inauguration. The point was that this made Flynn potentially vulnerable to blackmail by the Russians. because it was illegal for him to have done that as a civilian. The CIA Director and the Direction of National Intelligencve shared Yates' concern about Flynn.
As far as can be discerned at this time, the White House did nothing about it. Pence was obviously surprised when he learned that Flynn had lied to him. So did the White House counsel tell Trump? If so, then this implicates Trump in doing nothing.
In a side issue, it was also revealed Monday that a top Flynn aide was denied the security clearance he needed for the job and that the CIA Director had approved the denial. Is this in-fighting (CIA against Flynn?) or was there a good individual reason for the denial? No answer to that one yet.
PS: Late night update: Gen. Flynn has resigned. In his letter he acknowledges no wrong-doing except "inadvertently giving Vice President Pence incomplete information about my phone calls with the Russian ambassador."