This is Nancy Berryhill. Until recently, she was the Acting Commissioner of Social Security and had primary responsibility for administering programs under the Social Security Act. She still works at Social Security as the Deputy Commissioner of Operations. But she no longer holds the title of Acting Commissioner. The Washington Post reports:

The vacancy creates problems for attorneys who appeal unfavorable Social Security disability decisions to federal court. Who do you sue? Typically, it is the Commissioner of Social Security but there is no Commissioner currently, not even on eon deck.

The answer may lie in a Presidential Memorandum issued by President Obama in October 2014 which addresses the order of succession within the Social Security Administration.  The Deputy Commissioner of Operations is designated as the first official to act and perform the functions of the Commissioner of Social Security when there is no Commissioner.  Pursuant to this memorandum, Nancy A. Berryhill appears to be leading the Social Security Administration in her capacity as Deputy Commissioner of Operations.