The waivers policy, which President Obama says will help more welfare recipients find work, has been a major source of conflict between congressional Republicans and the White House.
Federal health officials touched off a firestorm last summer when they announced that states could apply for flexibility under the landmark 1996 welfare law that introduced work requirements.
HHS said states could apply for waivers in order to test new approaches for moving welfare recipients into jobs. The new approaches would have to be successful, officials warned, or states would lose their waivers.
GOP critics said the policy was designed to "gut" welfare's requirement that some recipients hold jobs, a charge the administration denies.
The attack gained traction during the 2012 presidential campaign, and House Republicans have twice passed legislation to prevent the waivers from moving forward.
As of March, no states had applied for waivers and none have been granted.
The memo released Tuesday provides a wide-ranging legal justification for a hypothetical waivers policy.
The author, whose name is redacted, told HHS Deputy Assistant Secretary for Policy Mark Greenberg that previously discussed changes to welfare "appear to be defensible exercises of the Secretary's discretion."
"However," the author added, "whether a particular project is legally supportable will depend on the facts and circumstances surrounding that project."
House Ways and Means Committee Chairman Dave Camp (R-Mich.) blasted the Obama administration for seeking to "increase benefits and disconnect them from work."
"This memo seriously undermines the Obama administration's timeline and credibility," Camp said in a statement.