The Trump transition team did not immediately return CNBC's phone call or emails requesting comment.
Earlier, the Trump transition team said Icahn "will be advising the President in his individual capacity and will not be serving as a federal employee or a Special Government Employee and will not have any specific duties."
Still, Norman Eisen, the former chief ethics lawyer to President Barack Obama, said Icahn should beware, because the situation may pose a violation of conflict of interest and other laws, some of which could carry a criminal penalty. He said Icahn's duties may go beyond informal advising, given that he has a formal title as special advisor, sweeping responsibilities and potentially influence over personnel choices.
"It appears that he may end up as a de facto special government employee. As such, he would be subject to the conflicts rules, including under 18 USC 208," Eisen told CNBC, referring to a law that restricts people with a financial interest from advising the president.
"If so, that would make him potentially criminally liable if he worked on a repeal of the ethanol mandate that would boost CVR's stock price and enrich him. Of course, that's a lot of ifs, and we have to see what his actual conduct is," said Eisen, now a fellow at the Brookings Institution.
In a statement to CNBC, Jesse Lynn, general counsel at Icahn Enterprises, said, "Mr. Icahn is well aware of his obligations under the law generally and with respect to 18 U.S.C. 208 specifically. He will follow the law as he always has."
He reiterated the Trump Team's message that "unlike a government employee, [Icahn] will have no official role or duties." He added that Icahn will not be in a position to set policy, but will instead offer suggestions.
Eisen said the press release announcing Icahn's appointment does not provide sufficient details about what Icahn "will and will not do, and how he will avoid tripping into" the role of special government employee.