Lerxst at Kautilyan has an interesting hypothesis

"Reading this article in Newsday by Knut Royce on Chalabi's ties to Iran, suddenly made me realize that it might have been Iran, which was behind a previous scheme to use forged documents concerning Iraq's nuclear ambitions, that might have been the ultimate source behind the Niger forgery."

However I am not convinced because the yellowcake dossier was too clumsy a fogery to be an official Iranian product. It was very obvious that the document was a forgery.

As Seymour Hersh wrote in The
New Yorker on 31 March 2003, "One letter, dated
October 10, 2000 was signed with the name of Allele Habibou, a [Nigerien] Minister of Foreign Affairs and Cooperation, who had been out of office since 1989."

I guessed that probably took the CIA about fifteen minutes to determine that the letter was a forgery using the weekly "Chiefs of State and Cabinet Members of Foreign Governments" recent issues of which they post on their web page.

in contrast the known Iranian forgeries were detected because " Some technical descriptions used terms that would only be used by an Iranian." Which is so much subtler that only the fact that Iraq had no bomb program makes me confident that the docutments were a forgery.

The clumsiness of the forgery supports the crook in the Nigerien embassy theory, since such a crook might want a document good enough to sell to "Panorama" but not good enough to cause a war. It would also fit the closely related crooked Italian spies hypothesis. Finally it fits with the CIA water cooler gossip reported by Hersh that the dossier was the product of disgruntled retired CIA analysts who wanted to see how far the obvious forgery would go up the Feith stove pipe but then were afraid to come clean when it went all the way to the State of the Union Address.

Iranian intelligence would have done a better job. They may be neomedieval theocrats but they are not total incompetents.