A spokesperson from the US Embassy in Paris, responding to an article at The Verge about the accusations:

We categorically deny these allegations from unnamed sources, published in L’Express, that the United States government has participated in a cyberattack against France. France is one of our best allies. Our cooperation is remarkable in the areas of intelligence, law enforcement, and cyberdefense. It has never been as strong and essential to our common fight against the threat of extremism.

L’Express is the French publication that first ran the accusations of US involvement in a cyber-espionage attack on the offices of former French President Nicolas Sarkozy (while he was still in office). They asked the US Secretary of Homeland Security Janet Napolitano why the US would do such a thing, and she reiterated her French visit’s mission to strengthen the long-standing alliance between the two nations.

I have no reason to doubt the Embassy’s (or Secretary Napolitano’s) sincerity, but it’s an interesting thought experiment to consider that the US Embassy in France, and even the Secretary of Homeland Security, may not be aware of any covert cyber espionage operations the US carries out in France.