On Narutopedia, it states that Kabuto is officially a genin and "acquired" the rank while being a spy in the chunin exams, but now we know that he was involved with Root long before, so shouldn't he be an ANBU?
As far as rank goes, he didn't even legitimately get the gennin rank. He basically has no official rank. He is comparable to a few higher ninja ranks but holds no rank.