Sounds illegal. But what do I know?
Comey said that the memo(s) he leaked were written as a private citizen.
The opposing view is that the memos are “government” memos and, therefore, they are technically government property and can only be released with official approval.
If the memos had an FBI Header on them or any other official writing or classification, it would be tough to show they were written by him as a “private citizen”.