I've been seeing a lot of commentary about that elsewhere, but I'm not sure what the source is supposed to be. It's supposed to have happened between 1970 and 1972, years during which there's some uncertainty (from what I've seen) about Kerry's military status.

Some are convinced that he was still a member of the Naval Reserve at that time, which would make the talks, IIANM, a court-martial offense.

His FBI files report that he met with Communist leaders from North Korea on behalf of the VVAW, hoping to get some POWs released.

Here is a link

The Hee-row did indeed try to get the North Viets to release a few of our POWs to VVAW. He did negotiate directly with the representatives of the DRV in contravention of Federal law, I believe it's called the Miller Act, prohibiting private citizens from unauthorised negotiations on the behalf of the USA with any foreign government.
The Nixon Administration, in the throes of Watergate, ignored it.
Kerry could not have been court martialled, though. He held a reserve commission but was not on active duty of any kind at the time. His legal status was that of a civilian. Had he been prosecuted the Federal Courts would have had jurisdiction, not the Judge Advocate General.

Straight from the horse's mouth:
"I have been to Paris. I have talked with both delegations at the peace talks, that is to say the Democratic Republic of Vietnam and the Provisional Revolutionary Government..."--John F. Kerry, from his 1971 Senate testimony.

I had thought that it was more widely known that he had at least one meeting in Paris with the enemy in an unofficial capacity while he was still an officer in the Naval Reserve.

Oh, and just meeting with foreign officials isn't a violation of the UCMJ, but it is a violation of Federal Law (and the UCMJ, if you are military or reserve) for ANY citizen to meet with them to carry on political negotiations without official authorization.

Thus, the Presidential imprimatur on official "envoys", and the warning to Congresscritters meeting with foreign officials that they "are neither representing the United States nor the President". I cannot ever remember a prosecution under that Law, but I've never looked closely...

I plead woefully ignorant on that. Thanks to all y'all for clearing that up for me.

Well, IIANM stands for "if I am not mistaken," and I was. Thanks, all, for correcting the record.

A lot of commentators had been more outraged over the idea of a man still in active service doing that, but that would be moral rather than legal outrage.

Is there such a thing as legal outrage? For that matter, how about illegal outrage? Hmmmm...

legal outrage = "I strenuously object!"

Okay, one more time. While the UCMJ may or may not, I don't have a copy in front of me, have the same prohibition against 'private negotiations' as does the US Code, it doesn't matter.
The UCMJ only applies to Reservists on some kind of active service. The Hee-row was not on any kind of active status. Therefore his actions were not subject to the UCMJ. Federal Law? Yes. Federal Court? Yes. Court Martial? No.
Does it make The Hee-row's actions less outrageous? Not in my book.