The FBI's called him the "Batman of the internet", but he calls himself the Jester.
An anonymous hacker on Saturday (NZ time) managed to put a message on the front page of the Russian Ministry of Foreign Affairs' website.
"Comrades! We interrupt regular scheduled Russian Foreign Affairs Website programming to bring you the following important message," he wrote. "Knock it off. You may be able to push around nations around you, but this is America. Nobody is impressed."
The US has pointed the finger at Russia over hacks into the Democratic Party's email systems, giving what they've found to groups like WikiLeaks.
WikiLeaks has published thousands of internal party emails in recent months, taking a strong stand against the party's presidential nominee, Hillary Clinton. Russia has denied being behind the hacks.
The Jester's message (supplied)
"Let's get real, I know it's you, even if by-proxy, and you know it's you," the Jester wrote. "Now, get to your room. Before I lose my temper."
The Jester told CNN he did it in response to a recent massive attack on the US internet, which disrupted access to services like Spotify and Reddit.
"I wanted to poke them in the eye and stop feeling like US is just taking it on the chin. Again. I'm not gonna sit around watching these f***ers laughing at us."
The Russian Ministry of Foreign Affairs said the site was never taken offline.
"The website of the Russian Foreign Ministry, which had been reportedly penetrated by an American hacker, according to CNN, is and has been functioning normally," spokeswoman Maria Zakharova said.
On his own site, the Jester said he used a "simple XSS injection" to put his words on the Russians' site.
"This was not meant to be an earth-shattering mega-hack. It was a gentle light hearted message, for which I do not apologise."