In 2013, a teenager named Jann Horn attended a reception in Berlin hosted by Chancellor Angela Merkel. He and 64 other young Germans had done well in a government-run competition designed to encourage students to pursue scientific research.
In Horn’s case, it worked. Last summer, as a 22-year-old Google cybersecurity researcher, he was first to report the biggest chip vulnerabilities ever discovered. The industry is still reeling from his findings, and processors will be designed differently from now on. That’s made him a reluctant celebrity, evidenced by the rousing reception and eager questions he received at an industry conference in Zurich last week.
Interviews with Horn and people who know him show how a combination of dogged determination and a powerful mind helped him stumble upon features and flaws that have been around for over a decade but had gone undetected, leaving most personal computers, internet servers and smartphones exposed to potential hacking.
Other researchers who found the same security holes months after Horn are amazed he worked alone. "We were several teams, and we had clues where to start. He was working from scratch," said Daniel Gruss, part of a team at Graz University of Technology in Austria that later uncovered what are now known as Meltdown and Spectre.
Horn wasn’t looking to discover a major vulnerability in the world’s computer chips when, in late April, he began reading Intel Corp. processor manuals that are thousands of pages long. He said he simply wanted to make sure the computer hardware could handle a particularly intensive bit of number-crunching code he’d created.
But Zurich-based Horn works at Project Zero, an elite unit of Alphabet Inc.’s Google, made up of cybersleuths who hunt for "zero day" vulnerabilities, unintended design flaws that can be exploited by hackers to break into computer systems.
So he started looking closely at how chips handle speculative execution — a speed-enhancing technique where the processor tries to guess what part of code it will be required to execute next and starts performing those steps ahead of time — and fetching the required data. Horn said the manuals stated that if the processor guessed wrong, the data from those misguided forays would still be stored in the chip’s memory. Horn realized that, once there, the information might be exposed by a clever hacker.
"At this point, I realized that the code pattern we were working on might potentially leak secret data," Horn said in emailed responses to Bloomberg questions. "I then realized that this could — at least in theory — affect more than just the code snippet we were working on."
That started what he called a "gradual process" of further investigation that led to the vulnerabilities. Horn said he was aware of other research, including from Gruss and the team at Graz, on how tiny differences in the time it takes a processor to retrieve information could let attackers learn where information is stored.
Horn discussed this with another young researcher at Google in Zurich, Felix Wilhelm, who pointed Horn to similar research he and others had done. This led Horn to what he called "a big aha moment." The techniques Wilhelm and others were testing could be "inverted" to force the processor to run new speculative executions that it wouldn’t ordinarily try. This would trick the chip into retrieving specific data that could be accessed by hackers.
Having come across these ways to attack chips, Horn said he consulted with Robert Swiecki, an older Google colleague whose computer he had borrowed to test some of his ideas. Swiecki advised him how best to tell Intel, ARM Holdings Plc. and Advanced Micro Devices Inc. about the flaws, which Horn did on June 1.
That set off a scramble by the world’s largest technology companies to patch the security holes. By early January, when Meltdown and Spectre were announced to the world, most of the credit went to Horn. The official online hub for descriptions and security patches lists more than ten researchers who reported the problems, and Horn is listed on top for both vulnerabilities.
Wolfgang Reinfeldt, Horn’s high school computer-science teacher at the Caecilienschule in the medieval city of Oldenburg about 20 miles from Germany’s north coast, isn’t surprised by his success. “Jann was in my experience always an outstanding mind,” he said. Horn found security problems with the school’s computer network that Reinfeldt admits left him speechless.
As a teenager he excelled at mathematics and physics. To reach the Merkel reception in 2013, he and a school friend conceived a way to control the movement of a double pendulum, a well-known mathematical conundrum. The two wrote software that used sensors to predict the movement, then used magnets to correct any unexpected or undesired movement. The key was to make order out of chaos. The pair placed fifth in the competition that took them to Berlin, but it was an early indicator of Horn’s ability.
Mario Heiderich, founder of Berlin-based cybersecurity consultancy Cure53, first noticed Horn in mid-2014. Not yet 20, Horn had posted intriguing tweets on a way to bypass a key security feature designed to prevent malicious code from infecting a user’s computer. Cure53 had been working on similar methods, so Heiderich shot Horn a message, and before long they were discussing whether Horn would like to join Cure53’s small team.
Heiderich soon discovered that Horn was still an undergraduate at the Ruhr University Bochum, where Heiderich was doing post-doctoral research. Ultimately, he became Horn’s undergraduate thesis supervisor, and Horn signed on at Cure53 as a contractor.
Cybersecurity specialist Bryant Zadegan and Ryan Lester, head of secure messaging startup Cyph, submitted a patent application alongside Horn in 2016. Zadegan had asked Horn, through Cure53, to audit Cyph’s service to check for hacking vulnerabilities. His findings ended up as part of the patent and proved so significant that Zadegan felt Horn more than merited credit as one of the inventors. The tool they built would ensure that, even if Cyph’s main servers were hacked, individual user data were not exposed.
“Jann’s skill set is that he would find an interesting response, some interesting pattern in how the computer works, and he’s just like ‘There’s something weird going on’ and he will dig,” Zadegan said. “That’s the magic of his brain. If something just seems a little bit amiss, he will dig further and find how something works. It’s like finding the glitch in the Matrix.”
Before long, Cure53’s penetration testers were talking about what they called "the Jann effect" — the young hacker consistently came up with extremely creative attacks. Meltdown and Spectre are just two examples of Horn’s brilliance, according to Heiderich. "He’s not a one-hit wonder. This is what he does."
After two years at Cure53 and completing his undergraduate program, Horn was recruited by Google to work on Project Zero. It was a bittersweet day for Heiderich when Horn asked him to write a recommendation letter for the job. "Google was his dream, and we didn’t try to prevent him from going there," he said. "But it was painful to let him go."
Horn is now a star, at least in cybersecurity circles. He received resounding applause from fellow researchers when he presented his Spectre and Meltdown findings to a packed auditorium at a conference in Zurich on Jan. 11, a week after the attacks became public.
With bowl-cut brown hair, light skin and a thin build, Horn walked his fellow researchers through the theoretical attacks in English with a German accent. He gave little away that wasn’t already known. Horn told the crowd that after informing Intel, he had no contact with the company for months until the chipmaker called him in early December to say other security researchers had also reported the same vulnerabilities. Aaron Stein, a Google spokesman, has a different account though: "Jann and Project Zero were in touch with Intel regularly after Jann reported the issue."
When a fellow researcher asked him about another possible aspect of processor design that might be vulnerable to attack, Horn said, with a brief-but-telling smile: "I’ve been wondering about it but I have not looked into it."