Teen's Side Hustle Became a Multi-Hundred-Million-Dollar Business

Entrepreneur

08-07-2025

It's the era of the side hustle, and if you've ever considered starting one to earn some extra cash outside of your 9-5, you're in good company. These days, more than one-third of U.S. adults have side hustles, and their supplemental gigs make an average of $891 a month, according to recent research from Bankrate. Of course, the most successful side-hustlers see much higher earnings, especially when they start a business that brings in nearly as much as — or significantly more than — their full-time sources of income.
Marcin Kleczynski, 35, is a Bay Area-based entrepreneur who was a teenager when he "unofficially" started the side hustle that would become a cybersecurity firm. Now the CEO, he and his co-founder Bruce Harrison have grown the venture into a multi-hundred-million-dollar business. Responses have been edited for length and clarity.
Image Credit: Courtesy of Malwarebytes. Marcin Kleczynski.
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What was your day job or primary occupation when you started your side hustle?
Before heading off to college, I worked at a local break-fix computer shop called Computer Express. It was a small, scrappy place, but it gave me invaluable hands-on experience fixing all kinds of machines. I got to see firsthand how often people were struggling with malware and tech problems, and that experience stuck with me. It's where the idea for Malwarebytes for me was planted and started to take shape.
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When did you start your side hustle, and where did you find the inspiration for it?
Malwarebytes unofficially started in 2004, when I was still a teenager, but things really took off around 2008. The inspiration came from when I downloaded a video game from a sketchy website and ended up infecting my family computer with malware. I tried everything to fix it, but none of the big-name antivirus tools could fully clean it. That moment lit a fire in me to build a better solution.
What were some of the first steps you took to get your side hustle off the ground?
I started by picking up one of those classic yellow For Dummies programming books — I figured if I could learn to build a calculator, I could build something useful. From there, I released my first malware removal tool on online forums where people helped each other clean infected computers. To my surprise, it took off and became a go-to resource, eventually used by tens of millions of people.
Are there any free or paid resources that have been especially helpful for you in starting and running this business?
Online forums were everything. I had amazing mentors who freely offered to give feedback, answer questions and point me in the right direction. I also relied on free websites and resources to teach myself how to code. It was basically like a virtual apprenticeship and crowdsourced education, which made all the difference.
If you could go back in your business journey and change one process or approach, what would it be, and how do you wish you'd done it differently?
I would have professionalized the business sooner. We were a group of passionate coders, but we waited too long to bring in people with experience scaling companies. Investing in experienced leadership earlier could have saved us a lot of stress and probably helped us grow faster, too.
Related: At Age 23, He Started a Side Hustle While on Welfare. It Led to a 7-Figure Business and a Stay on Richard Branson's Private Island.
When it comes to this specific business, what is something you've found particularly challenging and/or surprising that people who get into this type of work should be prepared for, but likely aren't?
Cybersecurity is not easy. Cybercriminals are just as scrappy as early-stage founders. They have time, money and skill, and they're constantly evolving. It's a constant back-and-forth. That makes the work high-stakes and fast-moving, which most people outside the industry don't fully appreciate.
Can you recall a specific instance when something went very wrong? How did you fix it?
Nearly a decade ago, we had a false positive that blocked a critical range of IP addresses, including our own update servers, as malicious. It was a major incident, but our team responded with incredible resilience. We spent days recovering and working with our customers. That experience really taught us the value of strong rollback processes and just how high the stakes are in cybersecurity.
How long did it take you to see consistent monthly revenue? What does growth and revenue look like now?
We started seeing revenue within days of launching our first product, and within four months, it became serious enough to make me rethink staying at university. It was an early indicator that we were solving a real problem, and people were willing to pay for it.
Malwarebytes is now a several-hundred-million-dollar business — growing, profitable and serving millions of users worldwide. We've expanded from consumers to enterprises and managed service providers (MSPs), and we're continuing to scale.
Related: After a 12-Year-Old's Side Hustle Made Over $4,000 in 1 Day, He and His Dad Grew the Business to Nearly $50,000 a Month: 'It Takes Commitment'
What do you enjoy most about running this business?
Honestly, what I enjoy most is building products and the speed at which we can scale them. Turning an idea into code, into a product, and seeing it used by millions of people within days — that's the magic. It's deeply fulfilling and never stops being exciting.
What is your best piece of specific, actionable business advice?
In the early days, it's all about the team. Find co-founders and early employees who are just as passionate as you are. Focus on building something great, and let the product lead the way. Then invest deeply in your early adopters — their feedback is the most valuable compass you'll have.
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