SAN FRANCISCO, April 15, 2026 /PRNewswire/ — AI has upended the foundation of open source security, and commercial open source applications must close their code to protect sensitive data.

“Open source security always relied on people to find and fix any problems,” said Peer Richelsen, co-founder of Cal.com, the world’s largest Next.js project. “Now AI attackers are flaunting that transparency.”

Anthropic’s Mythos model proved in early April that it was capable of breaking into some of the more secure software systems in the world, including OpenBSD, which has a huge emphasis on security.

“Open source code is basically like handing out the blueprint to a bank vault,” said Bailey Pumfleet, CEO of Cal.com. “And now there are 100× more hackers studying the blueprint.”

As one of the largest and fastest-growing open source startups globally, Cal.com has seen its security demands escalate in the last four months. This shift reflects a broader trend where open source projects are becoming high-value targets.

Third party security experts confirm the vulnerability inherent in open source.

Open source applications are 5–10× easier to exploit than closed, according to Huzaifa Ahmad, CEO of Hex Security.

The result is a fundamental shift in the software economy. Companies with open code will be forced to risk customer data or close public access to their code.

“We are committed to protecting sensitive data,” Pumfleet said. “We want to be a scheduling company, not a cybersecurity company.”

Today, Cal .com announced it is moving to a closed source model.

“Cal.com handles sensitive booking data for our users,” Pumfleet said. “We won’t risk that for our love of open source.”

Cal.com has also released Cal.diy, a fully open-source version of its platform for hobbyists. The open project will allow experimentation apart from the closed application which will handle high stakes data.

About Cal.com
Cal.com is one of the fastest-growing infrastructure companies in its category and maintainer of the world’s largest Next.js open source project. Founded in 2021, the company raised a $25M Series A backed by Alexis Ohanian, Obvious Ventures, Jack Altman, Tobi Lütke, Naval Ravikant, Chad Hurley, and Balaji Srinivasan.

Contact: [email protected]

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