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Over the past three years, Einaras Gravrock has turned his concerns as a parent into a fast-growing cybersecurity start-up.

Gravrock had grown anxious about his children’s privacy when he heard about baby monitors being hacked. By the time they started playing with iPads, he wanted protection. What he ended up with is Cujo, a bowl-size firewall device that hard-wires into a home router, providing for digital security what a guard dog brings to physical defense.

Gravrock believed enough in the idea that he divested from and stepped down as chief executive of Iconery, an online shop for jewelry that he co-founded.

Gravrock raised $330,000 off a Kickstarter crowdfunding page, opened headquarters in El Segundo, Calif., and agreed to a manufacturing deal with a factory in Illinois. Now, Cujo is nearing 100,000 users and 100 employees. The device is stocked at Best Buy and on Amazon.com for $250.

Cujo picked up $11 million from 27 investors in a recent financing round, including earlier loans converted into stock. Investors include TA Ventures in Ukraine, University of Southern California adjunct business professor Ivan Nikkhoo and cybersecurity expert Yuri Frayman.

Gravrock knows that what he’s selling won’t stop every computer virus or hacker. But he sees $250 as a necessary expense to deter the inconveniences that come with being hacked. The average user sees about five to seven suspicious connections blocked each week, he said.

“For someone installing ADT or a Ring, it’s the next frontier,” he said, referring to home security options. “Cujo is not that silver bullet, but being vigilant and educated about the problem, you have to apply best practices.”

Others in the industry back up that view.

“It’s a good step in the right direction for home consumers,” said Adriel Desautels, chief executive of Netragard, which tests security at companies in gaming, health and finance. “It’s going to eliminate the low-hanging fruit.”

Cujo has rivals on three fronts: Router makers, antivirus software developers and other hardware start-ups. Many are already moving in that direction.