According to Scheel, the problem is that the HbbTV standard, carried by DVB-T signals and supported by all smart TVS, allows the sending of commands that tell smart TVs to access and load a website in the background.

Knowing this, Scheel developed two exploits he hosted on his own website, which when loaded in the TV’s built-in browser would execute malicious code, gain root access, and effectively take over the device.

Source: About 90% of Smart TVs Vulnerable to Remote Hacking via Rogue TV Signals