The adverts appear every seven minutes and last for a few seconds.
Comcast says they’re injecting its own adverts to devices of people who are connected their public Wi-Fi hotspots across the US.
The internet service provider, which operates over 3.5m Wi-Fi hotspots, began the injections "months ago" but only came to light after a report by Wired’s former editor and now co-founder of tech startup ‘Contextly’, Ryan Singel.
According to Ars Technica, the injections can either be an alert to let users know they are connected to a Comcast hotspot, or inserted ads to promote Comcast’s Xfinity mobile apps.
"We think it’s a courtesy, and it helps address some concerns that people might not be absolutely sure they’re on a hotspot from Comcast," company spokesperson Charlie Douglas said.
The injection system was created by a third-party provider, Front Page, which also sells it to retailers, sporting venues and transit terminals.
When asked about the security implications, Comcast’s Douglas said that the system has multiple layers of security "based on industry best practices" to make sure hackers can’t exploit the Xfinity network.