A "benign" worm is scouring the net seeking out poorly protected smart gadgets.
CCTV systems, routers, digital video recorders and other internet-of-things (IoT) devices are now believed to be harbouring the Hajime worm.
The fast-moving worm is currently outpacing malicious equivalents seeking the same vulnerable gear.
Security researchers say they do not know who created Hajime or how it might ultimately be used.
Hajime was first discovered in October 2016 and, said security researchers, had been hunting down IoT devices with security vulnerabilities that could be exploited by a different worm, called Mirai.
Earlier the same month, a network of devices compromised by Mirai was responsible for knocking offline high-profile websites including Twitter, Spotify and Reddit.
Modest estimates suggested Hajime was now present on "tens of thousands" of devices, wrote Symantec researcher Waylon Grange in a blog.
Programs such as Hajime and Mirai must keep scouring the net for victims, because switching off a vulnerable device generally cleans out the infection.
Mr Grange noted that Hajime currently had no attack code built in so could not be used to mount the kinds of attacks Mirai had been implicated in.
The only action taken by Hajime is to regularly display a message from the worm's author on the internal interface for each device.
The message says, among other things: "Just a white hat, securing some systems."
The term "white hat" is typically applied to those hackers seeking to secure rather than exploit vulnerabilities.
Malicious or criminal hackers are known as "black hats".
"There is a question around trusting that the author is a true white hat and is only trying to secure these systems, as they are still installing their own backdoor on the system," wrote Mr Grange.
He added if the author's intentions changed they could "potentially" turn the infected devices into a "massive" attack network.