Malware that can build botnets out of IoT products has gone on to infect twice as many devices after its source code was publicly released.
The total number of IoT devices infected with the Mirai malware has reached 493,000, up from 213,000 bots before the source code was disclosed around Oct. 1, according to internet backbone provider Level 3 Communications.
"The true number of actual bots may be higher," Level 3 said in a Tuesday blog post.
Hackers have been taking advantage of the Mirai malware's source code, following its role in launching a massive DDOS (distributed denial-of-service) attack that took down the website of cybersecurity reporter Brian Krebs.
Unlike other botnets that rely on PCs, however, Mirai works by infecting internet-connected devices such as cameras and DVRs that come with weak default usernames and passwords.
Since Mirai's source code was released, hackers have been developing new variants of the malware, according to Level 3. It has identified four additional command-and-control servers associated with Mirai activity coming online this month.
About half of the infected bots Level 3 has observed resided in either the U.S. or Brazil. More than 80 percent of them were DVR devices.
Many of the DDOS attacks launched by Mirai botnets are used against game servers and residential IP addresses, Level 3 said.
"We have observed several attacks using more than 100 Gbps" of traffic, it said. "Large armies of bots participated in attacks, with several using over 100,000 bots against the same victim."
A few vendors that produce devices vulnerable to Mirai are encouraging their customers to take steps to mitigate the risk. Sierra Wireless, for instance, has issued a bulletin, advising users to reboot one of their products and change the default password.
However, it's unclear if other vendors are taking any steps to do the same. Security firm Flashpoint has identified Chinese company Hangzhou Xiongmai Technology as another maker of DVR products susceptible to the Mirai malware.
Potentially, half a million devices from the company are vulnerable partly due to their unchangeable default passwords, according to Flashpoint. But Xiongmai has not commented on the matter.