F-Secure, the Finnish security firm, also told its clients last week about the Russian hacking group, which Symantec has named “Dragonfly.”

In the past six months, researchers say the group has become more aggressive and sophisticated.

The Russian hackers have been breaking into the networks of industrial control software, or I.C.S., makers, inserting so-called Trojans into the software used by many oil and energy firms to allow employees to remotely get access to industrial control systems. So when oil and gas companies downloaded the latest version of the software, they inadvertently downloaded the hackers’ malware as well.

At least three industrial control software developers were affected, according to researchers at Symantec, F-Secure and CrowdStrike. The first was a maker of remote access tools for industrial control systems; the second, a European manufacturer of specialized industrial control devices; and the third, a European company that develops systems to manage wind turbines, natural gas plants and other energy infrastructure. They were not named by the security companies because of confidentiality agreements.

Security researchers estimate that more than 250 companies downloaded the infected software updates.

“These infections not only gave the attackers a beachhead in the targeted organizations’ networks, but also gave them the means to mount sabotage operations against infected I.C.S. computers,” Symantec wrote in its report Monday.

There was no evidence the Russian group intended to use its toehold in some networks to inflict damage, like blowing up an oil rig or power facility, said Kevin Haley, the director of security response at Symantec, in an interview. The apparent motive, Mr. Haley said, was to learn more about energy companies’ operations, strategic plans and technology. “But the potential for sabotage is there,” he added.

More recently, Energetic Bear has been targeting companies in the financial sector, said Adam Meyers, CrowdStrike’s head of threat intelligence. In particular, the group has been attacking, with the watering hole technique, some websites frequented by firms that invest in the energy sector.