In his epochal 1984 novel Neuromancer, William Gibson envisions attacking the computer systems of another country, as it happens, Russia:

"Screaming Fist, Case. You've heard the name."
"Some kind of run, wasn't it? Tried to burn this Russian nexus with virus programs. Yeah, I heard about it. And nobody got out."
He sensed abrupt tension. Armitagc walkcd to the window and looked out over Tokyo Bay. "That isn't true. One unit made it back to Helsinki, Case."
Case shrugged, sipped coffee.
"You're a console cowboy. The prototypes of the programs you use to crack industrial banks were developed for Screaming Fist. For the assault on the Kirensk computer nexus. Basic module was a Nightwing micro light, a pilot, a matrix deck, a jockey. We were running a virus called Mole. The Mole series was the first generation of real intrusion programs."
"Icebreakers," Case said, over the rim of the red mug.
"Ice from ICE, intrusion countermeasures electronics."
"Problem is, mister, I'm no jockey now, so I think I'll just be going...."
"I was there, Case; I was there when they invented your kind."

It is believed that Russia has already developed cyberwar software - some of it already trialled and tested in Ukraine.

There doesn't seem to be a lot of information about what has been happening; perhaps all sides have demurred, owing to the ease with which the same thing could happen to them.

President Joe Biden has been presented with a menu of options for the U.S. to carry out massive cyberattacks designed to disrupt Russia’s ability to sustain its military operations in Ukraine, four people familiar with the deliberations tell NBC News.

Two U.S. intelligence officials, one Western intelligence official and another person briefed on the matter say no final decisions have been made, but they say U.S. intelligence and military cyber warriors are proposing the use of American cyberweapons on a scale never before contemplated. Among the options: disrupting internet connectivity across Russia, shutting off electric power, and tampering with railroad switches to hamper Russia’s ability to resupply its forces, three of the sources said.

“You could do everything from slow the trains down to have them fall off the tracks,” one person briefed on the matter said...

... The most significant use of American cyber capability is believed to be the Stuxnet attack on the Iranian nuclear program from 2007 to 2010, which used computer malware to cause massive physical damage.

(Via NBC)