State-sponsored Russian hackers are actively seeking to hijack essential internet hardware, US and UK intelligence agencies say.
The UK's National Cyber Security Centre (NCSC), the FBI and the US Department of Homeland Security issued a joint alert warning of a global campaign.
The alert details methods used to compromise the networking equipment used to move traffic across the net.
This could be used be used to mount a future offensive, it warned.
Basic weakness
In a press conference about the alert, White House cyber-security co-ordinator Rob Joyce said the US and its allies had "high confidence" that Russia was behind the "broad campaign".
Intelligence gathered by the US and UK suggested that millions of machines directing data around the net were being targeted, he said.
Compromised devices were used to look at data passing through them, added Mr Joyce. Attackers also sought to undermine the firewalls and intrusion detection systems organisations used to spot malicious traffic before it reached users.
Source:BBC