Alerts have been issued by both the British and American governments as a growing number of hacking groups – potentially including spies and organised criminals – are exploiting the weakness to break into computer networks.
The British government says it is treating "this issue with the utmost seriousness" and the US have warned the vulnerability was "being widely exploited by a growing set of threat actors".
Experts in the private sector have said that "the potential for damage is incalculable" with one suggesting that the internet "is on fire right now".
The new vulnerability has been discovered in a piece of code known as Log4j.
It is not expected to impact people using personal devices but any data they have with organisations that operate web servers could be at risk.
Players of the video game Minecraft have been affected as hackers were able to post a short message in the chatbox to remotely execute commands on the computers of other players.
Microsoft says it has patched the issue for Minecraft players and told customers they would be protected if they applied the fix, although experts continue to warn of the dangers.
Adam Meyers, senior vice president of the cyber security company Crowdstrike, said: "The internet's on fire right now. People are scrambling to patch, and all kinds of people are scrambling to exploit it."
The danger has also been echoed by Lotem Finkelstein, director of threat intelligence for Check Point Software Technologies.
He warned: "I cannot overstate the seriousness of this threat."