Even correctly-typed addresses could be misdirected, thanks to the flaw
Computer experts have released software to tackle a security glitch in the internet's addressing system.
The flaw, discovered by accident, would allow criminals to redirect users to fake webpages, even if they typed the correct address into a browser.
Internet giants such as Microsoft are now distributing the security patch.
Security expert Dan Kaminsky said that the case was unprecedented, but added: "People should be concerned but they should not be panicking."
"We have bought you as much time as possible to test and apply the patch," he said. "Something of this scale has not happened before."
Mr Kaminsky discovered the error in the workings of the Domain Name System (DNS) about six months ago.
DNS is used to convert web addresses written in words - such as www.bbc.com - into the numerical sequences used by computers to route internet traffic around the world.
The flaw revolves around the way that the servers that translate words into numbers handle the requests they get.
Unresolved the flaw would make it simple to operate "phishing" scams, in which users are directed to fake webpages supposedly for genuine banks or businesses and are tricked into disclosing credit card details or other personal data.
Mr Kaminsky talked to Microsoft, Sun and Cisco and many others in March and has been part of a team engaged in secret research since then to develop the security patch which has now been released simultaneously.
"This hasn't been done before and it is a massive undertaking," said Mr Kaminsky.
Despite the scale of the operation few are expected to see any disruption to their web experience as the patch is applied. It is not thought that the flaw had been exploited prior to its discovery.
Technical details are being kept secret for another month to give companies a chance to update their computers, before malicious hackers try to unpick the patch.
Personal computers should pick up the patch through automated updates. Microsoft released its patch on 8 July as part of its regular security cycle.
Some readers have contacted the BBC saying that applying Microsoft patch had caused problems with their PC. However, a Microsoft spokesman said its call centre staff had not received any calls about such problems.