Internet security researchers have warned that hackers have caught on to a "critical" flaw that lets them control traffic on the Internet.

An elite squad of computer industry engineers that laboured in secret to solve the problem released a software "patch" two weeks ago and sought to keep details of the vulnerability hidden at least a month to give people time to protect computers from attacks.

"We are in a lot of trouble," said IOActive security specialist Dan Kaminsky, who stumbled upon the Domain Name System (DNS) vulnerability about six months ago and reached out to industry giants to collaborate on a solution.

"This attack is very good. This attack is being weaponised out in the field. Everyone needs to patch, please," Kaminsky said. "This is a big deal."

DNS is used by every computer that links to the Internet and works similar to a telephone system routing calls to proper numbers, in this case the online numerical addresses of websites.

The vulnerability allows "cache poisoning" attacks that tinker with data stored in computer memory caches that relay Internet traffic to its destination.

Attackers could use the vulnerability to route Internet users wherever the hackers wanted, no matter what website address is typed into a web browser.

The threat is greatest for business computers handling online traffic or hosting websites, according to security researchers.

The flaw is a boon for "phishing" cons that involve leading people to imitation web pages of businesses such as bank or credit card companies to trick them into disclosing account numbers, passwords and other information.