Tens of thousands of computer users around the world infected with malware last year may lose their Internet access Monday with the expiration of a fix by US authorities, security experts say. (See: Internet blackout Monday: Check if your PC is infected
The problem stems from malware known as DNS Changer, which was created by cybercriminals to redirect Internet traffic by hijacking the domain name systems of Web browsers.
The ring behind the DNS Changer virus, discovered in 2007, was shut down last year by the US Federal Bureau of Investigation (FBI), Estonian police and other law enforcement agencies.
Because the virus controlled so much Web traffic, authorities obtained a court order to allow the FBI to operate replacement servers which allow traffic to flow normally, even from infected computers.
But that order expires Monday, when experts say infected computers will face an "Internet doomsday."
The FBI, Facebook, Google, Internet service providers and security firms have been scrambling to warn users about the problem and direct them to fixes.
According to a working group set up by experts, more than 300,000 computers remained infected as of June 11.
The largest number were in the United States (69,000), but more than a dozen countries -- including Italy, Germany, India, Britain, Canada, France and Australia -- are also believed to have infected computers.
Security experts say it's not clear how many of those computers are active.
"Reaching victims is a very hard problem, and something we have had issues with for years," said Johannes Ullrich, a researcher with the SANS Security Institute.
But he said he expected the impact to be "minimal" because many of these systems are no longer used or maintained.
Users who think they are infected may perform a test at the DNS Changer Working Group's website http://www.dcwg.org/ or others operated by various security firms.
The security firm Internet Identity said last week that at least 58 of all Fortune 500 companies and two out of 55 major government entities had at least one computer or router that was infected with DNS Changer.
That's an improvement over January, when half of Fortune 500 companies and US federal agencies were infected.
"DNS Changer is an insidious form of malware affecting everyone from the everyday consumer to a large chunk of the Fortune 500," said IID chief executive Lars Harvey.
IID said that the malware also compromises computers by preventing antivirus software updates.
"This enables criminals to view any data, messages exchanged and more on a victim's computer, depending on what the victims' machines are infected with," the company said.
The security firm McAfee, which also offers a diagnostic tool at http://www.mcafee.com/dnscheck, said users must act before Monday to clean their computers.
"If users' computers have the wrong DNS settings for the servers, they will not be able to access websites, send e-mail or use Internet services," a McAfee statement said Thursday.
Google said in May it was seeking to notify 500,000 users of likely infections who were using the FBI servers.
Google spokesman Jay Nancarrow said Thursday it was not clear how many remain infected.
"We've notified many people and have seen some clean-up as a result, but we expect others with affected devices will likely encounter problems after the deadline passes," he told AFP.
For computers affected, the blackout will be total, experts say.
"Connectivity will be lost to the Internet PERIOD," said a blog posting from the security firm Symantec.
"If your computer is still using DNS entries that are pointing to the FBI servers on July 9, you will lose TOTAL access to the Internet. No connecting to the office from home, no updating Facebook, nothing until the DNS settings are fixed."
Six Estonians and a Russian were charged last November with infecting computers, including NASA machines, with the malware as part of an online advertising scam that reaped at least $14 million.
The Internet fraud, which took place between 2007 and October 2011, involved redirecting users searching for websites such as iTunes, Netflix and even the US tax collection agency.
At least four million computers located in over 100 countries may have been infected.