Hackers have released attack code for the flaw in the Domain Name System (DNS) software that sends messages between computers on the internet.
The code was created by the developers behind the Metasploit hacking toolkit and internet security experts warn it may give criminals a way to launch virtually undetectable phishing attacks against internet users whose ISPs have not installed the latest DNS server patches.
Attackers could also use the code to silently redirect users to fake software update servers in order to install malicious software on their computers, said Zulfikar Ramizan, a technical director with security vendor Symantec.
"What makes this whole thing really scary is that from an end-user perspective they may not notice anything," he said.
The bug was first disclosed by IOActive researcher Dan Kaminsky earlier this month, but technical details of the flaw were leaked onto the internet earlier this week, making the Metasploit code possible.
Kaminsky had worked for several months with major providers of DNS software such as Microsoft, Cisco and the Internet Systems Consortium (ISC) to develop a fix for the problem. The corporate users and ISPs who are the major users of DNS servers have had since July 8 to patch the flaw, but many have not yet installed the fix on all DNS servers.
The attack is a variation on what's known as a cache poisoning attack. It has to do with the way DNS clients and servers obtain information from other DNS servers on the internet. When the DNS software does not know the numerical IP address of a computer, it asks another DNS server for this information. With cache poisoning, the attacker tricks the DNS software into believing that legitimate domains map to malicious IP addresses.
In Kaminsky's attack a cache poisoning attempt also includes what is known as 'Additional Resource Record' data. By adding this data, the attack becomes much more powerful, security experts say.
An attacker could launch such an attack against an ISP's domain name servers and then redirect them to malicious servers. By poisoning the domain name record for www.citibank.com, for example, the attackers could redirect the ISP's users to a malicious phishing server every time they tried to visit the banking site with their web browser.
On Monday, security company Matasano accidentally posted details of the flaw on its website. Matasano quickly removed the post and apologised for its mistake, but it was too late. Details of the flaw soon spread around the internet.
Although a software fix is now available for most users of DNS software, it can take time for these updates to work their way through the testing process and actually get installed on the network.
"Most people have not patched yet," said ISC President Paul Vixie. "That's a gigantic problem for the world."
Metasploit's code looks "very real," and uses techniques that were not previously documented said Amit Klein, chief technology officer with Trusteer.
It will probably be used in attacks, he predicted. "Now that the exploit is out there, combined with the fact that not all DNS servers were upgraded, attackers should be able to poison the cache of some ISPs," he said. "The thing is, we may never know about such attacks, if the attackers work carefully and cover their tracks properly."