A computer security company inadvertently published details of a major DNS flaw several weeks before they were due to be disclosed.

The flaw was discovered several months ago by IOActive researcher Dan Kaminsky, who worked through the early part of this year with vendors such as Microsoft, Cisco and the Internet Systems Consortium to patch the issue.

The companies released a fix for the bug two weeks ago and encouraged corporate users and Internet service providers to patch their DNS systems as soon as possible. Although the problem could affect some home users, it is not considered to be a major issue for consumers, according to Kaminsky.

At the time he announced the flaw, Kaminsky asked members of the security research community to hold off on public speculation about its precise nature in order to give users time to patch their systems. Kaminsky had planned to disclose details of the flaw during a presentation at the Black Hat security conference set for 6 August.

Some researchers took the request as a personal challenge to find the flaw before Kaminsky's talk. Others complained at being kept in the dark about the technical details of his finding.

On Monday, Zynamics CEO Thomas Dullien (who uses the hacker name Halvar Flake) ttook a guess at the bug admitting that he knew very little about DNS.

His findings were quickly confirmed by Matasano Security, a vendor that had been briefed on the issue.

"The cat is out of the bag. Yes, Halvar Flake figured out the flaw Dan Kaminsky will announce at Black Hat," Matasano said in a blog posting that was removed within five minutes of its publication.

Matasano's post discusses the technical details of the bug, saying that by using a fast Internet connection, an attacker could launch what's known as a DNS cache poisoning attack against a Domain Name server and succeed, for example, in redirecting traffic to malicious websites within about 10 seconds.

Matasano Researcher Thomas Ptacek declined to comment on whether or not Flake had actually figured out the flaw, but said the item had been "accidentally posted too soon." Ptacek was one of the few security researchers who had been given a detailed briefing on the bug and had agreed not to comment on it before details were made public.

Matasano's post inadvertently confirmed that Flake had described the flaw correctly, Ptacek admitted.

Ptacek apologised to Kaminsky on his company blog. "We regret that it ran," he wrote. "We removed it from the blog as soon as we saw it. Unfortunately, it takes only seconds for Internet publications to spread."

Kaminsky's attack takes advantage of several known DNS bugs, combining them in a novel way, said Cricket Liu vice president of architecture with DNS appliance vendor Infoblox, after viewing the Matasano post.

The bug 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. "The combination of them is pretty bad," Liu said.

An attacker could launch such an attack against an Internet service provider'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.

Kaminsky declined to confirm that Flake had discovered his issue, but in a posting his own website he wrote "13>0," apparently a comment that the 13 days administrators have had to patch his flaw before its public disclosure is better than nothing.

"Patch. Today. Now. Yes, stay late," he wrote.

He has posted a test on his website that anyone can run to find our if their network's DNS software is patched