One day after a security company accidentally posted details of a serious flaw in the Internet's Domain Name System (DNS), hackers are saying that software that exploits this flaw is sure to pop up soon.

Several hackers are almost certainly already developing attack code for the bug, and it will most likely crop up within the next few days, said Dave Aitel, chief technology officer at security vendor Immunity. His company will eventually develop sample code for its Canvas security testing software too, a task he expects to take about a day, given the simplicity of the attack. "It's not that hard," he said. "You're not looking at a DNA-cracking effort."

The author of one widely used hacking tool said he expected to have an exploit by the end of the day Tuesday. In a telephone interview, HD Moore, author of the Metasploit penetration testing software, agreed with Aitel that the attack code was not going to be difficult to write.

The flaw, a variation on what's known as a cache poisoning attack, was announced on July 8 by IOActive researcher Dan Kaminsky, who planned to disclose full details of the bug during an Aug. 6 presentation at the Black Hat conference.

That plan was thwarted Monday, when someone at Matasano accidentally posted details of the flaw ahead of schedule. Matasano quickly removed the post and apologized for its mistake, but it was too late. Details of the flaw soon spread around the Internet.

And that's bad news, according to Paul Vixie, president of the company that is the dominant maker of DNS software, the Internet Systems Consortium. Vixie, like others who were briefed on Kaminsky's bug, did not confirm that it had been disclosed by Matasano. But if it had, "it's a big deal," he said in an e-mail message.

The attack can be used to redirect victims to malicious servers on the Internet by targeting the DNS servers that serve as signposts for all of the Internet's traffic. By tricking an Internet service provider's (ISPs) servers into accepting bad information, attackers could redirect that company's customers to malicious Web sites without their knowledge.

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," Vixie said. "That's a gigantic problem for the world."

Just how big of a problem is a matter of some debate.

Neal Krawetz, owner of computer security consultancy Hacker Factor Solutions, took a look at DNS servers run by major ISPs earlier this week and found that more than half of them were still vulnerable to the attack.

"I find it dumbfounding that the largest ISPs ... are still identified as vulnerable," he wrote in a blog posting. "When the [hackers] learn of the exploit, they will go playing. They are certain to start with the lowest hanging fruit -- large companies that are vulnerable and support a huge number of users."

He expects that users will see attacks within weeks, starting first with test attacks, and possibly even a widespread domain hijacking. "Finally will be the phishers, malware writers and organized attackers," he wrote in a Tuesday e-mail interview. "I really expect these to be very focused attacks."

Most ISPs will have probably applied the patch by the time any attacks start to surface, and that will protect the vast majority of home users, said Russ Cooper, a senior information security analyst with Verizon Business. And business users who use secure DNS-proxying software will also be "pretty much protected" from the attack at their firewall, Cooper said.

"If anyone actually tries to exploit this, the actual number of victims will end up being extremely small," he predicted.

HD Moore said he didn't exactly see things that way. Because the flaw affects nearly all of the DNS software being used on the Internet, he said that there could be lots of problems ahead.

"This is a bug we'll be worrying about a year from now," he said.