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A security researcher has found a serious vulnerability in an aging yet widely-used software program used for the Internet's addressing system, prompting the software's maintainers to retire the affected version.

The flaw within Berkeley Internet Name Domain 8 (Bind 8) software could misdirect users to a fraudulent Web site even if a user typed in the correct URL (uniform resource locator), wrote Amit Klein, chief technology officer for security vendor Trusteer Ltd. Klein discovered the problem.

Users are being advised to upgrade to Bind 9.4, the latest version of the software, which underwent an architecture rewrite to improve security. The software, looked after by the Internet Software Consortium (ISC), is free to download.

ISC issued an interim patch, but due to other weaknesses in the software, ISC is also ending support for Bind 8.

"It's never easy to retire a product," wrote the organization in an advisory. "The security issues of Bind 8 are many, and seven years after the release of Bind 9, ISC must devote our efforts to maintaining and enhancing the current version."

About 14 percent of the DNS servers on the Internet in 2006 were still using Bind 8, according to Infoblox Inc., which conducts an annual survey of DNS servers.

"Bind 8 is still a very popular DNS server nowadays, thus this attack applies to a big part of Internet users," Klein wrote.

In a research paper, Klein described a weakness in the algorithm Bind 8 uses to generate transaction IDs, apparently random serial numbers that allow it to spot whether someone is trying to supply it with false information in respose to queries. The weakness makes it possible to observe a few queries and then predict the transaction ID in the sequence, Klein wrote.

Using that information, an attacker can then send erroneous information to the DNS server, "poisoning" the address stored against a particular domain name in its memory cache. Thus, traffic intended for a certain Web site from users of that DNS server can be diverted to another server containing a fraudulent site, a deception known as "pharming."

Although Bind 9 has a better transaction ID algorithm, it could also be vulnerable, Klein wrote.