Sun Microsystems says it has verified the zero-day bug in Solaris 10 and 11, and company researchers are working on a fix that they hope to release within a few days.

The vulnerability -- which the SANS Institute on its site on Monday called a "major zero-day bug" -- allows hackers to easily gain remote access to computers running the operating systems. The problem lies in the way Telnet, a network protocol, uses parameters during the authentication process, said Johannes Ullrich in an interview with InformationWeek on Monday morning. Ullrich is the CTO for the Internet Storm Center, a cooperative cyberthreat monitoring and alert system.

Sun is preparing to issue an alert to its users later Monday, a company spokesman says.

"We're not aware that anyone has actually experienced this exploit," he adds. "It doesn't mean it hasn't happened but we haven't heard about it."

Ullrich explains that by simply adding what he calls a "trick" or simple text to the Telnet command, the system will skip asking for a user name and password. No exploit needs to be downloaded. If the systems are installed out of the box, they automatically come Telnet enabled.

Solaris 10 came out in 2005, and more than 7 million users have reportedly registered for it with Sun. Solaris 11, or what Sun code-named Solaris Next, is in beta.

The Sun spokesman says earlier versions of Solaris 10 had Telnet automatically enabled out of the box, but the latest version does not. He says for both the latest version of Solaris 10 and for the beta version of Solaris 11, users would have to specifically turn Telnet on to enable it. He adds that Sun is recommending that customers use Solaris Secure Shell as the protocol of choice.

He also says he does not know if Sun will take Telnet out of Solaris 11 all together.

Internet Storm Center analysts are recommending that Telnet be disabled on the Solaris systems.

While Ullrich calls Telnet out of date and problematic, he says this specific zero-day bug is caused by the way Solaris is designed.

Ullrich and other researchers at the Internet Storm Center are warning users to not use Telnet anymore -- on any system. "It's archaic at this point," says Ullrich. "Never use Telnet to log in to a system. Use SSH instead. There's just no reason to use Telnet. I don't know why they keep it enabled. They really shouldn't."