The SANS Institute is warning of a zero-day bug in Sun's Solaris 10 and 11 Telnet that allows hackers to easily gain remote access to the computes running the operating systems.

The vulnerability -- called a "major zero-day bug" -- has been verified, according to a release on the SANS' Internet Storm Center Web site. The problem lies in the way Telnet, which is a network protocol, uses parameters during the authentication process, says Johannes Ullrich, chief research officer at the SANS Institute and chief technology officer for the Storm Center, a cooperative cyberthreat monitoring and alert system.

Ullrich says 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. Every Solaris 10 and 11 system is at risk. If the systems are installed out of the box, they automatically come Telnet enabled.

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While Ullrich calls Telnet out of date and problematic, he says this specific zero-day bug is caused by the way Solaris is designed. "The funny or sad thing is that in 1994, AIX had a similar problem and they fixed it," he adds, saying Sun should have learned a lesson from IBM's mistake.

"I have not heard of it being used yet," he adds. "But I imagine it's going on because it's such a simple procedure."

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."