Security experts have warned of a zero-day vulnerability with Sun's Solaris 10 and 11 OSes.
Exploit code has been posted on the Internet, and no special tools are required to take advantage of it, according to an advisory from the SANS Institute.
The problem lies within Telnet, a networking protocol. The Telnet daemon - a process that runs in the background and waits for another Telnet client to connect - can allow a hacker to log in without a password.
SANS suggests a couple of solutions: disable Telnet or limit the IP (Internet Protocol) addresses that can connect to Telnet through the firewall.
In the past, SANS has warned that using Telnet poses a risk because data transferred between clients may not be encrypted. Telnet is also a frequent target for port scanners.
"In my opinion, no one should be running Telnet open to the Internet," wrote Donald Smith of SANS.