According to the Cupertino, Calif. company's alert, an exploit for the zero-day bug in Windows 2000's SMB (Server Message Block) protocol has been created by Immunity Security, the makers of the CANVAS exploit-creation platform.
By Immunity researcher Dave Aitel's account, the exploit leverages a flaw in the operating system's kernel that can be triggered through SMB, and will give an attacker full access to the PC.
Aitel claimed Immunity will make the exploit public in June.
"Immunity is considered to be a reliable source and we are of the opinion that this information should be treated as fact," read Symantec's warning. "An official security update from Microsoft will likely not be in development until after June when the information is released."
Until then, Symantec recommended that companies restrict SMB services to trusted hosts; if that's not possible, companies should consider upgrading to a newer version of Windows.
Windows 2000 was last patched against an SMB vulnerability in June 2005.
Although Windows 2000 has been in its last lifespan stage since June 2005 -- called "Extended Support" by Microsoft -- the Redmond, Wash. developer still patches bugs in the OS during its monthly security updates.