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"It is imperative that end-users apply the patch from Microsoft as soon as possible," the blog said.
Microsoft said the patch was "critical" for Windows XP and older versions and is rated as "important" for Windows Vista and newer versions.
Symantec has not seen wide-spread exploitation of this issue, but there have been reports of a certain file being downloaded on compromised computers.
This file copies another piece of malicious code onto the compromised computer.
Symantec has identified both these files as a Trojan horse called Infostealer, the blog said.