Microsoft has warned users of its Word software of a security hole that could allow viruses to get past standard checks. The bug appears to be limited to a small number of files.

The flaw only affects Word 97 and higher, and then only RTF files linked to a template. Standard Word files and unlinked RTF files are unaffected. Microsoft has issued a patch to fix the bug.

Macros have long been the province of annoying but usually not terribly destructive viruses.

The security bulletin and patches can be found at Microsoft's site.

The flaw concerns macros - small scripts that can be used in Word to automate or speed up a number of operations. Normally, when documents are opened, Word checks for macros and warns the user if any are present.

In the unlikely instance that a user opens an RTF (rich text format) file that references a template with a macro embedded in it, a macro can be run without warning the user. Such a macro could make some changes to Word, including enabling macros, even if the user had previously turned them off, according to Microsoft.