Microsoft has warned that an unpatched vulnerability in older versions of Excel is being actively exploited by hackers.

The bug, which has become the subject of targeted attacks, affects older versions of Excel. Newer versions Excel 2003 SP3 and Excel 2007 are said to be immune. Details of the vulnerability beyond what packages might be affected remain sketchy (http://secunia.com/advisories/28506/) at best.

Microsoft techies are investigating the flaw, which may become the topic of a future patch. In the meantime, Redmond is trying to play down concern by pointing out mitigating factors and suggesting defence. Its suggestion, however, that few hackers know about the bug rather misses the point that this hardly reduces the risk posed by targeted attacks against unpatched flaws.

"At this time, we are aware only of targeted attacks that attempt to use this vulnerability. Additionally, as the issue has not been publicly disclosed broadly, we believe the risk at this time to be limited," Microsoft's advisory explains (http://www.microsoft.com/technet/security/advisory/947563.mspx).

Pending the availability of a fix, security clearing house US-CERT advises (http://www.us-cert.gov/current/index.html#microsoft_office_excel_remote_code) users of older versions of Excel to avoid opening unfamiliar or unexpected email attachments. Sys admins should review Microsoft's suggested workarounds, it adds. ®

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