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If the Word document is in "protected view" then when a user clicks on a link they first get a warning about the dangers of the internet, and then Word tries to retrieve the document. In this case, the user agent string DOES NOT identify that it's coming from Word or Office - at least not in any way I can tell.
Protected View User Agent String:
Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 6.3; WOW64; Trident/7.0; .NET4.0E; .NET4.0C; .NET CLR 3.5.30729; .NET CLR 2.0.50727; .NET CLR 3.0.30729; Tablet PC 2.0)
Unprotected View User Agent String:
Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 6.3; WOW64; Trident/7.0; .NET4.0E; .NET4.0C; .NET CLR 3.5.30729; .NET CLR 2.0.50727; .NET CLR 3.0.30729; Tablet PC 2.0; ms-office)
You can see the "ms-office" part only arrives if the user has turned off protected view.
Since I can't force users to turn off protected view in a document (esp one they downloaded from the internet), I can't work out how to reliably tell if the "browser" is an older version of Internet Explorer or Microsoft Word.
Any ideas?