A Bing Community thread reports from one webmaster that someone is masking MSNBot around as a browser, either Internet Explorer or Mozilla Firefox. Here are the reported IPs and UserAgent's being reporting by this webmaster:
The following IP addresses are from MSN's search bots, but use what appears to be just normal user agents.
126.96.36.199 188.8.131.52 184.108.40.206 220.127.116.11 18.104.22.168 22.214.171.124 126.96.36.199 188.8.131.52 184.108.40.206 220.127.116.11 18.104.22.168 22.214.171.124 126.96.36.199 188.8.131.52 184.108.40.206 220.127.116.11
The user agent looks like this "Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; SLCC1; .NET CLR 1.1.4322; .NET CLR 2.0.50727; .NET CLR 3.0.30729; .NET CLR 3.5.30729; InfoPath.2)"
This is not the first time Microsoft Bing has been accused of masking their bots as browser useragents. We had a similar report back in September where MSNBot was using Mozilla by way of UserAgent.
Brett Yount, Bing's representative, said, "Could you send this information to email@example.com and I will have the crawling team investigate?"
So currently, this is not confirmed, but it is weird.
Forum discussion at Bing Community.