How secure is a password-protected Excel file?

How secure is a password-protected Excel document? Is it known what type of encryption is used? Has their been any cases of it being cracked when using a strong (Upper-and-lowercase, numbers, special character) password?
 

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Last updated November 14, 2019 Views 11,217 Applies to:
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Apparently it is better, here is a blog from the development team:

http://blogs.technet.com/b/office2010/archive/2009/10/16/enabling-password-rules-for-office-2010.aspx

One note, take the estimated time to hack with a huge grain of salt. Although the article is recent, it assumes using a single processor for hacking. That is no longer a reasonable assumption. Techniques using graphics cards and the cloud have sped up brute force attacks by orders of magnitude.  128 bit is much better than 40 bit on old Office, but still on the low end of acceptable. Go with long, complex passphrases.

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