According to Microsoft® Policy Regarding Missing or Invalid Passwords:

Microsoft engineers cannot help you break the passwords that are applied to the files.

[…]

If you need help to break or to reset a password, you can locate and contact a third-party company for this help.

Here it is:

Advanced Office Password Recovery (AOPR) is ElcomSoft's comprehensive recovery suite supporting all versions of Microsoft® Office (including international versions) and all other Microsoft® software using the new encryption method based on Cryptographic Service Provider (CSP) file protection for Office XP and later.

AOPR (a favourite of the FBI et al) is available in three editions, e.g., Home, Standard, and Professional. The key features of each AOPR edition are documented at Supported File Types and Passwords.

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