"Cannot open the file because the file format or extension is invalid" opening Office files

UPDATE:

Upon further analysis of the files that have been submitted to us for investigation, the analysts

have determined that the files are encrypted with a private and public key.

Unless the private key is available, the files will not be able to be recovered.

The private key is more than likely held by the attacker.The premise of ransom ware such as this is

that if a person pays the ransom, a key is provided to unlock the files.

The best course of action is to clean up the malware and then restore files from a backup.

  We have identified the malware as a new family of ransomware

 

 

Definition Information:

    Latest public Endpoint AM defs:  1.157.1542.0   (Expect several hours later for 1.157.1559.0 or higher)     https://www.microsoft.com/security/portal/definitions/whatsnew.aspx

    Latest Prerelease AM defs: 1.157.1542.0  (Expect several hours later for 1.157.1559.0 or higher)     https://www.microsoft.com/security/portal/shared/prereleasesignatures.aspx

 

The signatures are now updated. You should be able to use Safety Scanner to detect and clean it. It does NOT clean your documents, so you will want to restore from a backup after cleaning the malware from your system.



 

More Information:

 

We are currently investigating an ongoing situation where users may encounter an error when trying to open Office documents.

The error can happen opening any Office file type, not just Excel files as shown in the image below. The error says:

"Cannot open the file ... because the file format or extension is not valid. Verify, that the file has not been corrupted and that the file extension matches the format of the file."

 

 

  

   

It's recommended you update the definitions in your antivirus software and run a complete scan of the affected machine(s).

 

When opening binary Word (.DOC) files that are attributed with this issue, Word does not know how to open, so you receive a dialog for File conversion as shown below.

 

We've had reports of it affecting Word documents, Excel files, PowerPoint files and Access databases.

 

Other Support Blog links

 

 

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Security EssentialsMicrosoft Security Essentials  https://www.microsoft.com/security/pc-security/mse.aspx