A bug in Grammarly’s Google Chrome web browser extension was exposing all of your documents to virtually anyone or any website that wants to view them. If you’ve used Grammarly’s Chrome extension, all of the documents (including websites, documents, MS Word documents, including Microsoft Office documents) have been exposed to the public.
If any of the documents that you’ve used Grammarly on to check spelling or plagiarism are of a sensitive nature, then you probably should be concerned about this security breech.
Update: this has been fixed and user login details are no longer exposed. However, if you have any documents that are of a sensitive nature in your Grammarly account then you may want to remove them. Also, in any case like this, I urge you to go change your login details (change your password) at Grammarly.