SharePoint Online, folders shared by external link, no sign-in - is it safe?

Question

If I have document library with folders inside, and share folder1 to user1@gmail.com, folder2 to user2@gmail.com (send link to e-mails).

As I see, user1 cannot see folder2 and vice-versa, so they see only their shared folders. But how SP does it? It saves token in browser or what..

Can I be sure that user2 will never see folder1, if he does not have link?

This is how different users see different data. Folder names are guid's in my case:

user1:

user2:

share form:

Answer 1

The answer is "No" (Even though you excluded the knowledge of the link for the other user)

Using the Share option without sign in is something I would treat as unsecured data. Anyone with access to the link can access it. The information to access the item are encoded in the link itself.

You used "never" so the answer was harsh. They will only see the folder if they know the link. Anyone with the link can access the file.

Answer 2

There is no guarantee that user2 will not see folder1. If User2 gets the shared link to folder1 then user2 will for sure see folder1. Only way of making it totally secure is to share the folder in question to a specific user by entering that users email.