Can someone access a main folder which has not shared through a shared link of a subfolder in the main folder?

I came across a situation where someone has accessed my OneDrive main folder which I have not given shared access. But I have given shared access (anyone with this link can edit) to ONE subfolder in the main folder. An unauthorized user has accessed the shared subfolder but changed the permission settings so that he/ she can access my entire One Drive folder? How is that possible?