...
Currently the user and developer groups include members from many organizations. As you see in the screenshot below, members of these groups have permissions to add and edit content. This means that a large community of people will have access to edit your space unless you choose to either remove this group or restrict the default access.
NEW Default Space Permissions as of 1011/X21/2019
NOTE: The default permissions when creating new Confluence spaces will change on 1011/X21/2019. This will not impact existing spaces. The new defaults will be restrictive and will not automatically grant permissions to the users and developers groups. You will need to grant permissions to specific users. See "Set Permissions" for instructions on how to change security.
...