The WiredTiger database directory should have its permissions set to ensure database objects are not accessible to users without appropriate permissions. See WiredTiger Home Directory for more information.
WiredTiger creates file system objects readable and writable by the process owner, group and user, as modified by the process' umask value. The group ownership of created file system objects may vary depending on the system, and is not controlled by WiredTiger.
Consider security when configuring WiredTiger to use the WIREDTIGER_HOME environment variable, especially in applications which run with permissions other than the user's. Such applications are potentially vulnerable to allowing users access to databases they could not otherwise access. See WiredTiger Home Directory for more information.