Mantis Bug Tracker (MantisBT) is an open source issue tracker. In versions prior to 2.25.6, due to insufficient access-level checks, any logged-in user allowed to perform Group Actions can access to the Summary field of private Issues (i.e. having Private view status, or belonging to a private Project) via a crafted bug_arr[]
parameter in bug_actiongroup_ext.php. This issue is fixed in version 2.25.6. There are no workarounds.