In the beginning of April 2020, we were notified that subjects from one Zooniverse project were appearing in the subject set of a separate project where they did not belong. In our investigation of the issue, our team determined that this behavior was being caused by a Caesar configuration mistake that used an incorrect Subject Set ID. Project owners using Caesar were able to create Subject Rule Effects that added subjects to collections or subject sets, even without proper subject set editing permissions. We have rectified the issue surrounding Subject Rule Effects and eliminated this vulnerability, and would like to share the details for anyone who is interested.

The issue was raised by project lead James Perry (@JamesPerry), who reported that subjects that didn’t belong to his project were appearing in his subject sets.  Due to a mistyped subject set ID in a Caesar `add_to_subject_set` effect for an unrelated project, that Subject Rule Effect was sending subjects from that project to one of James’s subject sets instead of the correct target.

Our immediate course of action was to fix the project impacted by the vulnerability, and push out a temporary code fix to prevent the vulnerability from being exploited. 

1. To fix the affected project, we updated the incorrect subject set id for the project that was incorrectly sending subjects to the wrong project and removed the unwanted subjects from the set. 
2. On April 3rd we deployed a temporary code fix to disable Subject Rule Effect creation and modification for all but admin users (see PR #1109). This change was communicated to affected teams that were most impacted by the change, and teams that reached out after seeing our notification banner or encountering a Caesar interface error.

On May 15th we pushed out a permanent fix that checked the user has permissions to send data to the target subject set or collection. Specifically, the updated validation code checks that the user has update permissions on the project the subject set or collection is linked to. (see PRs #1115, #1129 and #1131). 

For anyone running their own hosted copy of Caesar, we recommend pulling these changes as soon as you’re able.