I have an odd use-case here. Administrator Alice enables cross-project issue relations, creates a private project and creates issue 1 (an issue User Bob can't see). Bob, who belongs to a public project, creates issue 2. Being the sneaky user that he is, he wants to see what tickets private trackers have. He adds an issue relation to issue 1 and sees the ticket subject. Oh noes!

Basically, cross-project issue relations aren't respecting user permissions to see the ticket (or its subject). The issue relation could be kept, for sure, just not displayed to that user.

I gather the fix is to restrict what issue relations show according the the viewing user's permissions, yeah?