If you discover a security vulnerability, we’d appreciate a non-public disclosure. systemd developers can be contacted privately on the firstname.lastname@example.org mailing list. The disclosure will be coordinated with distributions.
(The issue tracker and systemd-devel mailing list are fully public.)
Subscription to the systemd-security mailing list is open to regular systemd contributors and people working in the security teams of various distributions. Those conditions should be backed by publicly accessible information (ideally, a track of posts and commits from the mail address in question). If you fall into one of those categories and wish to be subscribed, submit a subscription request.