Spacebar Server before commit dcfd910 contains a missing authorization vulnerability tracked as CVE-2026-70617. Any authenticated attacker can exploit the unguarded PUT /channels/{channel_id}/recipients/{user_id} endpoint to add themselves to arbitrary group DM channels without membership verification. The vulnerability allows attackers to join private group DMs, read complete message history, post messages as a participant, and force-add third-party users without their consent. The fix was introduced in commit dcfd91035e3da42abf5f32d8d86a35219225b3d4. The issue is documented in a GitHub security advisory (GHSA-g38j-78fh-jm74) and VulnCheck. It represents a significant privacy and integrity risk for all users of affected Spacebar Server deployments. Users should update to the patched commit immediately to mitigate exposure.