CVE-2026-55066: Vikunja has cross-tenant IDOR in kanban move-task endpoint via unauthorized body task_id

The kanban endpoint POST /api/v1/projects/{project}/views/{view}/buckets/{bucket}/tasks moves a task into a bucket. The task is identified by task_id in the request body. The endpoint’s authorization check (TaskBucket.CanUpdate) only verifies that the caller may update the project/view/bucket named in the URL — it never checks any permission on task_id.

Any authenticated user can therefore supply another user’s task ID (task IDs are a global, sequential integer space) against a kanban bucket in their own project. The server loads that victim task with no authorization check, returns its full contents in the response, and — when the target bucket is a “done” bucket — writes to the victim task’s row.

This is the same vulnerability class Vikunja has already remediated for task relations (CVE-2026-33676), task attachments (CVE-2026-33678), task comments (CVE-2026-33313) and CalDAV task read (CVE-2026-35598). TaskBucket is the task-child operation that was missed.