CVE-2026-27802: Vaultwarden has Privilege Escalation via Bulk Permission Update to Unauthorized Collections by Manager

A Manager account (access_all=false) was able to escalate privileges by directly invoking the bulk-access API against collections that were not originally assigned to them. The API allowed changing assigned=false to assigned=true, resulting in unauthorized access.

Additionally, prior to the bulk-access call, the regular single-update API correctly returned 401 Unauthorized for the same collection. After executing the bulk-access API, the same update API returned 200 OK, confirming an authorization gap at the HTTP level.