FlashMQ is a MQTT broker/server, designed for multi-CPU environments. Prior to version 1.23.2, any authenticated user can create sessions and have them collect QoS messages. When not sent to a client, these are then not released upon (eventual) session expiration. Version 1.23.2 fixes the issue.
https://github.com/halfgaar/FlashMQ/security/advisories/GHSA-7mhp-22q4-r6vv
https://github.com/halfgaar/FlashMQ/issues/154
https://github.com/halfgaar/FlashMQ/commit/e86c49360ef4387440c97f591770cdb9284b4ee9