matrix-appservice-irc is a Node.js IRC bridge for Matrix. Prior to version 1.0.1, it was possible to craft an event such that it would leak part of a targeted message event from another bridged room. This required knowing an event ID to target. Version 1.0.1n fixes this issue. As a workaround, set the `matrixHandler.eventCacheSize` config value to `0`. This workaround may impact performance.
https://github.com/matrix-org/matrix-appservice-irc/security/advisories/GHSA-c7hh-3v6c-fj4q
https://github.com/matrix-org/matrix-appservice-irc/releases/tag/1.0.1
https://github.com/matrix-org/matrix-appservice-irc/commit/8bbd2b69a16cbcbeffdd9b5c973fd89d61498d75