matrix-appservice-irc is a Node.js IRC bridge for the Matrix messaging protocol. matrix-appservice-irc before version 2.0.0 can be exploited to leak the truncated body of a message if a malicious user sends a Matrix reply to an event ID they dont have access to. As a precondition to the attack, the malicious user needs to know the event ID of the message they want to leak, as well as to be joined to both the Matrix room and the IRC channel it is bridged to. The message reply containing the leaked message content is visible to IRC channel members when this happens. matrix-appservice-irc 2.0.0 checks whether the user has permission to view an event before constructing a reply. Administrators should upgrade to this version. Its possible to limit the amount of information leaked by setting a reply template that doesnt contain the original message. See these lines 601-604 in the configuration file linked.
The product does not handle or incorrectly handles when it has insufficient privileges to access resources or functionality as specified by their permissions. This may cause it to follow unexpected code paths that may leave the product in an invalid state.