Description
matrix-appservice-irc is a Node.js IRC bridge for Matrix. Prior to version 1.0.1, it is possible to craft a command with newlines which would not be properly parsed. This would mean you could pass a string of commands as a channel name, which would then be run by the IRC bridge bot. Versions 1.0.1 and above are patched. There are no robust workarounds to the bug. One may disable dynamic channels in the config to disable the most common execution method but others may exist.
Remediation
References
https://github.com/matrix-org/matrix-appservice-irc/commit/0afb064635d37e039067b5b3d6423448b93026d3
https://github.com/matrix-org/matrix-appservice-irc/releases/tag/1.0.1
https://github.com/matrix-org/matrix-appservice-irc/security/advisories/GHSA-3pmj-jqqp-2mj3
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