Redcarpet is a Ruby library for Markdown processing. In Redcarpet before version 3.5.1, there is an injection vulnerability which can enable a cross-site scripting attack. In affected versions no HTML escaping was being performed when processing quotes. This applies even when the `:escape_html` option was being used. This is fixed in version 3.5.1 by the referenced commit.
https://www.debian.org/security/2021/dsa-4831
https://rubygems.org/gems/redcarpet
https://lists.debian.org/debian-lts-announce/2021/01/msg00014.html
https://github.com/vmg/redcarpet/commit/a699c82292b17c8e6a62e1914d5eccc252272793
https://github.com/vmg/redcarpet/blob/master/CHANGELOG.md#version-351-security