More information about this security flaw is available in the following bug: https://bugzilla.redhat.com/show_bug.cgi?id=2302272 Disclaimer: Community trackers are created by Red Hat Product Security team on a best effort basis. Package maintainers are required to ascertain if the flaw indeed affects their package, before starting the update process.
Hm, since that security flaw is located inside rubygem-rexml, i.e. a dependency of rubygem-gist, shouldn't it be reported against rubygem-rexml?
More useful links regarding the fix of rubygem-rexml: Bug 2282493 (f39) Bug 2282494 (f40)