More information about this security flaw is available in the following bug: https://bugzilla.redhat.com/show_bug.cgi?id=2351766 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.
syncthing only serves HTTP on localhost for a local management GUI and does no HTTP over the network, so as far as I can tell it is not affected by this issue.