I'm not going to go into detail as to what spacewalk is and what this message is all about, you are here for a reason. You may have received an auto-generated email from Spacewalk to "inform you that your Spacewalk Certificate has expired on your [fqdn] server. After 4 day(s) the systems management services provided by your Spacewalk Server will be restricted for 24 days. After that the services will become inaccessible." The other is an alert on your Spacewalk logon page with a similar message. This issue effects Spacewalk 2.4 and below.
There are two fixes for this. One is a self-signed GPG resolution, which I have tried and works, the other the new official resolution as of July 16, 2018. This write-up is for the official resolution.
grep expires /usr/share/spacewalk/setup/spacewalk-public.cert <rhn-cert-field name="expires">2018-07-13 00:00:00</rhn-cert-field>
wget https://raw.githubusercontent.com/spacewalkproject/spacewalk/SPACEWALK-2.4/branding/setup/spacewalk-cert-key.gpg gpg --keyring /etc/webapp-keyring.gpg --no-default-keyring --import spacewalk-cert-key.gpg wget https://raw.githubusercontent.com/spacewalkproject/spacewalk/SPACEWALK-2.4/branding/setup/spacewalk-public.cert cp spacewalk-public.cert /usr/share/spacewalk/setup/ rhn-satellite-activate --sanity-only --rhn-cert=/usr/share/spacewalk/setup/spacewalk-public.cert rhn-satellite-activate --disconnected --rhn-cert=/usr/share/spacewalk/setup/spacewalk-public.cert
grep expires /usr/share/spacewalk/setup/spacewalk-public.cert <rhn-cert-field name="expires">2100-01-01 00:00:00</rhn-cert-field>