That means someone/something has corrupted or replaced the root CA cerificates your operating system uses to verify SSL cerificates. That means it either cant find them, access them or they have been modified/corrupted so that some or all of the root CAs certificates are missing. When this happens, yum cant confirm that SSL connections are valid. While this is an operating system level error, and something we'd advise you discuss with your OS vendor and/or hosting company, the following is the standard command to tell your OS to reinstall its SSL root CAs:
yum reinstall ca-certificates
If you are using any third party repos, we advise you disable them in case they are the source of this error. For example, if you are using EPEL:
yum --disablerepo="epel" reinstall ca-certificates
Note: You may need to disable additional repos, if those repos use SSL to download updates. yum will continue to fail as it tries to access repos that are accessed over SSL. Therefore, you will need to disable any repos that use SSL.
On some systems this may also be caused by an interrupted install of nss package updates from the OS vendor. Reinstallation of the nss packages may also be required:
yum reinstall nss*
If you would like our assistance with this OS level issue, we'd be happy to help you with and put a quote together for these services. Please contact support for a quote.