How to get list of security updates installed?

[Patil, Manjiri S]

Hi , 

I am running RHEL 4.0. I need to get a list of Security updates (Errata
) currently  installed on the machine. I couldn't find relevant option
in up2date? All I get through up2date or RHN website is security updates
that are required on the machine. I need updates which are already
installed. 

Is there a way to get those? 

Thanks