If you need to be up-to-date about security related information then subscribing to mailing lists is often the most effective solution.
More newsletters are available from the Informatikdienste, please subscribe to them if you're running machines in the ETH network:
Software vendors typically announce their security patches via mailing lists. When operating a system, you should subscribe to those lists to get up to date information about patches, updates etc. Here's a showcase collection of such lists. They're all low volume lists with typically less than one mail per week:
Two big mailing lists are currently discussing about security problems. Subscribe to them in case you like (or need to) be on the bleeding edge. You will get a lot of mails, quite often also mails which can mess with your mailclient (Outlook, Thunderbird, …). Do not subscribe unless you now what you are doing
Critical problems are currently usually cross posted between both lists and subscribing to BUGTRAQ should be enough.