I've worked on a number of WP sites, but often the client doesn't put much stake in keeping WordPress up to date, and sites are allowed to age ever more and more. This is usually because updating WordPress often requires at least a lot of testing to find any affected parts of the site, and sometimes requires updating code for the new version of WP.
If I were able to quickly see if any WP version upgrade included an important security update, I would be able to leverage that when trying to convince clients to update WP. However the change logs for WP updates are huge.
Is there any place where I can quickly see that a WP upgrade included a security fix, and which versions of WP before it are susceptible?