Apple has updated Safari to block older versions of Adobe Flash.
This comes after a the recent discovery of a vulnerability in the plug-in. Adobe has patched it since, but that doesn’t necessarily mean that everyone installs the updates. Those updates can be a bit annoying, because they always pop up when you are browsing the internet and you usually click “install later” so that it doesn’t interrupt your browsing.
Of course you can always go to Adobe’s website and install the update. This will allow Flash to work even if you download the latest version of Safari.