In the last few days, Hacking Team, an Italian company that sold commercial malware intended for use by governments and companies for spying, was comprehensively hacked. 400 GB of content from their company was stolen and put on the 'net as a bittorrent.
Among the things revealed in this massive data dump was an extremely severe Flash Player vulnerability that allows remote code execution as the logged-in user. No existing mitigation mechanism prevents this on any Windows OS in play. It affects all major browsers. There is no patch as yet.
And it is now being exploited in the wild by criminal malware - possibly the fastest known weaponization of a vulnerability by criminal malware teams, probably due to the detailed info about how to perform the exploit contained in the Hacking Team data trove.
The exploits involved are "drive-by downloads". If you have Flash content set to auto-play, you do not have to do anything but visit a compromised website. No clicking or downloading or opening attachments is required.
Any website can be compromised. Do not depend on high-profile or well-known sites being safe.
Here is where you can fix the exploit
http://www.pcadvisor.co.uk/how-to/internet/how-stop-autoplaying-ads-videos-media-on-web-pages-3497991/