“Waves of targeted email attacks, often called spear phishing, are exploiting client-side vulnerabilities in commonly used programs such as Adobe PDF Reader, QuickTime, Adobe Flash and Microsoft Office. This is currently the primary initial infection vector used to compromise computers that have Internet access. Those same client-side vulnerabilities are exploited by attackers when users visit infected web sites. (See Priority Two below for how they compromise the web sites). Because the visitors feel safe downloading documents from the trusted sites, they are easily fooled into opening documents and music and video that exploit client-side vulnerabilities. Some exploits do not even require the user to open documents. Simply accessing an infected website is all that is needed to compromise the client software. The victims’ infected computers are then used to propagate the infection and compromise other internal computers and sensitive servers incorrectly thought to be protected from unauthorized access by external entities. In many cases, the ultimate goal of the attacker is to steal data from the target organizations and also to install back doors through which the attackers can return for further exploitation. On average, major organizations take at least twice as long to patch client-side vulnerabilities as they take to patch operating system vulnerabilities. In other words the highest priority risk is getting less attention than the lower priority risk.”

(Source)