Dave Hull over at Trusted Signal has an interesting post on his blog right now about blended threats. (Unfortunately, I can't find a permalink for it, so I don't know how long you'll be able to read it.)
If it's not still there for you to read, let me give you the gist of it. There's been some recent research into and discussion of blended threat scenarios by some very smart people.
So what is a blended threat? It's where two or more lesser-severity vulnerabilities are exploited in conjunction with each other to lead to a greater compromise. An example would be a pen-test I did some years back where we found a SQL injection vulnerability in a low-value web app with no insert/delete grant to an older, unpatched version of Oracle. Individually, you wouldn't rank either vuln especially high. You could break the web app, but there wasn't sensitive data in there, and you couldn't tamper with the data itself. The Oracle database wasn't exposed to the Internet directly. But by using SQL injection to attack Oracle, I broke out into the server OS, reverse tunneled a command shell, and had the Administrator password in very short order. Which was also the Administrator password of the other servers I could talk to.
Myself and others have been predicting the emergence of wide scale blended threat attacks since at least about 2002/2003. And so far we've been wrong, which is good. For now, blended attacks are, as Dave points out, the stuff of professional pen-testers and other intelligent intruders. But frankly, I don't know why.
The problem with blended threats is that they're harder to identify and calculate risk for. CVSS doesn't provide a way for scoring vuln A when also in the presence of vuln B. And this has lead to vendors delaying patches or downplaying the severity of vulnerabilities based on the assumption that any vulnerability the only vulnerability present.
This creates an opening in the patching cycle for malware/botnet folks to capitalize on if the right blended threat comes along. Maybe we haven't seen it becauuse, to date, these folks simply haven't needed to go there in order to be successful.