Barracuda (the security company not the fish) has made public its threats database with entries going back to 2011.
You should probably be aware of this since their probing uses a "normal browser" - not sure how "normal". I don't block the barracuda IP ranges but instead allow them as proxies.
From a ThreatPost posting at http[://]threatpost[.]com/threatglass-tool-gives-deep-look-inside-compromised-sites/104844
(remove my brackets)
"Barracuda has been using its technology to scan millions of Web sites every
week, looking for malicious activity on legitimate sites ... The system
hits the sites using a normal browser and waits to see what kind of actions the
sites may take, looking for malicious activity like sites serving exploits or
trying to download files to visitors’ machines. Now, the company has built a GUI
for this system and exposed to the Web so that users and researchers can search
the database, dating back to 2011, looking for current or historic compromise data."
Threatglass is at http[://]www[.]threatglass[.]com/
