The Blackhole Exploit kit was good, so good in fact that criminals are having trouble finding anything with the capabilities to take its place, security firm Websense has noticed.

According to the firm’s monitoring, phishing campaigns sent via the important Cutwail bot that once made extensive use of the kit have been forced to experiment with a mixture of conventional Zip attachment spam and an alternative exploit kit, Magnitude.

The change happened in October, within days of the arrest of Blackhole’s Russian creator, dubbed ‘Paunch’. Since then, the gangs have been forced to chop and change new attack strategies, achieving mixed levels of success, Websense said.

It turns out that attachment spam is still effective when it gets past filters as are old-style phishing attacks using traditional lures such as ‘work at home’ and dieting. Often tactics are what distinguishes one gang form another on Cutwail, for instance the prominent ‘Zeus GameOver’ operation that specialises in collecting bank logins.

Given that Paunch’s arrest was recently confirmed, Blackhole won’t be coming back, leaving the way clear for another kit or some new innovation to take its place.

“We predict that in the next months, there will be a return to URL-based email attacks utilizing exploit kits that offer "malware as a service" on a larger scale,” concludes Websense.

“The use of exploit kits is simply a more effective delivery mechanism - especially with an increasingly security-aware target audience.”

Other firms have reported Blackhole’s sudden disappearance as causing a major headaches for criminals. Within days of Paunch’s rumoured arrest in October, Dell SecureWorks reorted that it was easy to see that something had happened to the previously hugely popular kit; the much smaller and less effective Magnitude spiked unexpectedly just at the moment Blackhole disappeared.

But the wider significance of Paunch's arrest remains unclear. Given that he was the most important software cybercriminal ever picked up by Russian police, does that bode ill for others in the same business?