It's the hottest-selling item, other than drugs, he said. The service providers encrypt the malware as many times as it takes until it is no longer detected by any of the standard anti-malware tools available on the market.
"it's why targeted attacks have become so prevalent," said Kellermann. "They will make sure their attacks cannot be stopped by perimeter defenses."
Prices range from $20 for a single crypt on one file to $1,000 per month for unlimited crypts on an unlimited number of files.
In addition, criminals can buy ongoing access to enterprises.
"In the Russian or Chinese underground, they won't sell you the back door into the system," Kellerman said. "That's a North American phenomenon. It's like, I broke into a house last night, I made a duplicate of the key. You want it, you got it."
It's not enough for homeowners to keep their doors and windows locked, he said. They also need to keep a Rottweiler inside the house.
The access is used to steal data, but also as a base for attacking other enterprises -- and for attacking the same enterprise over and over again.
"It really leads to the reality of so much hacking between networks occurring, and why so many organizations are suffering from secondary infections," he said. "They're getting hit again and again from in the inside out."
Sign up for CIO Asia eNewsletters.