"Containers do not make a promise of providing resilient, multi-tenant isolation," he said. "It is possible for malicious code to escape from a container to attack the operation system or the other containers on the machine."
If a company isn't looking to get maximum efficiency out of its containers, however, it can run just one container per virtual machine.
This is the case with Nashua, NH-based Pneuron, which uses containers to distribute its business application building blocks to customers.
"We wanted to have assigned resourcing in a virtual machine to be usable by a specific container, rather than having two containers fight for a shared set of resources," said Tom Fountain, the company's CTO. "We think it's simpler at the administrative level."
Plus, this gives the application a second layer of security, he said.
"The ability to configure a particular virtual machine will provide a layer of insulation and security," he said. "Then when we're deployed inside that virtual machine then there's one layer of security that's put around the container, and then within our own container we have additional layers of security as well."
But the typical use case is multiple containers inside a single machine, according to a survey of IT professionals released Wednesday by container security vendor Twistlock.
Only 15 percent of organizations run one container per virtual machine. The majority of the respondents, 62 percent, said that their companies run multiple containers on a single virtual machine, and 28 percent run containers on bare metal.
And the isolation issue is still not figured out, said Josh Bressers, security product manager at Red Hat.
"Every container is sharing the same kernel," he said. "So if someone can leverage a security flaw to get inside the kernel, they can get into all the other containers running that kernel. But I'm confident we will solve it at some point."
Bressers recommended that when companies think about container security, they apply the same principles as they would apply to a naked, non-containerized application -- not the principles they would apply to a virtual machine.
"Some people think that containers are more secure than they are," he said.
McCauley said that Docker is also working to address another security issue related to containers -- that of untrusted content.
According to BanyanOps, a container technology company currently in private beta, more than 30 percent of containers distributed in the official repositories have high priority security vulnerabilities such as Shellshock and Heartbleed.
Outside the official repositories, that number jumps to about 40 percent.
Of the images created this year and distributed in the official repositories, 74 percent had high or medium priority vulnerabilities.
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