A study published yesterday by Verizon Business offers a new look at one of security's oldest problems and arrives at a new conclusion: While most breaches are executed by external attackers, the attacks are usually facilitated by security failures that were overlooked by internal staff, often for a long period of time.

In the study, which was generated by analyzing data from more than 500 forensic investigations conducted between 2004 and 2007, Verizon reports that 73 percent of data breaches resulted from external sources. This includes breaches caused by business partners, a source of vulnerability that increased fivefold during the study. Only 18 percent of breaches were caused by insiders.

But that doesn't mean internal parties don't contribute to the problem, Verizon asserts. In fact, the study also reveals that 62 percent of data breaches can be attributed to a significant error in internal behavior. Sixty-six percent of the breaches involved data that the victim organization did not know was on the system, and 75 percent of breaches are discovered by a third party, rather than someone inside the organization.

These seemingly contradictory bits of evidence -- that most breaches are perpetrated by outsiders but facilitated by errors inside -- indicate that most security breaches are crimes of opportunity, in which a door is left open and attackers simply walk in, Verizon suggests.

In fact, the study states specifically that 83 percent of attacks resulting in breaches are "not highly difficult" for the attacker. Eighty-five percent are the result of "opportunistic attacks," rather than targeted schemes, and 87 percent of the breaches probably could have been avoided through the proper enforcement of security controls, Verizon says.

"This report clearly shows it’s not about clever or complex security protection measures," says Peter Tippett, vice president of research and intelligence for Verizon Business Security Solutions. "It really boils down to doing the basics, from planning to implementation to monitoring of the data."

The most common "errors" identified in the study are errors of omission, which account for 79 percent of the mistakes identified. "This often entailed standard security procedures or configurations which were believed to have been implemented, but in actuality were not," the study says.

A lack of knowledge about corporate data, and where it is stored, is another common element in data breaches, according to the study. In fact. 90 percent of the breaches could be attributed to one of four causes: a system that was operating without the organization's knowledge; a set of data that was stored on a particular system without the organization's knowledge; a system that had unknown access or network connections; or a system that had unknown accounts or user privileges.

"We refer to these recurring situations as 'unknown unknowns,' and they appear to be the Achilles heel in the data protection efforts of every organization -- regardless of industry, size, location, or overall security posture." The scenario in which sensitive data is stored on an unknown or unauthorized system is by far the most common, according to the report.

"Investigators concluded that nearly all breaches would likely have been prevented if basic security controls had been in place at the time of attack," the report states.