Is Perfect Access Control Possible?

By Bruce Schneier
Information Security
September 2009

This essay appeared as the second half of a point/counterpoint with Marcus Ranum. Marcus's half is here.

Access control is difficult in an organizational setting. On one hand, every employee needs enough access to do his job. On the other hand, every time you give an employee more access, there's more risk: he could abuse that access, or lose information he has access to, or be socially engineered into giving that access to a malfeasant. So a smart, risk-conscious organization will give each employee the exact level of access he needs to do his job, and no more.

Over the years, there's been a lot of work put into role-based access control. But despite the large number of academic papers and high-profile security products, most organizations don't implement it--at all--with the predictable security problems as a result.

Regularly we read stories of employees abusing their database access-control privileges for personal reasons: medical records, tax records, passport records, police records. NSA eavesdroppers spy on their wives and girlfriends. Departing employees take corporate secrets.

A spectacular access control failure occurred in the UK in 2007. An employee of Her Majesty's Revenue & Customs had to send a couple of thousand sample records from a database on all children in the country to National Audit Office. But it was easier for him to copy the entire database of 25 million people onto a couple of disks and put it in the mail than it was to select out just the records needed. Unfortunately, the discs got lost in the mail, and the story was a huge embarrassment for the government.

Eric Johnson at Dartmouth's Tuck School of Business has been studying the problem, and his results won't startle anyone who has thought about it at all. RBAC is very hard to implement correctly. Organizations generally don't even know who has what role. The employee doesn't know, the boss doesn't know--and these days the employee might have more than one boss -- and senior management certainly doesn't know. There's a reason RBAC came out of the military; in that world, command structures are simple and well-defined.

Even worse, employees' roles change all the time--Johnson chronicled one business group of 3,000 people that made 1,000 role changes in just three months--and it's often not obvious what information an employee needs until he actually needs it. And information simply isn't that granular. Just as it's much easier to give someone access to an entire file cabinet than to only the particular files he needs, it's much easier to give someone access to an entire database than only the particular records he needs.

This means that organizations either over-entitle or under-entitle employees. But since getting the job done is more important than anything else, organizations tend to over-entitle. Johnson estimates that 50 percent to 90 percent of employees are over-entitled in large organizations. In the uncommon instance where an employee needs access to something he normally doesn't have, there's generally some process for him to get it. And access is almost never revoked once it's been granted. In large formal organizations, Johnson was able to predict how long an employee had worked there based on how much access he had.

Clearly, organizations can do better. Johnson's current work involves building access-control systems with easy self-escalation, audit to make sure that power isn't abused, violation penalties (Intel, for example, issues "speeding tickets" to violators), and compliance rewards. His goal is to implement incentives and controls that manage access without making people too risk-averse.

In the end, a perfect access control system just isn't possible; organizations are simply too chaotic for it to work. And any good system will allow a certain number of access control violations, if they're made in good faith by people just trying to do their jobs. The "speeding ticket" analogy is better than it looks: we post limits of 55 miles per hour, but generally don't start ticketing people unless they're going over 70.

earlier essay: Offhand but On Record
later essay: The Battle Is On Against Facebook and Co to Regain Control of Our Files
categories: Business of Security, Computer and Information Security
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Photo of Bruce Schneier by Per Ervland.

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