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Analysis of Topical Vulnerabilities

29 July 2003

We figure our readers must know as much about new gadgets as any group in the world. So we are going to ask you to keep an eye out for one (we'll describe it later) that we think might prevent vulnerabilities like the one under the microscope today.

The flaw was first reported by security firm ThreeZee. The full text of their advisory is available at http://www.threezee.com/sections/security/tzt001.txt. (As always, we encourage you to read the original advisory in full. There's always more to the story than we cover here.)

ThreeZee points out a problem with a particular mobile phone service provider's messaging software. It turns out that any visitor to the provider's web site can predict the ID numbers associated with text messages, also known as Short Message Service (SMS) messages. That simple ability opens up a gaping security hole. You could (but please don't) obtain for yourself the delivery reports intended for message senders. You could get the email addresses of the recipients, too.

What's the big deal about that? Well, by combining that information with a couple of other flaws, an attacker could eavesdrop on new text messages sent to the cell phones. Potentially, one could gain at least partial control of a cell phone account. It's a classic example of step-by-step system compromise, where each new plateau reached yields information making further compromise possible. If we were the each-new-dawn-a-miracle type, we would call it beautiful.

But the bug itself is quite a curiosity, too. The principal flaw: message ID's are coined in a predictable sequence. Once you know one, you can deduce a practically unlimited number of them; and knowing those message ID's unlocks all that information you're not supposed to be able to get to. Here's how the advisory explains the prediction method.
"While the Tracking, or message ID may look foreign in ways, it's quite simple.

Think of the way an odometer turns on a car. That is the basic idea of the ID.

Example 1: MsgID4_A54GKVHD

Example 2: MsgID4_3M5GKVHD

Starting after the '_', the message ID will progress in the order of A - Z, and 0 ? 9. There seems to be no association with the time sent, or who it was sent to. Like the odometer, when a character/digit of the ID reaches the end (9), it will restart at A, and the preceding character will increase by 1."
Does this seem familiar? Where have we seen this before?

Well, for starters, Robert Tappan Morris described a similar vulnerability in his 1985 paper [1] at AT&T Bell Labs. The problem he unearthed there had to do with predicting sequence numbers used in the TCP protocol. In Chapter 4 of Secure Coding, we cite a conceptually similar problem [2] in version 4 of MIT's Kerberos system. In that case, the designers really tried to make the initial sequence numbers "random" (hence, unpredictable) but still came up short.

Now those design errors were made in the 70's and '80s, the bad old days. How could such a problem get introduced into a web site "in this day and age"? Well, it's easy, really. Let's ask instead: how could it have been avoided?

WHAT CAN GO WRONG?

In some of the better Software Engineering curricula we are familiar with, the value and power of a process known as "domain analysis" is taught. It's basically a fancy way to learn from the mistakes of others. The point is to locate, study and analyze during the design process similar problems that cropped up in earlier projects. We recommend the practice. (In fact, as we write this, one of us is undertaking just such a study for an important design we are participating in.)

There's not always time for domain analysis. Even if there were, our profession is so new--and the world we deal with still so abstract--that we don't have the great body of disasters other engineers do to draw upon for inspiration. (To get a start, you might try Perrow [3], Neumann [4], and Reason [5] for stories, respectively, of catastrophic engineering errors; computer-related disasters; and common human errors in risk evaluation. In addition to Peter Neumann's above book, his superb on-line RISKS Digest, http://catless.ncl.ac.uk/Risks, is a great forum to study and learn.) So what is the best way for a programmer operating under real-world constraints to identify lurking design-level errors?

Train yourself to ask the question, "What can go wrong?" When designing a piece of software, the design team should be considering the ramifications of their design choices from exactly that perspective. What would happen if someone were able to guess the value of any arbitrary message ID on the text-messaging portal? What could an attacker do with that information? It's our experience that once you start down that road, you'll often find yourself rooting out one potential design weakness after another. In this case, maybe it would have sufficed to pass onto the implementer a note that message ID's should be reasonably unpredictable.

A MIRACLE OF A RARE DEVICE

We opened this analysis with a mention of a device we had an idea for that could help prevent vulnerabilities like this.

We would like this gizmo to hover above our shoulder all of the time. (Or you could build it into, let's say, the kind of pith helmet worn by jungle explorers. That would be OK.) Our main requirement is that it must sound a loud gong whenever we need to ask the question, "What can go wrong?" Once a project should be enough. Hey, by the way, solar power would be a neat add-on feature.

Until we have one, we'll try to remember to ask the question ourselves--or, better yet, use a checklist to help remind us to ask the questions.

Cheers,

Mark G. Graff
Kenneth R. van Wyk
29 July 2003

REFERENCES

[1] Morris, Robert T. "A Weakness in the 4.2BSD Unix TCP/IP Software". AT&T Bell Laboratories, 1985. http://www.pdos.lcs.mit.edu/~rtm/papers/117.pdf

[2] Dole, Bryn, Steve Lodin, and Eugene Spafford. "Misplaced Trust: Kerberos 4 Session Keys." Proceedings of the 1997 ISOC Conference. 1997. Available online at http://www.isoc.org/isoc/conferences/ndss/97/dole_sl.pdf

[3] Perrow, Charles. Normal Accidents. New York, NY: Princeton University Press, 1999. ISBN 0691004129.

[4] Neumann, Peter. Computer-Related Risks, New York: Addison-Wesley/ACM Press, 1995. ISBN 0-201-55805-X.

[5] Reason, James. Human Error. New York: Cambridge University Press, 1990. ISBN 052131494.

Copyright (C) 2003, Mark G. Graff and Kenneth R. van Wyk. Permission granted to reproduce and distribute in entirety with credit to authors.


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