Vulnerability analysis site
For those interested, my co-author (Mark Graff) and I have been posting and
maintaining a free repository of analyses of some recent/topical
vulnerabilities on our book's web page, at http://www.securecoding.org --
or go directly to the analyses at
In these periodic "columns", we analyze the root causes (no pun intended) of
some recent vulnerabilities and discuss ways of preventing similar mistakes
in the future. We also look at where in the development process (e.g.,
architecture, design, implementation, operations) the flaws were likely to
have been introduced. I've attached our most recent write-up below as an
Although the book isn't free, we're maintaining the web site* as a free
"book companion" resource. No registration, subscription, etc., is
* The web site is graciously hosted by our friends at http://www.nidhog.com.
Ken van Wyk
Co-authors of "Secure Coding: Principles and Practices" (O'Reilly, 2003)
Copyright (C) 2003, Mark G. Graff and Kenneth R. van Wyk. Permission granted
to reproduce and distribute in entirety with credit to authors.
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
"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  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  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
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 , Neumann , and Reason  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
Mark G. Graff
Kenneth R. van Wyk
29 July 2003
 Morris, Robert T. "A Weakness in the 4.2BSD Unix† TCP/IP
Software". AT&T Bell Laboratories,
 Dole, Bryn, Steve Lodin, and Eugene Spafford. "Misplaced Trust:
Kerberos 4 Session Keys." Proceedings of the 1997 ISOC
Conference. 1997. Available online at
 Perrow, Charles. Normal Accidents. New York, NY: Princeton
University Press, 1999. ISBN 0691004129.
 Neumann, Peter. Computer-Related Risks, New York:
Addison-Wesley/ACM Press, 1995. ISBN 0-201-55805-X.
 Reason, James. Human Error. New York: Cambridge University Press,
1990. ISBN 052131494.
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