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Friday, August 14, 2009

Costly Software Bugs

Software Bugs Cost U.S. Economy $59.6 Billion Annually, RTI Study Finds
Research Triangle Park, NC -- Software bugs are costly both to software producers and users.
Extrapolating from estimates of the costs in several software-intensive industries, bugs may be
costing the U.S. economy $59.5 billion a year; about 0.6 percent of gross domestic product, says a
study conducted by RTI for the U.S. Department of Commerce's National Institute of Standards and
Technology (NIST).

"More than half of the costs are borne by software users, and the remainder by software developers
and vendors," NIST said in summarizing the findings. "More than a third of these costs … could be
eliminated by an improved testing infrastructure that enables earlier and more effective identification
and removal of software defects."

History's Worst Software Bugs

What seems certain is that bugs are here to stay. Here, in chronological order, is the Wired News list
of the 10 worst software bugs of all time … so far.

July 28, 1962 -- Mariner I space probe. A bug in the flight software for the Mariner 1 causes the
rocket to divert from its intended path on launch. Mission control destroys the rocket over the
Atlantic Ocean. The investigation into the accident discovers that a formula written on paper in
pencil was improperly transcribed into computer code, causing the computer to miscalculate the
rocket's trajectory.

1982 -- Soviet gas pipeline. Operatives working for the Central Intelligence Agency allegedly (.pdf)
plant a bug in a Canadian computer system purchased to control the trans-Siberian gas pipeline.
The Soviets had obtained the system as part of a wide-ranging effort to covertly purchase or steal
sensitive U.S. technology. The CIA reportedly found out about the program and decided to make it
backfire with equipment that would pass Soviet inspection and then fail once in operation. The
resulting event is reportedly the largest non-nuclear explosion in the planet's history.

1985-1987 -- Therac-25 medical accelerator. A radiation therapy device malfunctions and delivers
lethal radiation doses at several medical facilities. Based upon a previous design, the Therac-25 was
an "improved" therapy system that could deliver two different kinds of radiation: either a low-power
electron beam (beta particles) or X-rays. The Therac-25's X-rays were generated by smashing high-
power electrons into a metal target positioned between the electron gun and the patient. A second
"improvement" was the replacement of the older Therac-20's electromechanical safety interlocks with
software control, a decision made because software was perceived to be more reliable.

What engineers didn't know was that both the 20 and the 25 were built upon an operating system
that had been kludged together by a programmer with no formal training. Because of a subtle bug
called a "race condition," a quick-fingered typist could accidentally configure the Therac-25 so the
electron beam would fire in high-power mode but with the metal X-ray target out of position.
At least five patients die; others are seriously injured.

1988 -- Buffer overflow in Berkeley Unix finger daemon. The first internet worm (the so-called
Morris Worm) infects between 2,000 and 6,000 computers in less than a day by taking advantage
of a buffer overflow. The specific code is a function in the standard input/output library routine called
gets() designed to get a line of text over the network. Unfortunately, gets() has no provision to limit
its input, and an overly large input allows the worm to take over any machine to which it can connect.

Programmers respond by attempting to stamp out the gets() function in working code, but they
refuse to remove it from the C programming language's standard input/output library, where it remains
to this day.

1988-1996 -- Kerberos Random Number Generator. The authors of the Kerberos security system
neglect to properly "seed" the program's random number generator with a truly random seed. As a
result, for eight years it is possible to trivially break into any computer that relies on Kerberos for
authentication. It is unknown if this bug was ever actually exploited.

January 15, 1990 -- AT&T Network Outage. A bug in a new release of the software that controls
AT&T's #4ESS long distance switches causes these mammoth computers to crash when they receive
a specific message from one of their neighboring machines -- a message that the neighbors send out
when they recover from a crash.

One day a switch in New York crashes and reboots, causing its neighboring switches to crash, then
their neighbors' neighbors, and so on. Soon, 114 switches are crashing and rebooting every six
seconds, leaving an estimated 60 thousand people without long distance service for nine hours. The
fix: engineers load the previous software release.

1993 -- Intel Pentium floating point divide. A silicon error causes Intel's highly promoted Pentium
chip to make mistakes when dividing floating-point numbers that occur within a specific range. For
example, dividing 4195835.0/3145727.0 yields 1.33374 instead of 1.33382, an error of 0.006 percent.
Although the bug affects few users, it becomes a public relations nightmare. With an estimated 3
million to 5 million defective chips in circulation, at first Intel only offers to replace Pentium chips for
consumers who can prove that they need high accuracy; eventually the company relents and agrees
to replace the chips for anyone who complains. The bug ultimately costs Intel $475 million.

1995/1996 -- The Ping of Death. A lack of sanity checks and error handling in the IP fragmentation
reassembly code makes it possible to crash a wide variety of operating systems by sending a
malformed "ping" packet from anywhere on the internet. Most obviously affected are computers
running Windows, which lock up and display the so-called "blue screen of death" when they receive
these packets. But the attack also affects many Macintosh and Unix systems as well.

June 4, 1996 -- Ariane 5 Flight 501. Working code for the Ariane 4 rocket is reused in the Ariane 5,
but the Ariane 5's faster engines trigger a bug in an arithmetic routine inside the rocket's flight
computer. The error is in the code that converts a 64-bit floating-point number to a 16-bit signed
integer. The faster engines cause the 64-bit numbers to be larger in the Ariane 5 than in the Ariane
4, triggering an overflow condition that results in the flight computer crashing.

First Flight 501's backup computer crashes, followed 0.05 seconds later by a crash of the primary
computer. As a result of these crashed computers, the rocket's primary processor overpowers the
rocket's engines and causes the rocket to disintegrate 40 seconds after launch.

November 2000 -- National Cancer Institute, Panama City. In a series of accidents, therapy
planning software created by Multidata Systems International, a U.S. firm, miscalculates the proper
dosage of radiation for patients undergoing radiation therapy.

Multidata's software allows a radiation therapist to draw on a computer screen the placement of
metal shields called "blocks" designed to protect healthy tissue from the radiation. But the software
will only allow technicians to use four shielding blocks, and the Panamanian doctors wish to use five.

The doctors discover that they can trick the software by drawing all five blocks as a single large
block with a hole in the middle. What the doctors don't realize is that the Multidata software gives
different answers in this configuration depending on how the hole is drawn: draw it in one direction
and the correct dose is calculated, draw in another direction and the software recommends twice
the necessary exposure.

At least eight patients die, while another 20 receive overdoses likely to cause significant health
problems. The physicians, who were legally required to double-check the computer's calculations
by hand, are indicted for murder.

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