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Trojan Horses

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This chapter is from the book

This chapter is from the book

Poisoning the Source

Most software sucks.

—Jim McCarthy, founder of a software quality training company

So, we've seen a variety of techniques bad guys use to squeeze Trojan horse functionality into our systems. However, perhaps the most worrisome Trojan horse vector involves inserting malicious code into a software product before it's even released. Attackers could Trojanize programs during the software vendor's development and testing processes. Suppose an attacker hires on as an employee at a major software development shop or volunteers to contribute code to an open source software project. The target could be anything; a major operating system, a widely used enterprise resource planning tool, or even a very esoteric program used by banks to manage their funds transfer would all make juicy targets. As a developer or even a tester, the attacker could insert a relatively small backdoor of less than 100K of code inside of hundreds of megabytes of legitimate code. That's really a needle in a haystack! Any users purchasing the product would then unwittingly be buying a Trojan horse and installing it on their systems. The whole software product itself becomes the Trojan horse, doing something useful (that's why you buy or download it), yet masking this backdoor.

Ken Thompson, noted UNIX cocreator and C programming language guru, discussed the importance of controlling source code and the possibility of planting backdoors in it in his famous 1984 paper titled "Reflections on Trusting Trust." In that classic paper, Thompson described modifying the source code for a compiler so that it built a backdoor into all code that it compiles [6]. The proposed attack was particularly insidious, as even a brand new compiler that is compiled with a Trojan version of the old compiler would have the backdoor in it, too. This avenue of attack has long been a concern, and is an even bigger potential problem today.

This concern is even more disturbing than the Trojaning of software distribution sites that we discussed in the last section. When an attacker Trojanizes a software distribution site, the developers of the software at least have a clean version of the software that they can compare against to detect the subterfuge. Backing out problems is relatively easier after discovery, as a clean version of the software can be placed on the Web site for distribution. On the other hand, if an attacker embeds a Trojan horse during the software development process, the vendor might not even have a clean copy. If the attackers are particularly clever, they will intertwine a small, inconspicuous backdoor throughout the normal code, making eradication extremely difficult. The software developer would have to scan enormous quantities of code to ensure the integrity of a whole product. The larger the software product, the more difficult detection and eradication become. Let's analyze why this is so.

Code Complexity Makes Attack Easier

Most modern software tools are vast in scope. Detecting bugs in code, let alone backdoors, is very difficult and costly. To Trojanize a software product, an evil employee doesn't even have to actually write an entire backdoor into the product. Instead, the malicious developer could purposefully write code that contains an exploitable flaw, such as a buffer overflow, that would let an attacker take over the machine. Effectively, such a purposeful flaw acts just like a backdoor. If the flaw sneaks past the software testing team, the developer would be the only one who knows about the hole initially. By exploiting that flaw, the developer could control any systems using his or her code.

To get a feel for how easily such an intentional flaw or even a full Trojan horse could squeak past software development quality processes, let's consider the quality track record of the information technology industry over time. Software quality problems have plagued us for decades. With the introduction of higher density chips, fiber-optic technology, and better hard drives, hardware continues to get more reliable over time. Software, on the other hand, remains stubbornly flawed. Watts Humphrey, a software quality guru and researcher from Carnegie Mellon University, has conducted surveys into the number of errors software developers commonly make when writing code [6]. Various analyses have revealed that, on average, a typical developer accidentally introduces between 100 and 150 defects per 1,000 lines of code. These issues are entirely accidental, but a single intentional flaw could be sneaked in as well.

Although many of these errors are simple syntactical problems easily discovered by a compiler, a good deal of the remaining defects often result in gaping security holes. In fact, in essence, a security vulnerability is really just the very controlled exploitation of a bug to achieve an attacker's specific goal. If the attacker can make the program fail in a way that benefits the attacker (by crashing the system, yielding access, or displaying confidential information), the attacker wins. Estimating very conservatively, if only one in 10 of the defects in software has security implications, that leaves between 10 and 15 security defects per 1,000 lines of code. These numbers just don't look very heartening.

A complex operating system like Microsoft Windows XP has approximately 45 million lines of code, and this gigantic number is growing as new features and patches are released [7]. Other operating systems and applications have huge amounts of code as well. Doing the multiplication for XP, there might be about 450,000 security defects in Windows XP alone. Even if our back-of-the-envelope calculation is too high by a factor of 100, that could still mean 4,500 security flaws. Ouch! Indeed, the very same day that Windows XP was launched in October 2001, Microsoft released a whopping 18 megabytes of patches for it.

Don't get me wrong; I love Windows XP. It's far more reliable and easier to use than previous releases of Windows. It's definitely a move in the right direction from these perspectives. However, this is just an illustration of the security problem inherent in large software projects. It isn't just a Microsoft issue either; the entire software industry is introducing larger, more complex, ultra-feature-rich (and sometimes feature-laden) programs with tons of security flaws. Throughout the software industry, we see very fertile soil for an attacker to plant a subtle Trojan horse.

Test? What Test?

Despite these security bugs, some folks still think that the testing process employed by developers will save us and find Trojan horses before the tainted products hit the shelves. I used to assuage my concerns with that argument as well. It helped me sleep better at night. But there is another dimension here to keep in mind to destroy your peaceful slumber: Easter eggs. According to The Easter Egg Archive™, an Easter egg is defined as:

Any amusing tidbit that creators hid in their creations. They could be in computer software, movies, music, art, books, or even your watch. There are thousands of them, and they can be quite entertaining, if you know where to look.

Easter eggs are those unanticipated goofy little "features" squirreled away in your software (or other products) that pop up under very special circumstances. For example, if you run the program while holding down the E, F, and S keys, you might get to see a dorky picture of the program developer. The Easter Egg Archive maintains a master list of these little gems at http://www.eeggs.com, with more than 2,775 software Eas-ter eggs on record as of this writing.

What do Easter eggs have to do with Trojan horses in software? A lot, in fact. If you think about our definition of a Trojan horse from early in this chapter, an Easter egg is really a form of Trojan horse, albeit a (typically) benign one. However, if software developers can sneak a benign Easter egg past the software testing and quality assurance teams, there's no doubt in my mind that they could similarly pass a Trojan horse or intentional buffer overflow as well. In fact, the attacker could even put the backdoor inside an Easter egg embedded within the main program. If the testing and quality assurance teams don't notice the Eas-ter egg or even notice it but let it through, they likely won't check it for such hidden functionality. To me, the existence of Easter eggs proves quite clearly that a malicious developer or tester could put nasty hidden functionality inside of product code and get it through product release without being noticed.

To get a feel for an Easter egg, let's look at one embedded within a popular product, Microsoft's Excel spreadsheet program. Excel is quite famous for its Easter eggs. An earlier version of the program,

Figure 8Figure 6.8 The game hidden inside of the Microsoft Excel 2000 spreadsheet application.

Excel 97, included a flight simulator game. A more recent version, Excel 2000, includes a car-driving game called Dev Hunter, which is shown in Figure 6.8.

For this Easter egg to work, you must have Excel 2000 (pre Service Release 1), Internet Explorer, and DirectX installed on your computer. To activate the Easter egg and play the game, you must do the following:

  • Run Excel 2000.

  • Under the File menu, select Save as Web Page.

  • On the Save interface, select Publish and then click the Add Interactivity box.

  • Click Publish to save the resulting HTM page on your drive.

  • Next, open the HTM page you just created with Internet Explorer. The blank spreadsheet will appear in the middle of your Internet Explorer browser window.

  • Here's the tricky part. Scroll down to row 2000, and over to column WC.

  • Now, select the entirety of row 2000 by clicking on the 2000 number at the left of the row.

  • Hit the Tab key to make WC the active column. This column will be white, while the other columns in the row will be darkened.

  • Hold down Shift+Crtl+Alt and, at the same time, click the Microsoft Office logo in the upper left corner of the spreadsheet.

  • In a second or two, the game will run.

  • Use the arrow keys to drive and steer and the spacebar to fire. The O key drops oil slicks to confound the other cars. When it gets dark, you can use the H key to turn on your headlights.

If the game isn't invoked on your system, it is likely because you have Service Release 1 or a later version of Microsoft Excel installed on your machine, which doesn't include the Easter egg. You could hunt down an earlier version of Microsoft Excel, or just take my word for it.

Now, mind you, this "feature" is in a spreadsheet, an office productivity program. Depending on your mindset, it might be quirky and fun. However, how does such a thing get past the software quality process (which should include code reviews) and testing team? Maybe the quality assurance and testing personnel didn't notice it. Or, perhaps the quality assurance folks and testers were in cahoots with the developers to see that the game got included into the production release. Either way, I'm concerned with the prospects of a Trojan horse being inserted in a similar way at other vendors.

Again, I'm not picking on just Microsoft here. In fact, Microsoft has gotten better over the past couple of years with respect to these concerns. New service packs or hot fixes frequently and quickly squash any Easter eggs included in earlier releases. Microsoft's Trusted Computing initiative, although often derided, is beginning to bear some fruit as fewer and fewer security vulnerabilities and Easter eggs appear to be coming to market in Microsoft programs. However, I say this with great hesitation, as another huge gaping egg could be discovered any day. Still, underscoring that this is not a Microsoft-only issue, many other software development shops have Easter eggs included in their products, including Apple Computer, Norton, Adobe, Quark, the open source Mozilla Web browser, and the Opera browser. The list goes on and on, and is spelled out for the world to see at http://www.eeggs.com.

The Move Toward International Development

A final area of concern regarding malicious software developers and Trojan horses is associated with code being developed around the world. Software manufacturers are increasingly relying on highly distributed teams around the planet to create code. And why not? From an economic perspective, numerous countries have citizens with top-notch software development skills and much lower labor rates. Although the economics make sense, the Trojan horse security issue looms much larger with this type of software development.

Suppose you buy or download a piece of software from Vendor X. That vendor, in turn, contracts with Vendors Y and Z to develop certain parts of the code. Vendor Z subcontracts different subcomponents of the work to three different countries around the globe. By the time the product sits on your hard drive, thousands of hands distributed across the planet could have been involved in developing it. Some of those hands might have planted a nasty backdoor. Worse yet, the same analysis applies to the back-end financial systems used by your bank and the database programs housing your medical records. Information security laws and product liability rules vary significantly from country to country, with many nations not having very robust regulations at all.

This concern is not associated with the morality of the developers in various countries. Instead, the concern deals with the level of quality control that can be applied with limited contract and regulatory supporting structures. Also, the same economic effects that are driving development to countries with less expensive development personnel could exacerbate the problem. An attacker might be able to bribe a developer making $100 a week or month into putting a backdoor into code for very little money. "Here's 10 years' salary ... please change two lines of code for me" might be all that it would take. We don't want to be xenophobic here; international software development is a reality with significant benefits in today's information technology business. However, we must also recognize that it does increase the security risks of Trojan horses or intentional software flaws.

Defenses Against Poisoning the Source

How can you defend yourself from a Trojan horse planted by an employee of your software development house? This is a particularly tough question, as you have little control over the development of the vast majority of the software on your systems. Still, there are things we can all do as a community to improve this situation.

First, you can encourage your commercial vendors to have robust integrity controls and testing regimens for their products. If they don't, beat them up1 and threaten to use other products. When the marketplace starts demanding more secure code, we'll gradually start inching in that direction. Additionally, if you use a lot of open source software, support that community with your time and effort in understanding software flaws. If you have the skills, help out by reviewing open source code to make sure it is secure.

Next, when you purchase or download new software, test it first to make sure it doesn't include any obvious Trojan horse capability. Use the software tests we described in Chapter 5 to look for unusual open ports, strange communication across the network, and suspect files on your machine. With a thorough software test and evaluation process in house, you might just find some Trojan horses in your products before anyone else notices them. Communicate this information to the vendor to help resolve the issue.

If your organization develops any code in house, make sure your software testing team is aware of the problems of Easter eggs, Trojan horses, and intentional flaws. Sadly, software testers are often viewed as the very bottom tier of importance in the software development hierarchy, usually getting little respect, recognition, or pay. Yet, their importance to the security of our products is paramount. Train these folks so that they can quickly spot code that doesn't look right and report it to appropriate management personnel. Reward your testers when they find major security problems before you ship software. Be careful, though. You don't want to have testers working with developers to game the system and plant bugs so they can make more money. That's like having a lottery where people can print their own winning tickets. Carefully monitor any bug reward programs you create for such subterfuge.

Furthermore, ensure that your testers and developers can report security concerns without reprisals from desperate managers trying to meet a strict software deadline. Depending on the size of your organization and its culture, you might even have to introduce an anonymous tipline for your developers to report such concerns. By giving this much-needed additional attention to your software testers, you can help to squelch problems with Trojan horses as well as improve the overall quality of your products.

To infuse this mindset throughout the culture of your software development teams, consider transforming your test organization into a full-fledged quality assurance function. The quality assurance organization should be chartered with software security responsibility as a facet of quality. Build your quality assurance process into the entire cycle of software development, including design, code reviews, and testing. You should also impose careful controls on your source code, requiring developers to authenticate before working on any modules. All changes should be tracked and reviewed by another developer. Only with thorough quality processes and source code control can we improve the situation associated with untrustworthy source code.

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