to notify a broad range of would-be attackers that a specific vulnerability exists. And if the patch is not installed, a broader range of attackers is likely to have knowledge of the vulnerability than if the patch had not been distributed at all. And patches are not always installed when the vendor issues them because patch installation will from time to time damage existing functionality on a system (e.g., causing a critical application to stop working until it can be made compatible with the patch to be installed).

As a rule, vulnerabilities resulting from design errors or insecure design choices are harder to fix than those resulting from implementation errors. Perhaps still more difficult are vulnerabilities introduced by unintended functionality (the euphemism for adding a function to software that helps an attacker but that is not desired by the authorized user or developer)—the classic “back-door” vulnerability.2 Most system evaluation checks the extent to which a product meets the formal requirements, and not whether it does more than intended. Whereas vulnerabilities due to faulty design and implementation may be uncovered during the testing process or exposed during system operation and then fixed, vulnerabilities associated with unintended functionality may go undetected because the problem is tantamount to proving a negative.

Today, applications and operating systems are made up of millions of lines of code, not all of which can possibly be audited for every changed line of source code. A widely used program might have vulnerabilities deliberately introduced into it by a “rogue” programmer employed by the software vendor but planted by the attacker. (One of the most plausible vectors for the surreptitious introduction of hostile code is a third-party device driver. In some operating systems, drivers almost always require the calling system to delegate to them privileges higher than those granted


As an example of a back door that is harmless, most versions of Microsoft Word from Word 97 to Word 2003 contain some unexpected functionality—typing “=rand()” in a Word document and then pressing the ENTER key results in three paragraphs of five repetitions of the sentence “The quick brown fox jumps over the lazy dog.” This particular back door is harmless and is even documented by Microsoft (see “How to Insert Sample Text into a Document in Word,” available at Such functionality could easily not be documented, and could easily be harmful functionality as well. For example, a security interface to a computer might be designed to require the user to enter a password and to insert a physical “smart card” into a slot before granting her access. But the interface could easily be programmed to ignore the smart-card requirement when a special password is entered, and then to grant the user many more privileges than would be normal. On the other hand, the in-advance installation of a back-door vulnerability always runs a risk of premature exposure—that is, it may be discovered and fixed before the attacker can use it. Even worse from the attacker’s standpoint, it may be fixed in such a way that the attacked system appears vulnerable but is in fact not vulnerable to that particular attack. Thus, the attacker may attack and believe he was successful, even though he was not.

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