31.6 Testing, Maintenance, and Operation
Testing provides an informal validation of the design and implementation of the program. The goal of testing is to show that the program meets the stated requirements. When design and implementation are driven by the requirements, as in the method used to create the program under discussion, testing is likely to uncover only minor problems, but if the developers do not have well-articulated requirements, or if the requirements are changed during development, testing may uncover major problems, requiring changes up to a complete redesign and reimplementation of a program. The worst mistake managers and developers can make is to take a program that does not meet the security requirements and add features to it to meet those requirements. The problem is that the basic design does not meet the security requirements. Adding security features will not ameliorate this fundamental flaw.
Once the program has been written and tested, it must be installed. The installation procedure must ensure that when a user starts the process, the environment in which the process is created matches the assumptions embodied in the design. This constrains the configuration of the program parameters as well as the manner in which the system is configured to protect the program. Finally, the installers must enable trusted users to modify and upgrade the program and the configuration files and parameters.
The results of testing a program are most useful if the tests are conducted in the environment in which the program will be used (the production environment). So, the first step in testing a program is to construct an environment that matches the production environment. This requires the testers to know the intended production environment. If there are a range of environments, the testers must test the programs in all of them. Often there is overlap between the environments, so this task is not so daunting as it might appear.
The production environment should correspond to the environment for which the program was developed. A symptom of discrepancies between the two environments is repeated failures resulting from erroneous assumptions. This indicates that the developers have implicitly embedded information from the development environment that is inconsistent with the testing environment. This discrepancy must be reconciled.
The testing process begins with the requirements. Are they appropriate? Do they solve the problem? This analysis may be moot (if the task is to write a program meeting the given requirements), but if the task is phrased in terms of a problem to be solved, the problem drives the requirements. Because the requirements drive the design of the program, the requirements must be validated before designing begins.
As many of the software life cycle models indicate, this step may be revisited many times during the development of the program. Requirements may prove to be impossible to meet, or may produce problems that cannot be solved without changing the requirements. If the requirements are changed, they must be reanalyzed and verified to solve the problem.
Then comes the design. Section 31.4 discusses the stepwise refinement of the program. The decomposition of the program into modules allows us to test the program as it is being implemented. Then, once it has been completed, the testing of the entire program should demonstrate that the program meets its requirements in the given environment.
The general philosophy of testing is to execute all possible paths of control and compare the results with the expected results. In practice, the paths of control are too numerous to test exhaustively. Instead, the paths are analyzed and ordered. Test data is generated for each path, and the testers compare the results obtained from the actual data with the expected results. This continues until as many paths as possible have been tested.
For security testing, the testers must test not only the most commonly used paths but also the least commonly used paths.64 The latter often create security problems that attackers can exploit. Because they are relatively unused, traditional testing places them at a lower priority than that of other paths. Hence, they are not as well scrutinized, and vulnerabilities are missed.
The ordering of the paths relies on the requirements. Those paths that perform multiple security checks are more critical than those that perform single (or no) security checks because they introduce interfaces that affect security requirements. The other paths affect security, of course, but there are no interfaces.
First, we examine a module that calls no other module. Then we examine the program as a composition of modules. We conclude by testing the installation, configuration, and use instructions.
188.8.131.52 Testing the Module
The module may invoke one or more functions. The functions return results to the caller, either directly (through return values or parameter lists) or indirectly (by manipulation of the environment). The goal of this testing is to ensure that the module exhibits correct behavior regardless of what the functions returns.
The first step is to define “correct behavior.” During the design of the program, the refinement process led to the specification of the module and the module’s interface. This specification defines “correct behavior,” and testing will require us to check that the specification holds.
We begin by listing all interfaces to the module. We will then use this list to execute four different types of tests. The types of test are as follows:
Normal data tests. These tests provide unexceptional data. The data should be chosen to exercise as many paths of control through the module as possible.
Boundary data tests. These tests provide data that tests any limits to the interfaces. For example, if the module expects a string of up to 256 characters to be passed in, these tests invoke the module and pass in arrays of 255, 256, and 257 characters. Longer strings should also be used in an effort to overflow internal buffers. The testers can examine the source code to determine what to try. Limits here do not apply simply to arrays or strings. In the program under discussion, the lowest allowed UID is 0, for root. A good test would be to try a UID of –1 to see what happens. The module should report an error.
EXAMPLE: One UNIX system had UIDs of 16 bits. The system used a file server that would not allow a client’s root user to access any files. Instead, it remapped root’s UID to the public UID of –2. Because that UID was not assigned to any user, the remapped root could access only those files that were available to all users. The limit problem arose because one user, named Mike, had the UID 65534. Because 65534 = –2 in two’s complement 16-bit arithmetic, the remote root user could access all of Mike’s files—even those that were not publicly available.
Exception tests. These tests determine how the program handles interrupts and traps. For example, many systems allow the user to send a signal that causes the program to trap to a signal handler, or to take a default action such as dumping the contents of memory to a core file. These tests determine if the module leaves the system in a nonsecure state—for example, by leaving sensitive information in the memory dump. They also analyze what the process does if ordinary actions (such as writing to a file) fail.
EXAMPLE: An FTP server ran on a system that kept its authentication information confidential. An attacker found that she could cause the system to crash by sending an unexpected sequence of commands, causing multiple signals to be generated before the first signal could be handled. The crash resulted in a core dump. Because the server would be restarted automatically, the attacker simply connected again and downloaded the core dump. From that dump, she extracted the authentication information and used a dictionary attack65 to obtain the passwords of several users.
Random data tests. These tests supply inputs generated at random and observe how the module reacts. They should not corrupt the state of the system. If the module fails, it should restore the system to a safe state.66
EXAMPLE: In a study of UNIX utilities , approximately 30% crashed when given random inputs. In one case, an unprivileged program caused the system to crash. In 1995, a retest showed some improvement, but still “significant rates of failure” [1346, p. 1]. Other tested systems fared little better [705, 1344].
Throughout the testing, the testers should keep track of the paths taken. This allows them to determine how complete the testing is. Because these tests are highly informal, the assurance they provide is not as convincing as the techniques discussed in Chapter 20. However, it is more than random tests, or no tests, would provide.
31.6.2 Testing Composed Modules
Now consider a module that calls other modules. Each of the invoked modules has a specification describing its actions. So, in addition to the tests discussed in the preceding section, one other type of test should be performed.
Error handling tests. These tests assume that the called modules violate their specifications in some way. The goal of these tests is to determine how robust the caller is. If it fails gracefully, and restores the system to a safe state, then the module passes the test. Otherwise, it fails and must be rewritten.
EXAMPLE: Assume that a security-related program, running with root privileges, logs all network connections to a UNIX system. It also sends mail to the network administrator with the name of the connecting host on the subject line. To do this, it executes a command such as
mail -s hostname netadmin
where hostname is the name of the connecting host. This module obtains hostname from a different module that is passed the connecting host’s IP address and uses the Domain Name Service to find the corresponding host name. A serious problem arose because the DNS did not verify that hostname was composed of legal characters. The effects were discovered when one attacker changed the name of his host to
hi nobody; rm -rf *; true
causing the security-related program to delete critical files. Had the calling module expected failure, and checked for it, the error would have been caught before any damage was done.
31.6.3 Testing the Program
Once the testers have assembled the program and its documentation, the final phase of testing begins. The testers have someone follow the installation and configuration instructions. This person should not be a member of the testing team, because the testing team has been working with the program and is familiar with it. The goal of this test is to determine if the installation and configuration instructions are correct and easy to understand. The principle of least astonishment67 requires that the tool be as easy to install and use as possible. Because most installers and users will not have experience with the program, the testers need to evaluate how they will understand the documentation and whether or not they can install the program correctly by following the instructions. An incorrectly installed security tool does not provide security; it may well detract from it. Worse, it gives people a false sense of security.