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2.5. Types of Secure Programs

Many different types of programs may need to be secure programs (as the term is defined in this book). Some common types are:

This book merges the issues of these different types of program into a single set. The disadvantage of this approach is that some of the issues identified here don't apply to all types of programs. In particular, setuid/setgid programs have many surprising inputs and several of the guidelines here only apply to them. However, things are not so clear-cut, because a particular program may cut across these boundaries (e.g., a CGI script may be setuid or setgid, or be configured in a way that has the same effect), and some programs are divided into several executables each of which can be considered a different ``type'' of program. The advantage of considering all of these program types together is that we can consider all issues without trying to apply an inappropriate category to a program. As will be seen, many of the principles apply to all programs that need to be secured.

There is a slight bias in this book toward programs written in C, with some notes on other languages such as C++, Perl, PHP, Python, Ada95, and Java. This is because C is the most common language for implementing secure programs on Unix-like systems (other than CGI scripts, which tend to use languages such as Perl, PHP, or Python). Also, most other languages' implementations call the C library. This is not to imply that C is somehow the ``best'' language for this purpose, and most of the principles described here apply regardless of the programming language used.