A buffer overflow occurs when data is written outside of the boundaries of the memory allocated to a particular data structure. Buffer overflows occur frequently in C and C++ because these languages (a) define strings as a null-ter-minated arrays of characters, (b) do not perform implicit bounds checking, and (c) provide standard library calls for strings that do not enforce bounds checking. These properties have proven to be a highly reactive mixture when combined with programmer ignorance about vulnerabilities caused by buffer overflows.
Buffer overflows are troublesome in that they can go undetected during the development and testing of software applications. Common C and C++ compilers do not identify possible buffer overflow conditions at compilation time or report buffer overflow exceptions at runtime. Dynamic analysis tools can be used to discover buffer overflows only as long as the test data precipitates a detectable overflow.
Not all buffer overflows lead to an exploitable software vulnerability. However, a buffer overflow can cause a program to be vulnerable to attack when the program's input data is manipulated by a (potentially malicious) user. Even buffer overflows that are not obvious vulnerabilities can introduce risk.
Buffer overflows are a primary source of software vulnerabilities. Type unsafe languages, such as C and C++, are especially prone to such vulnerabilities. Exploits can and have been written for Windows, Linux, Solaris, and other common operating systems and for most common hardware architectures, including Intel, Sparc, and Motorola.
A common mitigation strategy is to adopt a new library that provides an alternative, more secure approach to string manipulation. There are a number of replacement libraries and functions of this kind with varying philosophies and the choice of a particular library depends on your requirements. The safe library functions from ISO/IEC TR 24731 are not especially secure but are designed as easy drop-in replacement functions for existing calls. As a result, these functions may be used in preventive maintenance to reduce the likelihood of vulnerabilities in an existing, legacy code base. There is often a tradeoff between convenience and security involved in selecting an appropriate approach. More-secure functions often have more error conditions while less-secure functions try harder to provide a valid result for a given set of inputs. The choice of libraries is also constrained by language choice, platform, and portability issues.
There are practical mitigation strategies that can be used to help eliminate vulnerabilities resulting from buffer overflows. It is not practical to use all of the avoidance strategies because each has a cost in effort, schedule, or licensing fees. However, some strategies complement each other nicely. Static analysis can be used to identify potential problems to be evaluated during source code audits. Source code audits share common analysis with testing, so it is possible to split some costs. Dynamic analysis can be used in conjunction with testing to identify overflow conditions.
Runtime solutions such as bounds checkers, canaries, and safe libraries also have a runtime performance cost and may conflict. For example, it may not make sense to use a canary in conjunction with safe libraries because each performs more or less the same function in a different way.
Buffer overflows are the most frequent source of software vulnerabilities and should not be taken lightly. If possible, we recommend a defense-in-depth strategy of applying multiple strategies. The first and foremost strategy for avoiding buffer overflows, however, is to educate developers about how to avoid creating vulnerable code.