Prohibiting the use of banned APIs is a good way to remove a significant number of code vulnerabilities — this practice is reflected in Stage 6 of The Microsoft Security Development Lifecycle: "Establish and Follow Best Practices for Development." It can also be referenced in Chapter 11 of the Microsoft Press Book The Security Development Lifecycle.

When the C runtime library (CRT) was first created about 25 years ago, the threats to computers were different; machines were not as interconnected as they are today, and attacks were not as prevalent. With this in mind, a subset of the C runtime library must be deprecated for new code and, over time, removed from earlier code. It's just too easy to get code wrong that uses these outdated functions. Even some of the classic replacement functions are prone to error, too.

This list is the SDL view of what comprises banned APIs; it is derived from experience with real-world security bugs and focuses almost exclusively on functions that can lead to buffer overruns (Howard, LeBlanc, and Viega 2005). Any function in this section's tables must be replaced with a more secure version. Obviously, you cannot replace a banned API with another banned API. For example, replacing strcpy with strncpy is not valid because strncpy is banned, too.

Also note that some of the function names might be a little different, depending on whether the function takes ASCII, Unicode, _T (ASCII or Unicode), or multibyte chars. Some function names might include A or W at the end of the name. For example, the StrSafe StringCbCatEx function is also available as StringCbCatExW (Unicode) and StringCbCatExA (ASCII).