PRE09-C. Do not replace secure functions with deprecated or obsolescent functions

Macros are frequently used in the remediation of existing code to globally replace one identifier with another, for example, when an existing API changes. While there is always some risk involved, this practice becomes particularly dangerous if a function name is replaced with the function name of a deprecated or obsolescent functions. Deprecated functions are defined by the C99 standard and Technical Corrigenda. Obsolescent functions are defined by rule MSC34-C. Do not use deprecated or obsolescent functions.

While compliance with rule MSC34-C. Do not use deprecated or obsolescent functions guarantees compliance with this recommendation, the emphasis of this recommendation is the extremely risky and deceptive practice of replacing functions with less secure alternatives.

Noncompliant Code Example

The Internet Systems Consortium's (ISC) Dynamic Host Configuration Protocol (DHCP) contained a vulnerability that introduced several potential buffer overflow conditions VU#654390. ISC DHCP makes use of the vsnprintf() function for writing various log file strings, which is defined in the Open Group Base Specifications Issue 6 [Open Group 2004] as well as C99 [ISO/IEC 9899:1999]. For systems that do not support vsnprintf(), a C include file was created that defines the vsnprintf() function to vsprintf(), as shown in this noncompliant code example:

#define vsnprintf(buf, size, fmt, list) \
vsprintf(buf, fmt, list)

The vsprintf() function does not check bounds. Consequently, size is discarded, creating the potential for a buffer overflow when untrusted data is used.

Compliant Solution

The solution is to include an implementation of the missing function vsnprintf() to eliminate the dependency on external library functions when they are not available. This compliant solution assumes that __USE_ISOC99 is not defined on systems that fail to provide a vsnprintf() implementation.

#include <stdio.h>
#ifndef __USE_ISOC99
  /* reimplements vsnprintf() */
  #include "my_stdio.h"
#endif

Risk Assessment

Replacing secure functions with less secure functions is a very risky practice because developers can be easily fooled into trusting the function to perform a security check that is absent. This may be a concern, for example, as developers attempt to adopt more secure functions, like the ISO/IEC TR 24731-1 functions that might not be available on all platforms. (See recommendation STR07-C. Use C11 Annex K bounds-checking interfaces for remediation of existing string manipulation code.)