PRE09-A. Do not replace secure functions with less secure functions

Macros are frequently used in the remediation of existing code to globally replace on 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 a less secure function.

Non-Compliant Code Example

The Internet Systems Consortium's (ISC) Dynamic Host Configuration Protocol (DHCP) contained a vulnerability that introduced several potential buffer overflow conditions. ISC DHCP makes use of the vsnprintf() function for writing various log file strings, which is defined in in the Open Group Base Specifications Issue 6 [[Open Group 04]] 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 non-compliant 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 to the above problem 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
  #include "my_stdio.h" /* reimplements vsnprintf() */
#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 (see STR07-A. Use TR 24731 for remediation of existing string manipulation code) that might not be available on all platforms.

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this rule on the CERT website.

References

[[ISO/IEC 9899:1999]] Section 7.19.6.12, "The vsnprintf function"
[[Open Group 04]] vsnprintf()
[[Seacord 05]] Chapter 6, "Formatted Output"
[[VU#654390]]

PRE08-A. Guarantee that header file names are unique      01. Preprocessor (PRE)       PRE10-A. Wrap multi-statement macros in a do-while loop