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While it might seem this recommendation unnecessary, experience has proven otherwise.

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 \[[Open Group 04|AA. C References#Open Group 04]\]. 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:

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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

Include an implementation of the missing function vsnprintf() in this case) with the executable to eliminate the dependency on an external library.

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 STR00-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

\[[Open Group 04|AA. C References#Open Group 04]\] [{{vsnprintf()}}|http://www.opengroup.org/onlinepubs/009695399/functions/vsnprintf.html]
\[[Seacord 05|AA. C References#Seacord 05]\] Chapter 6, "Formatted Output"
\[[VU#654390|AA. C References#VU#654390]\]

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