The CERT C Secure Coding Standard was developed specifically for versions of the C programming language defined by
- ISO/IEC 9899:2011 ISO/IEC. Programming Languages—C, 3rd ed. [ISO/IEC 9899:2011]
- ISO/IEC 9899:2011/Cor.1:2012, Technical Corrigendum 1
Most of the material included in this standard can also be applied to earlier versions of the C programming language.
Noncompliant examples in CERT C Coding Standard guidelines are operating system and platform independent. However, the best solutions to secure coding problems are often platform specific. In most cases, this standard provides appropriate compliant solutions for POSIX-compliant and Windows operating systems. Language and library extensions that have been published as ISO/IEC Technical Reports or Technical Specifications are frequently given precendence, such has those described by ISO/IEC TR 24731-2 Extensions to the C Library—Part II: Dynamic Allocation Functions [ISO/IEC TR 24731-2:2010]. In many cases, compliant solutions are also provided for specific platforms such as Linux or OpenBSD. Occasionally, we also point out implementation-specific behaviors when these behaviors are of interest.
Rationale
A secure coding standard for the C programming language can create the highest value for the longest period of time by focusing on the C Standard (C11) and the relevant post-C11 technical reports.
The C Standard documents existing practice where possible. That is, most features must be tested in an implementation before being included in the standard. The CERT C Coding Standard has a different purpose. When existing practice serves this purpose, that is fine, but the goal is to create a new set of best practices, and that includes introducing some concepts that are not yet widely known. To put it a different way, the CERT C Coding Standard attempts to drive change rather than just document it.
For example, the optional but normative Annex K, "Bounds-Checking Interfaces" introduced in C11 is gaining support but at present is implemented by only a few vendors. It introduces functions such as memcpy_s(), which serve the purpose of security by adding the destination buffer size to the API. A forward-looking document could not reasonably ignore these functions simply because they are not yet widely implemented.
The base C Standard is more widely implemented than Annex K, but even if it were not, it is the direction in which the industry is moving. Developers of new C code, especially, need guidance that is usable on and makes the best use of the compilers and tools that are now being developed and are being supported into the future.
Some vendors have extensions to C, and some also have implemented only part of the C Standard before stopping development. Consequently, it is not possible to back up and discuss only C95, C90, or C99. The vendor support equation is too complicated to draw a line and say that a certain compiler supports exactly a certain standard. Whatever demarcation point is selected, different vendors are on opposite sides of it for different parts of the language. Supporting all possibilities would require testing the cross-product of each compiler with each language feature. Consequently, we have selected a demarcation point that is the most recent in time so that the rules and recommendations defined by the standard will be applicable for as long as possible. As a result of the variations in support, source-code portability is enhanced when the programmer uses only the features specified by C99. This is one of many trade-offs between security and portability inherent to C language programming.
The value of forward-looking information increases with time before it starts to decrease. The value of backward-looking information starts to decrease immediately.
For all of these reasons, the priority of this standard is to support new code development using C11 and the post-C11 technical reports that have not been incorporated into the C Standard. A close-second priority is supporting remediation of old code using C99 and the technical reports.
This standard does make contributions to support older compilers when these contributions can be significant and doing so does not compromise other priorities. The intent is not to capture all deviations from the standard but to capture only a few important ones.
Issues Not Addressed
A number of issues are not addressed by this secure coding standard.
Coding Style
Coding style issues are subjective, and it has proven impossible to develop a consensus on appropriate style guidelines. Consequently, the CERT C Secure Coding Standard does not require any particular coding style to be enforced but only that the user defines style guidelines and apply these guidelines consistently. The easiest way to consistently apply a coding style is to use a code-formatting tool. Many interactive development environments (IDEs) provide such capabilities.
Tools
As a federally funded research and development center (FFRDC), the SEI is not in a position to recommend particular vendors or tools to enforce the restrictions adopted. The user of this document is free to choose tools, and vendors are encouraged to provide tools to enforce the rules.
Controversial Rules
In general, the CERT secure coding standards try to avoid the inclusion of controversial rules that lack a broad consensus.