ISO/IEC TS 17961 C Secure Coding Rules

The purpose of ISO/IEC TS 17961 [ISO/IEC TS 17961:2013] is to establish a baseline set of requirements for analyzers, including static analysis tools and C language compilers, to be applied by vendors that wish to diagnose insecure code beyond the requirements of the language standard. All rules are meant to be enforceable by static analysis. The criterion for selecting these rules is that analyzers that implement these rules must be able to effectively discover secure coding errors without generating excessive false positives.

To date, the application of static analysis to security has been performed in an ad-hoc manner by different vendors, resulting in non-uniform coverage of significant security issues. ISO/IEC TS 17961 enumerates secure coding rules and requires analysis engines to diagnose violations of these rules as a matter of conformance to the specification [ISO/IEC TS 17961:2013]. These rules may be extended in an implementation- dependent manner, which provides a minimum coverage guarantee to customers of any and all conforming static analysis implementations.

ISO/IEC TS 17961 specifies rules for secure coding in the C programming language and includes code examples for each rule. Noncompliant code examples demonstrate language constructs that have weaknesses with potentially exploitable security implications; such examples are expected to elicit a diagnostic from a conforming analyzer for the affected language construct. Compliant examples are expected not to elicit a diagnostic. ISO/IEC TS 17961 does not specify the mechanism by which these rules are enforced or any particular coding style to be enforced [ISO/IEC TS 17961:2013].

The following table shows how ISO/IEC TS 17961 relates to other standards and guidelines. Of the publications listed, ISO/IEC TS 17961 is the only one for which the immediate audience is analyzers and not developers.

ISO/IEC TS 17961 Compared with Other Standards

Coding Standard	C Standard	Security Standard	Safety Standard	International Standard	Whole Language
CWE	None/all	Yes	No	No	N/A
MISRA C2	C89	No	Yes	No	No
MISRA C3	C99	No	Yes	No	No
CERT C99	C99	Yes	No	No	Yes
CERT C11	C11	Yes	No	No	Yes
ISO/IEC TS 17961	C11	Yes	No	Yes	Yes

A conforming analyzer must be capable of producing a diagnostic for each distinct rule in the technical specification upon detecting a violation of that rule in isolation. If the same program text violates multiple rules simultaneously, a conforming analyzer may aggregate diagnostics but must produce at least one diagnostic. The diagnostic message might be of the form

     Accessing freed memory in function abc, file xyz.c, line nnn.

ISO/IEC TS 17961 does not require an analyzer to produce a diagnostic message for any violation of any syntax rule or constraint specified by the C Standard [ISO/IEC TS 17961:2013]. Conformance is defined only with respect to source code that is visible to the analyzer. Binary-only libraries, and calls to them, are outside the scope of these rules.

An interesting aspect of the technical specification is the portability assumptions, known within the group as the “San Francisco rule” because the assumptions evolved at a meeting hosted by Coverity at its headquarters. The San Francisco rule states that a conforming analyzer must be able to diagnose violations of guidelines for at least one C implementation but does not need to diagnose a rule violation if the result is documented for the target implementation and does not cause a security flaw. Variations in quality of implementation permit an analyzer to produce diagnostics concerning portability issues. For example, the following program fragment can produce a diagnostic, such as the mismatch between %d and lon g int:

     long i; printf ("i = %d", i);

This mismatch might not be a problem for all target implementations, but it is a portability problem because not all implementations have the same representation for int and long.

In addition to other goals already stated, the CERT C Coding Standard has been updated for consistency with ISO/IEC TS 17961. Although the documents serve different audiences, consistency between the documents should improve the ability of developers to use ISO/IEC TS 17961– conforming analyzers to find violations of rules from this coding standard. The Secure Coding Validation Suite is a set of tests developed by CERT to validate the rules defined in ISO/IEC TS 17961. These tests are based on the examples in this technical specification and are distributed with a BSD-style license.