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A good security considerations section should explain how the feature can be misused to violate each general security requirement in the list. Each explanation should be accompanied by instructions the reader should follow in order to avoid a violation. When referencing potential vulnerabilities described in the Secure Programming Practices manual page, sprog(7), likewise cross-reference that document rather than replicating information. Whenever possible, refer to this document rather than reproducing the material it contains.

This is an example security considerations section for the strcpy(3) manual page:

The strcpy() function is easily misused in a manner which enables malicious users to arbitrarily change a running program's functionality through a buffer overflow attack.

Avoid using strcpy(). Instead, use strncpy() and ensure that no more characters are copied to the destination buffer than it can hold. Do not forget to NUL-terminate the destination buffer, as strncpy() will not terminate the destination string if it is truncated.

Note that strncpy() can also be problematic. It may be a security concern for a string to be truncated at all. Since the truncated string will not be as long as the original, it may refer to a completely different resource and usage of the truncated resource could result in very incorrect behavior. Example:

void
foo(const char *arbitrary_string)
{
        char onstack[8];

#if defined(BAD)
        /*
         * This first strcpy is bad behavior.  Do not use strcpy()!
         */
        (void)strcpy(onstack, arbitrary_string);     /* BAD! */
#elif defined(BETTER)
        /*
         * The following two lines demonstrate better use of
         * strncpy().
         */
        (void)strncpy(onstack, arbitrary_string, sizeof(onstack) - 1);
        onstack[sizeof(onstack - 1)] = '\0';
#elif defined(BEST)
        /*
         * These lines are even more robust due to testing for
         * truncation.
         */
        if (strlen(arbitrary_string) + 1 > sizeof(onstack))
                err(1, "onstack would be truncated");
        (void)strncpy(onstack, arbitrary_string, sizeof(onstack));
#endif
}

Security considerations sections for tools and commands are apt to be less formulaic. Let your list of potentially-violated security requirements be your guide; explain each one and list a solution in as concise a manner as possible.

This is an example security considerations section for the rtld(1) manual page:

Using the LD_LIBRARY_PATH and LD_PRELOAD environment variables, malicious users can cause the dynamic linker to link shared libraries of their own devising into the address space of processes running non-set-user-ID/group-ID programs. These shared libraries can arbitrarily change the functionality of the program by replacing calls to standard library functions with calls to their own. Although this feature is disabled for set-user-ID and set-group-ID programs, it can still be used to create Trojan horses in other programs.

All users should be aware that the correct operation of non set-user-ID/group-ID dynamically-linked programs depends on the proper configuration of these environment variables, and take care to avoid actions that might set them to values which would cause the run-time linker to link in shared libraries of unknown pedigree.

Edward Amoroso, AT&T Bell Laboratories, Fundamentals of Computer Security Technology, P T R Prentice Hall, 1994.

Ken Thompson, Vol. 27, No. 8, Communications of the ACM, 761-763, Association for Computing Machinery, Inc., Reflections on Trusting Trust, August, 1984.