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The C Standard Annex K functions are still capable of overflowing a buffer if the maximum length of the destination buffer and number of characters to copy are incorrectly specified. ISO/IEC TR 24731 Part II functions can make it more difficult to keep track of memory that must be freed, leading to memory leaks. As a result, the C Standard Annex K and the ISO/IEC TR 24731 Part II functions are not particularly secure but may be useful in preventive maintenance to reduce the likelihood of vulnerabilities in an existing legacy code base.

Noncompliant Code Example

This noncompliant code overflows its buffer if msg is too long, and it has undefined behavior if msg is a null pointer:

void complain(const char *msg) {
  static const char prefix[] = "Error: ";
  static const char suffix[] = "\n";
  char buf[BUFSIZ];

  strcpy(buf, prefix);
  strcat(buf, msg);
  strcat(buf, suffix);
  fputs(buf, stderr);
}

Compliant Solution (Runtime)

This compliant solution will not overflow its buffer:

void complain(const char *msg) {
  errno_t err;
  static const char prefix[] = "Error: ";
  static const char suffix[] = "\n";
  char buf[BUFSIZ];

  err = strcpy_s(buf, sizeof(buf), prefix);
  if (err != 0) {
    /* Handle error */
  }

  err = strcat_s(buf, sizeof(buf), msg);
  if (err != 0) {
    /* Handle error */
  }

  err = strcat_s(buf, sizeof(buf), suffix);
  if (err != 0) {
    /* Handle error */
  }

  fputs(buf, stderr);
}

Compliant Solution (Partial Compile Time)

This compliant solution performs some of the checking at compile time using a static assertion. (See DCL03-C. Use a static assertion to test the value of a constant expression.)

void complain(const char *msg) {
  errno_t err;
  static const char prefix[] = "Error: ";
  static const char suffix[] = "\n";
  char buf[BUFSIZ];

  /* 
   * Ensure that more than one character
   * is available for msg
   */
  static_assert(sizeof(buf) > sizeof(prefix) + sizeof(suffix),
                "Buffer for complain() is too small");
  strcpy(buf, prefix);

  err = strcat_s(buf, sizeof(buf), msg);
  if (err != 0) {
    /* Handle error */
  }

  err = strcat_s(buf, sizeof(buf), suffix);
  if (err != 0) {
    /* Handle error */
  }
  fputs(buf, stderr);
}

Risk Assessment

String-handling functions defined in the C Standard, subclause 7.24, and elsewhere are susceptible to common programming errors that can lead to serious, exploitable vulnerabilities. Proper use of the C11 Annex K functions can eliminate most of these issues.

Automated Detection

Related Vulnerabilities

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

Related Guidelines

Bibliography

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