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ISO/IEC TR 24731 The C Standard, Annex K (normative), defines alternative versions of C standard string-handling functions that are designed to be safer replacements for existing functions. For example, ISO/IEC TR 24731 Part I (24731-1)  it defines the strcpy_s(), strcat_s(), strncpy_s(), and strncat_s() functions as replacements for strcpy(), strcat(), strncpy(), and strncat(), respectively.

The ISO/IEC TR 24731-1 The Annex K functions were created by Microsoft to help retrofit its existing , legacy code base in response to numerous, well-publicized security incidents over the past decade. These functions were then subsequently proposed to the international standardization working group for the programming language C (ISO/IEC JTC1/SC22/WG14) for standardization.

The strcpy_s() function, for example, has this signature:

errno_t strcpy_s(
   char * restrict s1,
   rsize_t s1max,
   const char * restrict s2
);

The signature is similar to strcpy() but takes an extra argument of type rsize_t that specifies the maximum length of the destination buffer. ( Functions that accept parameters of type rsize_t diagnose a constraint violation if the values of those parameters are greater than RSIZE_MAX. Extremely large object sizes are frequently a sign that an object's size was calculated incorrectly. For example, negative numbers appear as very large positive numbers when converted to an unsigned type like size_t. For those reasons, it is sometimes beneficial to restrict the range of object sizes to detect errors. For machines with large address spaces, ISO/IEC TR 24731-1 , the C Standard, Annex K, recommends that RSIZE_MAX be defined as the smaller of the size of the largest object supported or (SIZE_MAX >> 1), even if this limit is smaller than the size of some legitimate, but very large, objects (see also INT01-C. Use rsize_t or size_t for all integer values representing the size of an object).

The semantics of strcpy_s() are also similar to the semantics of strcpy(). When there are no input validation errors, the strcpy_s() function copies characters from a source string to a destination character array up to and including the terminating null character. The function returns zero returns 0 on success.

The strcpy_s() function succeeds only succeeds when the source string can be fully copied to the destination without overflowing the destination buffer. The Specifically, the following conditions are treated as a constraint violationchecks are made:

• The source and destination pointers are checked to see if they are null NULL.
• The maximum length of the destination buffer is checked to see if it is equal to zero0, greater than RSIZE_MAX, or less than or equal to the length of the source string.
• Copying is not allowed between objects that overlap.

When a runtime-constraint violation is detected, the destination string is set to the null string (as long as it is not a null pointer, and the maximum length of the destination buffer is greater than 0 and not greater than RSIZE_MAX), and the function returns a nonzero value. In the following example, the strcpy_s() function is used to copy src1 to dst1.:

char src1[100] = "hello";
char src2[78] =  {'g','o','o','d','b','y','e','\0'};
char dst1[6],;
char dst2[5];
int r1,;
int r2;

r1 = strcpy_s(dst1, sizeof(dst1), src1);
r2 = strcpy_s(dst2, sizeof(dst2), src2);

Wiki MarkupHowever, the call to copy {{src2}} to {{dst2}} fails because there is insufficient space is available to copy the entire string, which consists of seven eight characters, to the destination buffer. As a result, {{r2}} is assigned a nonzero value and {{dst2\[0\]}} is set to "\0." (The {{src2}} string is also not null-terminated, but this is actually irrelevant, since the length provided to {{strcpy_s}} is computed using {{siceof}}.)to the null character.

Users of the C Standard Annex K Users of the ISO/IEC TR 24731-1 functions are less likely to introduce a security flaw because the size of the destination buffer and the maximum number of characters to append must be specified.   ISO/IEC TR 24731 Part II ([ISO/IEC TR 24731-2, in progress) will offer :2010] offers another approach, supplying functions that allocate enough memory for their results.   ISO/IEC TR 24731 Part II functions also ensure null termination of the destination string.

ISO/IEC TR 24731-1 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 -2 Part II functions may 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 especially particularly secure but may be useful in preventive maintenance to reduce the likelihood of vulnerabilities in an existing legacy code base.

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Noncompliant Code Example

The following 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 (

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Runtime)

The following 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];

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

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

  fputs(buf, stderr);
}

Compliant Solution (

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Partial Compile Time)

The following This compliant solution performs some of the checking at compile time using a static assertion (see DCL03-AC. 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);

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

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

Risk Assessment

String-handling functions defined in C99 Section the C Standard, subclause 7.21 24, and elsewhere are susceptible to common programming errors that can lead to serious, exploitable vulnerabilities. Proper use of TR 24731 the C11 Annex K functions can eliminate the majority eliminate most of these issues.

Automated Detection

The LDRA tool suite V 7.6.0 is able to detect violations of this recommendation.

Related Vulnerabilities

Automated Detection

Related Vulnerabilities

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

References

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Related Guidelines

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Bibliography

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Image Added Image Added Image Added TR 24731-1-2007]\] \[[ISO/IEC 9899-1999|AA. C References#ISO/IEC 9899-1999]\] Section 7.21, "String handling <string.h>" \[[Seacord 05a|AA. C References#Seacord 05a]\] Chapter 2, "Strings" \[[Seacord 05b|AA. C References#Seacord 05b]\]07. Characters and Strings (STR)      07. Characters and Strings (STR)       STR01-A. Use managed strings for development of new string manipulation code