STR03-C. Do not inadvertently truncate a null-terminated byte string

Alternative functions that limit the number of bytes copied are often recommended to mitigate buffer overflow vulnerabilities. Examples include

• strncpy() instead of strcpy()
• strncat() instead of strcat()
• fgets() instead of gets()
• snprintf() instead of sprintf()

These functions truncate strings that exceed the specified limits. Additionally, some functions such as strncpy() do not guarantee that the resulting string is null-terminated. (See STR32-C. Null-terminate byte strings as required.)

Unintentional truncation results in a loss of data and in some cases leads to software vulnerabilities.

Noncompliant Code Example

The standard functions strncpy() and strncat() copy a specified number of characters n from a source string to a destination array. In the case of strncpy(), if there is no null character in the first n characters of the source array, the result will not be null-terminated and any remaining characters are truncated.

char *string_data;
char a[16];
/* ... */
strncpy(a, string_data, sizeof(a));

Compliant Solution (Adequate Space)

Either the strcpy() or strncpy() function can be used to copy a string and a null character to a destination buffer, provided there is enough space. The programmer must be careful to ensure that the destination buffer is large enough to hold the string to be copied and the null byte to prevent errors, such as data truncation and buffer overflow.

char *string_data = NULL;
char a[16];

/* ... */

if (string_data == NULL) {
  /* Handle null pointer error */
}
else if (strlen(string_data) >= sizeof(a)) {
  /* Handle overlong string error */
}
else {
  strcpy(a, string_data);
}

This solution requires that string_data is null-terminated; that is, a null byte can be found within the bounds of the referenced character array. Otherwise, strlen() will stray into other objects before finding a null byte.

Compliant Solution (TR 24731-1)

The strcpy_s() function defined in Extensions to the C Library—Part I [ISO/IEC TR 24731-1:2007], which provides additional safeguards, including accepting the size of the destination buffer as an additional argument. (See STR07-C. Use the bounds-checking interfaces for remediation of existing string manipulation code.) Also, strnlen_s() accepts a maximum-length argument for strings that may not be null-terminated.

char *string_data = NULL;
char a[16];

/* ... */

if (string_data == NULL) {
  /* Handle null pointer error */
}
else if (strnlen_s(string_data, sizeof(a)) >= sizeof(a)) {
  /* Handle overlong string error */
}
else {
  strcpy_s(a, sizeof(a), string_data);
}

If a runtime-constraint error is detected by either the call to strnlen_s() or strcpy_s(), the currently registered runtime-constraint handler is invoked. See ERR03-C. Use runtime-constraint handlers when calling the bounds-checking interfaces for more information on using runtime-constraint handlers with TR 24731-1 functions.

Exceptions

STR03-EX1: The intent of the programmer is to intentionally truncate the null-terminated byte string.

Risk Assessment

Truncating strings can lead to a loss of data.

Automated Detection

Related Vulnerabilities

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

Related Guidelines

Bibliography