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Alternative functions that limit the number of bytes copied are often recommended to mitigate buffer overflow vulnerabilities. For exampleExamples 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 character sequence is null-terminated. (see See STR32-C. Null-terminate byte strings as required).Do not pass a non-null-terminated character sequence to a library function that expects a string.)

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. If 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 (

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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. Care The programmer must be taken 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 (

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strcpy_s(), C11 Annex K)

The Wiki MarkupThe {{strcpy_s()}} function defined in \C11 Annex K  [[ISO/IEC TR 24731-1:2007|AA. References#ISO/IEC TR 24731-1-2007]\] provides additional safeguards, including accepting the size of the destination buffer as an additional argument (see [9899:2011] provides additional safeguards, including accepting the size of the destination buffer as an additional argument. (See STR07-C. Use TR 24731 for remediation of existing string manipulation code|STR07-C. Use TR 24731 for remediation of existing string manipulation code]). Also, {{Use the bounds-checking interfaces for string manipulation.) 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 either strnlen_s() or strcpy_s(), the currently registered runtime-constraint handler is invoked. See ERR03-C. Use runtime-constraint handlers when calling functions defined by TR24731-1the bounds-checking interfaces for more information on using runtime-constraint handlers with TR 24731-1 C11 Annex K functions.

Exceptions

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

Risk Assessment

Truncating strings can lead to a loss of data.

Automated Detection

The LDRA tool suite Version 7.6.0 can detect violations of this recommendation.

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Related Vulnerabilities

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

Other Languages

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Related Guidelines

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References

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Bibliography

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Image Added Image Added 9899:1999|AA. References#ISO/IEC 9899-1999]\] Section 7.21, "String handling <string.h>" \[[ISO/IEC PDTR 24772|AA. References#ISO/IEC PDTR 24772]\] "CJM String Termination" \[[ISO/IEC TR 24731-1:2007|AA. References#ISO/IEC TR 24731-1-2007]\] \[[MITRE 07|AA. References#MITRE 07]\] [CWE ID 170|http://cwe.mitre.org/data/definitions/170.html], "Improper Null Termination," [CWE ID 464|http://cwe.mitre.org/data/definitions/464.html], "Addition of Data Structure Sentinel" \[[Seacord 05a|AA. References#Seacord 05a]\] Chapter 2, "Strings"Image Removed      07. Characters and Strings (STR)      Image Modified