Alternative functions that limit the number of bytes copied are often recommended to mitigate buffer overflow vulnerabilities, for example:

These function truncate strings that exceed the specified limits. Additionally, some functions such as {{strncpy()}} do not guarantee that the resulting string is null-terminated \[[STR33-C|STR33-C. Guarantee that all strings are null-terminated]\].
Truncation results in a loss of data, and in some cases, leads to software vulnerabilities.

Non-Compliant Code Example

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

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

Compliant Solution 1

Truncation resulting from a string copy operation should be treated as an error condition.

#define A_SIZE 16
char *string_data;
char a[A_SIZE];
...
if (string_data) {
  if (strlen(string_data) < sizeof(a)) {
    strcpy(a, string_data);
  }
  else {
    /* handle string too large condition */
  }
}
else {
  /* handle null string condition */
}

Compliant Solution 2

The {{strcpy_s()}} function provides additional safeguards including accepting the size of the destination buffer as an additional argument \[[STR00-A|STR00-A. Use TR 24731 for remediation of existing string manipulation code]\].

#define A_SIZE 16
char *string_data;
char a[A_SIZE];
...
if (string_data) {
  if (strlen(string_data) < sizeof(a)) {
    strcpy(a,  sizeof(a), string_data);
  }
  else {
    /* handle string too large condition */
  }
}
else {
  /* handle null string condition */
}

Exception

An exception to this rule applies if the intent of the programmer was to intentionally truncate the null-terminated byte string. To be compliant with this standard, this intent must be made clear statement in comments.

Priority: P2 Level: L1

Truncating strings can lead to a loss of data and exploitable vulnerabilities in some cases.

Component

Value

Severity

1 (medium)

Likelihood

1 (probable)

Remediation cost

2 (medium)

References