CERT C: Rule STR32-C

Invalid use of standard library string routine occurs when a string library function is called with invalid arguments.

The risk depends on the type of invalid arguments. For instance, using the strcpy function with a source argument larger than the destination argument can result in buffer overflows.

The fix depends on the standard library function involved in the defect. In some cases, you can constrain the function arguments before the function call. For instance, if the strcpy function:

char * strcpy(char * destination, const char* source)

See examples of fixes below.

If you do not want to fix the issue, add comments to your result or code to avoid another review. See:

 #include <string.h>
 #include <stdio.h>
 
 char* Copy_String(void)
 {
  char *res;
  char gbuffer[5],text[20]="ABCDEFGHIJKL";

  res=strcpy(gbuffer,text);  //Noncompliant
  /* Error: Size of text is less than gbuffer */

  return(res);
 }

The string text is larger in size than gbuffer. Therefore, the function strcpy cannot copy text into gbuffer.

One possible correction is to declare the destination string gbuffer with equal or larger size than the source string text.

#include <string.h>
 #include <stdio.h>
 
 char* Copy_String(void)
 {
  char *res;
  /*Fix: gbuffer has equal or larger size than text */
  char gbuffer[20],text[20]="ABCDEFGHIJKL";

  res=strcpy(gbuffer,text);

  return(res);
 }

Tainted NULL or non-null-terminated string looks for strings from unsecure sources that are being used in string manipulation routines that implicitly dereference the string buffer. For example, strcpy or sprintf.

Tainted NULL or non-null-terminated string raises no defect for a string returned from a call to scanf-family variadic functions. Similarly, no defect is raised when you pass the string with a %s specifier to printf-family variadic functions.

If a string is from an unsecure source, it is possible that an attacker manipulated the string or pointed the string pointer to a different memory location.

If the string is NULL, the string routine cannot dereference the string, causing the program to crash. If the string is not null-terminated, the string routine might not know when the string ends. This error can cause you to write out of bounds, causing a buffer overflow.

Validate the string before you use it. Check that:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#define SIZE128 128
#define MAX 40
extern void print_str(const char*);
void warningMsg(void)
{
	char userstr[MAX];
	read(0,userstr,MAX);
	char str[SIZE128] = "Warning: ";
	strncat(str, userstr, SIZE128-(strlen(str)+1)); //Noncompliant
	print_str(str);
}

In this example, the string str is concatenated with the argument userstr. The value of userstr is unknown. If the size of userstr is greater than the space available, the concatenation overflows.

One possible correction is to check the size of userstr and make sure that the string is null-terminated before using it in strncat. This example uses a helper function, sansitize_str, to validate the string. The defects are concentrated in this function.

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#define SIZE128 128
#define MAX 40
extern void print_str(const char*);
int sanitize_str(char* s) {
	int res = 0; 
	if (s && (strlen(s) > 0)) { // TAINTED_STRING only flagged here //Noncompliant
		// - string is not null
		// - string has a positive and limited size
		// - TAINTED_STRING on strlen used as a firewall
		res = 1;
	}
	return res; 
}
void warningMsg(void)
{
	char userstr[MAX];
	read(0,userstr,MAX);
	char str[SIZE128] = "Warning: ";
	if (sanitize_str(userstr))	
		strncat(str, userstr, SIZE128-(strlen(str)+1));
	print_str(str);
}

Another possible correction is to call function errorMsg and warningMsg with specific strings.

#include <stdio.h>
#include <string.h>
#include <stdlib.h>

#define SIZE128 128

extern void print_str(const char*);

void warningMsg(char* userstr)
{
    char str[SIZE128] = "Warning: ";
    strncat(str, userstr, SIZE128-(strlen(str)+1));
    print_str(str);
}

void errorMsg(char* userstr)
{
  char str[SIZE128] = "Error: ";
  strncat(str, userstr, SIZE128-(strlen(str)+1));
  print_str(str);
}

int manageSensorValue(int sensorValue) {
  int ret = sensorValue;
  if ( sensorValue < 0 ) {
    errorMsg("sensor value should be positive");
    exit(1);
  } else if ( sensorValue > 50 ) {
    warningMsg("sensor value greater than 50 (applying threshold)...");
    sensorValue = 50;
  }
  
  return sensorValue;
}