Unexpected Sign Extension

The software performs an operation on a number that causes it to be sign extended when it is transformed into a larger data type. When the original number is negative, this can produce unexpected values that lead to resultant weaknesses.


Demonstrations

The following examples help to illustrate the nature of this weakness and describe methods or techniques which can be used to mitigate the risk.

Note that the examples here are by no means exhaustive and any given weakness may have many subtle varieties, each of which may require different detection methods or runtime controls.

Example One

The following code reads a maximum size and performs a sanity check on that size. It then performs a strncpy, assuming it will not exceed the boundaries of the array. While the use of "short s" is forced in this particular example, short int's are frequently used within real-world code, such as code that processes structured data.

int GetUntrustedInt () {
  return(0x0000FFFF);
}

void main (int argc, char **argv) {

  char path[256];
  char *input;
  int i;
  short s;
  unsigned int sz;

  i = GetUntrustedInt();
  s = i;
  /* s is -1 so it passes the safety check - CWE-697 */
  if (s > 256) {
    DiePainfully("go away!\n");
  }

  /* s is sign-extended and saved in sz */
  sz = s;

  /* output: i=65535, s=-1, sz=4294967295 - your mileage may vary */
  printf("i=%d, s=%d, sz=%u\n", i, s, sz);

  input = GetUserInput("Enter pathname:");

  /* strncpy interprets s as unsigned int, so it's treated as MAX_INT
  (CWE-195), enabling buffer overflow (CWE-119) */
  strncpy(path, input, s);
  path[255] = '\0'; /* don't want CWE-170 */
  printf("Path is: %s\n", path);

}

This code first exhibits an example of CWE-839, allowing "s" to be a negative number. When the negative short "s" is converted to an unsigned integer, it becomes an extremely large positive integer. When this converted integer is used by strncpy() it will lead to a buffer overflow (CWE-119).

See Also

SEI CERT C Coding Standard - Guidelines 04. Integers (INT)

Weaknesses in this category are related to the rules and recommendations in the Integers (INT) section of the SEI CERT C Coding Standard.

SFP Secondary Cluster: Glitch in Computation

This category identifies Software Fault Patterns (SFPs) within the Glitch in Computation cluster (SFP1).

Comprehensive CWE Dictionary

This view (slice) covers all the elements in CWE.

Weaknesses Introduced During Implementation

This view (slice) lists weaknesses that can be introduced during implementation.

Weaknesses in Software Written in C++

This view (slice) covers issues that are found in C++ programs that are not common to all languages.


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