Access of Memory Location Before Start of Buffer

Description

This typically occurs when a pointer or its index is decremented to a position before the buffer, when pointer arithmetic results in a position before the beginning of the valid memory location, or when a negative index is used.

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

In the following C/C++ example, a utility function is used to trim trailing whitespace from a character string. The function copies the input string to a local character string and uses a while statement to remove the trailing whitespace by moving backward through the string and overwriting whitespace with a NUL character.

char* trimTrailingWhitespace(char *strMessage, int length) {

  char *retMessage;
  char *message = malloc(sizeof(char)*(length+1));

  // copy input string to a temporary string
  char message[length+1];
  int index;
  for (index = 0; index < length; index++) {
    message[index] = strMessage[index];
  }
  message[index] = '\0';

  // trim trailing whitespace
  int len = index-1;
  while (isspace(message[len])) {
    message[len] = '\0';
    len--;
  }

  // return string without trailing whitespace
  retMessage = message;
  return retMessage;

}

However, this function can cause a buffer underwrite if the input character string contains all whitespace. On some systems the while statement will move backwards past the beginning of a character string and will call the isspace() function on an address outside of the bounds of the local buffer.

Example Two

The following example asks a user for an offset into an array to select an item.

int main (int argc, char **argv) {
  char *items[] = {"boat", "car", "truck", "train"};
  int index = GetUntrustedOffset();
  printf("You selected %s\n", items[index-1]);
}

The programmer allows the user to specify which element in the list to select, however an attacker can provide an out-of-bounds offset, resulting in a buffer over-read (CWE-126).

Example Three

The following is an example of code that may result in a buffer underwrite. This code is attempting to replace the substring "Replace Me" in destBuf with the string stored in srcBuf. It does so by using the function strstr(), which returns a pointer to the found substring in destBuf. Using pointer arithmetic, the starting index of the substring is found.

int main() {
  ...
  char *result = strstr(destBuf, "Replace Me");
  int idx = result - destBuf;
  strcpy(&destBuf[idx], srcBuf);
  ...
}

In the case where the substring is not found in destBuf, strstr() will return NULL, causing the pointer arithmetic to be undefined, potentially setting the value of idx to a negative number. If idx is negative, this will result in a buffer underwrite of destBuf.

See Also