CWE-786: Access of Memory Location Before Start of Buffer

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.

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.

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;                      }

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

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).

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

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.

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.

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