Example - 1

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.

// // copy input string to a temporary string// 
message[index] = strMessage[index];
// // trim trailing whitespace// 

message[len] = '\0';len--;
// // return string without trailing whitespace// 
char *retMessage;char *message = malloc(sizeof(char)*(length+1));char message[length+1];int index;for (index = 0; index < length; index++) {}message[index] = '\0';int len = index-1;while (isspace(message[len])) {}retMessage = message;return retMessage;char* trimTrailingWhitespace(char *strMessage, int length) {}

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 - 2

The following is an example of code that may result in a buffer underwrite, if find() returns a negative value to indicate that ch is not found in srcBuf:

...strncpy(destBuf, &srcBuf[find(srcBuf, ch)], 1024);...int main() {}

If the index to srcBuf is somehow under user control, this is an arbitrary write-what-where condition.