Example - 1

The following code attempts to save four different identification numbers into an array.

int id_sequence[3];/* Populate the id array. */id_sequence[0] = 123;id_sequence[1] = 234;id_sequence[2] = 345;id_sequence[3] = 456;

Since the array is only allocated to hold three elements, the valid indices are 0 to 2; so, the assignment to id_sequence[3] is out of bounds.

Example - 2

In the following code, it is possible to request that memcpy move a much larger segment of memory than assumed:

// /* if chunk info is valid, return the size of usable memory,// 
// * else, return -1 to indicate an error// 
// */// 
...

...memcpy(destBuf, srcBuf, (returnChunkSize(destBuf)-1));...int returnChunkSize(void *) {}int main() {}

If returnChunkSize() happens to encounter an error it will return -1. Notice that the return value is not checked before the memcpy operation (CWE-252), so -1 can be passed as the size argument to memcpy() (CWE-805). Because memcpy() assumes that the value is unsigned, it will be interpreted as MAXINT-1 (CWE-195), and therefore will copy far more memory than is likely available to the destination buffer (CWE-787, CWE-788).

Example - 3

This code takes an IP address from the user and verifies that it is well formed. It then looks up the hostname and copies it into a buffer.

// /*routine that ensures user_supplied_addr is in the right format for conversion */// 
struct hostent *hp;in_addr_t *addr;char hostname[64];in_addr_t inet_addr(const char *cp);validate_addr_form(user_supplied_addr);addr = inet_addr(user_supplied_addr);hp = gethostbyaddr( addr, sizeof(struct in_addr), AF_INET);strcpy(hostname, hp->h_name);void host_lookup(char *user_supplied_addr){}

This function allocates a buffer of 64 bytes to store the hostname. However, there is no guarantee that the hostname will not be larger than 64 bytes. If an attacker specifies an address which resolves to a very large hostname, then the function may overwrite sensitive data or even relinquish control flow to the attacker.

Note that this example also contains an unchecked return value (CWE-252) that can lead to a NULL pointer dereference (CWE-476).

Example - 4

This code applies an encoding procedure to an input string and stores it into a buffer.

die("user string too long, die evil hacker!");

dst_buf[dst_index++] = '&';dst_buf[dst_index++] = 'a';dst_buf[dst_index++] = 'm';dst_buf[dst_index++] = 'p';dst_buf[dst_index++] = ';';

// /* encode to < */// 

if( '&' == user_supplied_string[i] ){}else if ('<' == user_supplied_string[i] ){}else dst_buf[dst_index++] = user_supplied_string[i];
int i, dst_index;char *dst_buf = (char*)malloc(4*sizeof(char) * MAX_SIZE);if ( MAX_SIZE <= strlen(user_supplied_string) ){}dst_index = 0;for ( i = 0; i < strlen(user_supplied_string); i++ ){}return dst_buf;char * copy_input(char *user_supplied_string){}

The programmer attempts to encode the ampersand character in the user-controlled string. However, the length of the string is validated before the encoding procedure is applied. Furthermore, the programmer assumes encoding expansion will only expand a given character by a factor of 4, while the encoding of the ampersand expands by 5. As a result, when the encoding procedure expands the string it is possible to overflow the destination buffer if the attacker provides a string of many ampersands.

Example - 5

In the following C/C++ code, 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 - 6

The following code allocates memory for a maximum number of widgets. It then gets a user-specified number of widgets, making sure that the user does not request too many. It then initializes the elements of the array using InitializeWidget(). Because the number of widgets can vary for each request, the code inserts a NULL pointer to signify the location of the last widget.

ExitError("Incorrect number of widgets requested!");
WidgetList[i] = InitializeWidget();
int i;unsigned int numWidgets;Widget **WidgetList;numWidgets = GetUntrustedSizeValue();if ((numWidgets == 0) || (numWidgets > MAX_NUM_WIDGETS)) {}WidgetList = (Widget **)malloc(numWidgets * sizeof(Widget *));printf("WidgetList ptr=%p\n", WidgetList);for(i=0; i<numWidgets; i++) {}WidgetList[numWidgets] = NULL;showWidgets(WidgetList);

However, this code contains an off-by-one calculation error (CWE-193). It allocates exactly enough space to contain the specified number of widgets, but it does not include the space for the NULL pointer. As a result, the allocated buffer is smaller than it is supposed to be (CWE-131). So if the user ever requests MAX_NUM_WIDGETS, there is an out-of-bounds write (CWE-787) when the NULL is assigned. Depending on the environment and compilation settings, this could cause memory corruption.

Example - 7

The following code 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.