Example - 1

The following example retrieves the sizes of messages for a pop3 mail server. The message sizes are retrieved from a socket that returns in a buffer the message number and the message size, the message number (num) and size (size) are extracted from the buffer and the message size is placed into an array using the message number for the array index.

// /* capture the sizes of all messages */// 

// // read values from socket and added to sizes array// 

// // continue read from socket until buf only contains '.'// 
break;
sizes[num - 1] = size;if (DOTLINE(buf))else if (sscanf(buf, "%d %d", &num, &size) == 2)......char buf[BUFFER_SIZE];int ok;int num, size;while ((ok = gen_recv(sock, buf, sizeof(buf))) == 0){}int getsizes(int sock, int count, int *sizes) {}

In this example the message number retrieved from the buffer could be a value that is outside the allowable range of indices for the array and could possibly be a negative number. Without proper validation of the value to be used for the array index an array overflow could occur and could potentially lead to unauthorized access to memory addresses and system crashes. The value of the array index should be validated to ensure that it is within the allowable range of indices for the array as in the following code.

// /* capture the sizes of all messages */// 

// // read values from socket and added to sizes array// 

// // continue read from socket until buf only contains '.'// 
break;
sizes[num - 1] = size;

// /* warn about possible attempt to induce buffer overflow */// 
report(stderr, "Warning: ignoring bogus data for message sizes returned by server.\n");if (num > 0 && num <= (unsigned)count)elseif (DOTLINE(buf))else if (sscanf(buf, "%d %d", &num, &size) == 2) {}......char buf[BUFFER_SIZE];int ok;int num, size;while ((ok = gen_recv(sock, buf, sizeof(buf))) == 0){}int getsizes(int sock, int count, int *sizes) {}

Example - 2

In the following example the method displayProductSummary is called from a Web service servlet to retrieve product summary information for display to the user. The servlet obtains the integer value of the product number from the user and passes it to the displayProductSummary method. The displayProductSummary method passes the integer value of the product number to the getProductSummary method which obtains the product summary from the array object containing the project summaries using the integer value of the product number as the array index.

// // Method called from servlet to obtain product information// 

String productSummary = getProductSummary(index);
String productSummary = new String("");try {} catch (Exception ex) {...}return productSummary;
return products[index];public String displayProductSummary(int index) {}public String getProductSummary(int index) {}

In this example the integer value used as the array index that is provided by the user may be outside the allowable range of indices for the array which may provide unexpected results or cause the application to fail. The integer value used for the array index should be validated to ensure that it is within the allowable range of indices for the array as in the following code.

// // Method called from servlet to obtain product information// 

String productSummary = getProductSummary(index);
String productSummary = new String("");try {} catch (Exception ex) {...}return productSummary;

productSummary = products[index];

System.err.println("index is out of bounds");throw new IndexOutOfBoundsException();
String productSummary = "";if ((index >= 0) && (index < MAX_PRODUCTS)) {}else {}return productSummary;public String displayProductSummary(int index) {}public String getProductSummary(int index) {}

An alternative in Java would be to use one of the collection objects such as ArrayList that will automatically generate an exception if an attempt is made to access an array index that is out of bounds.

productSummary = (String) productArray.get(index);ArrayList productArray = new ArrayList(MAX_PRODUCTS);...try {} catch (IndexOutOfBoundsException ex) {...}

Example - 3

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

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

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).