Operation on a Resource after Expiration or Release

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

The following code shows a simple example of a use after free error:

char* ptr = (char*)malloc (SIZE);
if (err) {
  abrt = 1;
  free(ptr);
}
...
if (abrt) {
  logError("operation aborted before commit", ptr);
}

When an error occurs, the pointer is immediately freed. However, this pointer is later incorrectly used in the logError function.

Example Two

The following code shows a simple example of a double free error:

char* ptr = (char*)malloc (SIZE);
...
if (abrt) {
  free(ptr);
}
...
free(ptr);

Double free vulnerabilities have two common (and sometimes overlapping) causes:

Although some double free vulnerabilities are not much more complicated than the previous example, most are spread out across hundreds of lines of code or even different files. Programmers seem particularly susceptible to freeing global variables more than once.

Example Three

In the following C/C++ example the method processMessage is used to process a message received in the input array of char arrays. The input message array contains two char arrays: the first is the length of the message and the second is the body of the message. The length of the message is retrieved and used to allocate enough memory for a local char array, messageBody, to be created for the message body. The messageBody is processed in the method processMessageBody that will return an error if an error occurs while processing. If an error occurs then the return result variable is set to indicate an error and the messageBody char array memory is released using the method free and an error message is sent to the logError method.

#define FAIL 0
#define SUCCESS 1
#define ERROR -1
#define MAX_MESSAGE_SIZE 32

int processMessage(char **message)
{

  int result = SUCCESS;

  int length = getMessageLength(message[0]);
  char *messageBody;

  if ((length > 0) && (length < MAX_MESSAGE_SIZE)) {


    messageBody = (char*)malloc(length*sizeof(char));
    messageBody = &message[1][0];

    int success = processMessageBody(messageBody);

    if (success == ERROR) {
      result = ERROR;
      free(messageBody);
    }

  }
  else {
    printf("Unable to process message; invalid message length");
    result = FAIL;
  }

  if (result == ERROR) {
    logError("Error processing message", messageBody);
  }

  return result;

}

However, the call to the method logError includes the messageBody after the memory for messageBody has been released using the free method. This can cause unexpected results and may lead to system crashes. A variable should never be used after its memory resources have been released.

...
messageBody = (char*)malloc(length*sizeof(char));
messageBody = &message[1][0];

int success = processMessageBody(messageBody);

if (success == ERROR) {
  result = ERROR;
  logError("Error processing message", messageBody);
  free(messageBody);
}
...

See Also