A simple, yet effective way to avoid double-free and access-freed-memory vulnerabilities is to set pointers to NULL after they have been freed. Calling free() on a NULL pointer results in no action being taken by free(). As a result, it is recommended that freed pointers be set to NULL to help eliminate memory related vulnerabilities.

Non-compliant Code Example 1

may result in accessing freed memory

if (!strcmp(message,value_1)) {
  process_message(message);
  free(message);
}
if (!strcmp(message,value_2)) {
  process_message(message);
  free(message);
}

Non-compliant Code Example 1

memory set to NULL to correct this

if (!strcmp(message,value_1)) {
  process_message(message);
  free(message);
  message = NULL;
}
if (!strcmp(message,value_2)) {
  process_message(message);
  free(message);
  message = NULL;
}

References