Multiple Operations on Resource in Single-Operation Context

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 double free vulnerability.

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 this 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 Two

This code binds a server socket to port 21, allowing the server to listen for traffic on that port.

void bind_socket(void) {


  int server_sockfd;
  int server_len;
  struct sockaddr_in server_address;

  /*unlink the socket if already bound to avoid an error when bind() is called*/

  unlink("server_socket");
  server_sockfd = socket(AF_INET, SOCK_STREAM, 0);

  server_address.sin_family = AF_INET;
  server_address.sin_port = 21;
  server_address.sin_addr.s_addr = htonl(INADDR_ANY);
  server_len = sizeof(struct sockaddr_in);

  bind(server_sockfd, (struct sockaddr *) &s1, server_len);

}

This code may result in two servers binding a socket to same port, thus receiving each other's traffic. This could be used by an attacker to steal packets meant for another process, such as a secure FTP server.

See Also