Multiple Binds to the Same Port

Description

On most systems, a combination of setting the SO_REUSEADDR socket option, and a call to bind() allows any process to bind to a port to which a previous process has bound with INADDR_ANY. This allows a user to bind to the specific address of a server bound to INADDR_ANY on an unprivileged port, and steal its UDP packets/TCP connection.

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

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