OpenSSL CCS Attack

The buggy code is the following one (the red numbers follow the above description):

int ssl3_do_change_cipher_spec(SSL *s)
 {
 int i;
 const char *sender;
 int slen;

 if (s->state & SSL_ST_ACCEPT)
  i=SSL3_CHANGE_CIPHER_SERVER_READ;
 else
  i=SSL3_CHANGE_CIPHER_CLIENT_READ;

 if (s->s3->tmp.key_block == NULL)1
  {
  if (s->session == NULL)
   {
   /* might happen if dtls1_read_bytes() calls this */
         SSLerr(SSL_F_SSL3_DO_CHANGE_CIPHER_SPEC,SSL_R_CCS_RECEIVED_EARLY);
   return (0);
   }

  s->session->cipher=s->s3->tmp.new_cipher;
  if (!s->method->ssl3_enc->setup_key_block(s)) return(0); 2
  }

 if (!s->method->ssl3_enc->change_cipher_state(s,i))
  return(0);

 /* we have to record the message digest at
  * this point so we can get it before we read
  * the finished message */
 if (s->state & SSL_ST_CONNECT)
  {
  sender=s->method->ssl3_enc->server_finished_label;
  slen=s->method->ssl3_enc->server_finished_label_len;
  }
 else
  {
  sender=s->method->ssl3_enc->client_finished_label;
  slen=s->method->ssl3_enc->client_finished_label_len;
  }

 i = s->method->ssl3_enc->final_finish_mac(s,
  sender,slen,s->s3->tmp.peer_finish_md); 3
 if (i == 0)
  {
  SSLerr(SSL_F_SSL3_DO_CHANGE_CIPHER_SPEC, ERR_R_INTERNAL_ERROR);
  return 0;
  }
 s->s3->tmp.peer_finish_md_len = i;

 return(1);
 }

Fortunately a patch is available here and a simple go tool to check the bug presence is here. For having more detailed infos, please visit (this post, and the original post).