CURLcode Curl_disconnect(struct Curl_easy *data,
                         struct connectdata *conn, bool dead_connection)
{
  if(!conn)
    return CURLE_OK; /* this is closed and fine already */

  if(!data) {
    DEBUGF(infof(data, "DISCONNECT without easy handle, ignoring\n"));
    return CURLE_OK;
  }

  /*
   * If this connection isn't marked to force-close, leave it open if there
   * are other users of it
   */
  if(CONN_INUSE(conn) && !dead_connection) {
    DEBUGF(infof(data, "Curl_disconnect when inuse: %zu\n", CONN_INUSE(conn)));
    return CURLE_OK;
  }

  conn->data = data;
  if(conn->dns_entry != NULL) {
    Curl_resolv_unlock(data, conn->dns_entry);
    conn->dns_entry = NULL;
  }

  Curl_hostcache_prune(data); /* kill old DNS cache entries */

#if !defined(CURL_DISABLE_HTTP) && defined(USE_NTLM)
  /* Cleanup NTLM connection-related data */
  Curl_http_ntlm_cleanup(conn);
#endif

  if(conn->handler->disconnect)
    /* This is set if protocol-specific cleanups should be made */
    conn->handler->disconnect(conn, dead_connection);

    /* unlink ourselves! */
  infof(data, "Closing connection %ld\n", conn->connection_id);
  Curl_conncache_remove_conn(conn, TRUE);

  free_fixed_hostname(&conn->host);
  free_fixed_hostname(&conn->conn_to_host);
  free_fixed_hostname(&conn->http_proxy.host);
  free_fixed_hostname(&conn->socks_proxy.host);

  DEBUGASSERT(conn->data == data);
  /* this assumes that the pointer is still there after the connection was
     detected from the cache */
  Curl_ssl_close(conn, FIRSTSOCKET);

  conn_free(conn);
  return CURLE_OK;
}

The product reuses or references memory after it has been freed. At some point afterward, the memory may be allocated again and saved in another pointer, while the original pointer references a location somewhere within the new allocation. Any operations using the original pointer are no longer valid because the memory belongs to the code that operates on the new pointer.