do not allow filesystem fallback in server download (#15429)

It is possible for anyone with admin access to relatively
to get any content of any random OS location by simply
providing the file with 'mc admin update alias/ /etc/passwd`.

Workaround is to disable 'admin:ServiceUpdate' action. Everyone
is advised to upgrade to this patch.

Thanks to @Alevsk for finding this bug.

cmd/update.go

@@ -291,63 +291,50 @@ func getUserAgent(mode string) string {
 }
 
 func downloadReleaseURL(u *url.URL, timeout time.Duration, mode string) (content string, err error) {
-	var reader io.ReadCloser
-	if u.Scheme == "https" || u.Scheme == "http" {
-		req, err := http.NewRequest(http.MethodGet, u.String(), nil)
-		if err != nil {
-			return content, AdminError{
-				Code:       AdminUpdateUnexpectedFailure,
-				Message:    err.Error(),
-				StatusCode: http.StatusInternalServerError,
-			}
+	req, err := http.NewRequest(http.MethodGet, u.String(), nil)
+	if err != nil {
+		return content, AdminError{
+			Code:       AdminUpdateUnexpectedFailure,
+			Message:    err.Error(),
+			StatusCode: http.StatusInternalServerError,
 		}
-		req.Header.Set("User-Agent", getUserAgent(mode))
+	}
+	req.Header.Set("User-Agent", getUserAgent(mode))
 
-		client := &http.Client{Transport: getUpdateTransport(timeout)}
-		resp, err := client.Do(req)
-		if err != nil {
-			if xnet.IsNetworkOrHostDown(err, false) {
-				return content, AdminError{
-					Code:       AdminUpdateURLNotReachable,
-					Message:    err.Error(),
-					StatusCode: http.StatusServiceUnavailable,
-				}
-			}
+	client := &http.Client{Transport: getUpdateTransport(timeout)}
+	resp, err := client.Do(req)
+	if err != nil {
+		if xnet.IsNetworkOrHostDown(err, false) {
 			return content, AdminError{
-				Code:       AdminUpdateUnexpectedFailure,
+				Code:       AdminUpdateURLNotReachable,
 				Message:    err.Error(),
-				StatusCode: http.StatusInternalServerError,
+				StatusCode: http.StatusServiceUnavailable,
 			}
 		}
-		if resp == nil {
-			return content, AdminError{
-				Code:       AdminUpdateUnexpectedFailure,
-				Message:    fmt.Sprintf("No response from server to download URL %s", u),
-				StatusCode: http.StatusInternalServerError,
-			}
+		return content, AdminError{
+			Code:       AdminUpdateUnexpectedFailure,
+			Message:    err.Error(),
+			StatusCode: http.StatusInternalServerError,
 		}
-		reader = resp.Body
-		defer xhttp.DrainBody(resp.Body)
-
-		if resp.StatusCode != http.StatusOK {
-			return content, AdminError{
-				Code:       AdminUpdateUnexpectedFailure,
-				Message:    fmt.Sprintf("Error downloading URL %s. Response: %v", u, resp.Status),
-				StatusCode: resp.StatusCode,
-			}
+	}
+	if resp == nil {
+		return content, AdminError{
+			Code:       AdminUpdateUnexpectedFailure,
+			Message:    fmt.Sprintf("No response from server to download URL %s", u),
+			StatusCode: http.StatusInternalServerError,
 		}
-	} else {
-		reader, err = os.Open(u.Path)
-		if err != nil {
-			return content, AdminError{
-				Code:       AdminUpdateURLNotReachable,
-				Message:    err.Error(),
-				StatusCode: http.StatusServiceUnavailable,
-			}
+	}
+	defer xhttp.DrainBody(resp.Body)
+
+	if resp.StatusCode != http.StatusOK {
+		return content, AdminError{
+			Code:       AdminUpdateUnexpectedFailure,
+			Message:    fmt.Sprintf("Error downloading URL %s. Response: %v", u, resp.Status),
+			StatusCode: resp.StatusCode,
 		}
 	}
 
-	contentBytes, err := ioutil.ReadAll(reader)
+	contentBytes, err := ioutil.ReadAll(resp.Body)
 	if err != nil {
 		return content, AdminError{
 			Code:       AdminUpdateUnexpectedFailure,