[v8.3.x] Security: Fix directory traversal issue (#42838)

* security: fix dir traversal issue

* Improve comments and error message.

* Fix lint

Co-authored-by: Kyle Brandt <kyle@grafana.com>
Co-authored-by: Sofia Papagiannaki <1632407+papagian@users.noreply.github.com>

pkg/api/plugins_test.go

@@ -23,9 +23,13 @@ func Test_GetPluginAssets(t *testing.T) {
 	pluginDir := "."
 	tmpFile, err := ioutil.TempFile(pluginDir, "")
 	require.NoError(t, err)
+	tmpFileInParentDir, err := ioutil.TempFile("..", "")
+	require.NoError(t, err)
 	t.Cleanup(func() {
 		err := os.RemoveAll(tmpFile.Name())
 		assert.NoError(t, err)
+		err = os.RemoveAll(tmpFileInParentDir.Name())
+		assert.NoError(t, err)
 	})
 	expectedBody := "Plugin test"
 	_, err = tmpFile.WriteString(expectedBody)
@@ -61,6 +65,29 @@ func Test_GetPluginAssets(t *testing.T) {
 			})
 	})
 
+	t.Run("Given a request for a relative path", func(t *testing.T) {
+		p := plugins.PluginDTO{
+			JSONData: plugins.JSONData{
+				ID: pluginID,
+			},
+			PluginDir: pluginDir,
+		}
+		service := &fakePluginStore{
+			plugins: map[string]plugins.PluginDTO{
+				pluginID: p,
+			},
+		}
+		l := &logger{}
+
+		url := fmt.Sprintf("/public/plugins/%s/%s", pluginID, tmpFileInParentDir.Name())
+		pluginAssetScenario(t, "When calling GET on", url, "/public/plugins/:pluginId/*", service, l,
+			func(sc *scenarioContext) {
+				callGetPluginAsset(sc)
+
+				require.Equal(t, 404, sc.resp.Code)
+			})
+	})
+
 	t.Run("Given a request for an existing plugin file that is not listed as a signature covered file", func(t *testing.T) {
 		p := plugins.PluginDTO{
 			JSONData: plugins.JSONData{