[v8.2.x] Security: Fix directory traversal issue (#42845)

* security: fix dir traversal issue

* Improve comments and error message.

* lint

Co-authored-by: Kyle Brandt <kyle@grafana.com>
Co-authored-by: Sofia Papagiannaki <1632407+papagian@users.noreply.github.com>
Co-authored-by: Malcolm Holmes <mdh@odoko.co.uk>

pkg/api/plugins.go

@@ -264,14 +264,27 @@ func (hs *HTTPServer) getPluginAssets(c *models.ReqContext) {
 		return
 	}
 
-	requestedFile := filepath.Clean(macaron.Params(c.Req)["*"])
-	pluginFilePath := filepath.Join(plugin.PluginDir, requestedFile)
+	// prepend slash for cleaning relative paths
+	requestedFile := filepath.Clean(filepath.Join("/", macaron.Params(c.Req)["*"]))
+	rel, err := filepath.Rel("/", requestedFile)
+	if err != nil {
+		// slash is prepended above therefore this is not expected to fail
+		c.JsonApiErr(500, "Failed to get the relative path", err)
+		return
+	}
 
-	if !plugin.IncludedInSignature(requestedFile) {
+	if !plugin.IncludedInSignature(rel) {
 		hs.log.Warn("Access to requested plugin file will be forbidden in upcoming Grafana versions as the file "+
 			"is not included in the plugin signature", "file", requestedFile)
 	}
 
+	absPluginDir, err := filepath.Abs(plugin.PluginDir)
+	if err != nil {
+		c.JsonApiErr(500, "Failed to get plugin absolute path", nil)
+		return
+	}
+
+	pluginFilePath := filepath.Join(absPluginDir, rel)
 	// It's safe to ignore gosec warning G304 since we already clean the requested file path and subsequently
 	// use this with a prefix of the plugin's directory, which is set during plugin loading
 	// nolint:gosec

pkg/api/plugins_test.go

@@ -24,9 +24,13 @@ 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)
@@ -60,6 +64,30 @@ func Test_GetPluginAssets(t *testing.T) {
 			})
 	})
 
+	t.Run("Given a request for a relative path", func(t *testing.T) {
+		p := &plugins.PluginBase{
+			Id:        pluginID,
+			PluginDir: pluginDir,
+			SignedFiles: map[string]struct{}{
+				requestedFile: {},
+			},
+		}
+		service := &pluginManager{
+			plugins: map[string]*plugins.PluginBase{
+				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.PluginBase{
 			Id:        pluginID,