[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.go

@@ -281,14 +281,27 @@ func (hs *HTTPServer) getPluginAssets(c *models.ReqContext) {
 		return
 	}
 
-	requestedFile := filepath.Clean(web.Params(c.Req)["*"])
-	pluginFilePath := filepath.Join(plugin.PluginDir, requestedFile)
+	// prepend slash for cleaning relative paths
+	requestedFile := filepath.Clean(filepath.Join("/", web.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