diff --git a/backend/handlers.go b/backend/handlers.go
index fc0c07bd2..1f8e1966d 100644
--- a/backend/handlers.go
+++ b/backend/handlers.go
@@ -2,6 +2,7 @@ package main
 
 import (
 	"encoding/json"
+	"errors"
 	"fmt"
 	"html/template"
 	"io"
@@ -10,6 +11,7 @@ import (
 	"path"
 	"path/filepath"
 	"strings"
+	"syscall"
 
 	"github.com/gorilla/mux"
 	log "github.com/sirupsen/logrus"
@@ -99,10 +101,15 @@ func serveFilesHandler(fs http.FileSystem) http.Handler {
 		upath = path.Clean(upath)
 		fileInfo, err := os.Stat(fmt.Sprintf("%v%s", fs, upath))
 		if err != nil {
-			if os.IsNotExist(err) {
+			// A path below a regular file (ENOTDIR, e.g. /index.html/foo) or with an
+			// overlong component (ENAMETOOLONG) cannot exist, so treat it as not found.
+			if os.IsNotExist(err) || errors.Is(err, syscall.ENOTDIR) || errors.Is(err, syscall.ENAMETOOLONG) {
 				notFoundHandler(w, r)
 				return
 			}
+			// Let http.FileServer report any other error (e.g. 403 on permission denied).
+			fsh.ServeHTTP(w, r)
+			return
 		}
 
 		if fileInfo.IsDir() {
diff --git a/backend/main_test.go b/backend/main_test.go
index c03115264..2cc178c81 100644
--- a/backend/main_test.go
+++ b/backend/main_test.go
@@ -3,6 +3,9 @@ package main
 import (
 	"net/http"
 	"net/http/httptest"
+	"os"
+	"path/filepath"
+	"strings"
 	"testing"
 )
 
@@ -112,3 +115,35 @@ func TestStaticFileServer(t *testing.T) {
 		t.Errorf("Wrong content type, expected %s, got %s", expectedContentType, contentType)
 	}
 }
+
+func TestServeFilesHandler(t *testing.T) {
+	root := t.TempDir()
+	if err := os.WriteFile(filepath.Join(root, "page.html"), []byte("<html>page</html>"), 0o644); err != nil {
+		t.Fatal(err)
+	}
+
+	h := serveFilesHandler(http.Dir(root))
+
+	cases := []struct {
+		name string
+		path string
+		want int
+	}{
+		{name: "existing file", path: "/page.html", want: http.StatusOK},
+		{name: "missing file", path: "/missing.html", want: http.StatusNotFound},
+		{name: "path below a regular file", path: "/page.html/foo", want: http.StatusNotFound},
+		{name: "overlong path component", path: "/" + strings.Repeat("a", 300), want: http.StatusNotFound},
+		{name: "directory without index", path: "/", want: http.StatusNotFound},
+	}
+
+	for _, tc := range cases {
+		t.Run(tc.name, func(t *testing.T) {
+			rec := httptest.NewRecorder()
+			h.ServeHTTP(rec, httptest.NewRequest(http.MethodGet, tc.path, nil))
+
+			if rec.Code != tc.want {
+				t.Fatalf("unexpected status for %s: got %d want %d", tc.path, rec.Code, tc.want)
+			}
+		})
+	}
+}