fix(plugins): reject relative plugin dirs to prevent cwd fallback (CWE-426)

pkg/plugins/plugins.go

@@ -20,28 +20,40 @@ func getPluginDir() (string, error) {
 	dir := os.Getenv(pluginDirEnv)
 	// if TKN_PLUGINS_DIR is set, follow it
 	if dir != "" {
+		if !filepath.IsAbs(dir) {
+			return "", fmt.Errorf("plugin dir %q is not an absolute path", dir)
+		}
 		return dir, nil
 	}
 	// Respect XDG_CONFIG_HOME if set
 	if xdgHome := os.Getenv("XDG_CONFIG_HOME"); xdgHome != "" {
+		if !filepath.IsAbs(xdgHome) {
+			return "", fmt.Errorf("XDG_CONFIG_HOME %q is not an absolute path", xdgHome)
+		}
 		return filepath.Join(xdgHome, "tkn", "plugins"), nil
 	}
 	// Fallback to default pluginDir (~/.config/tkn/plugins)
-	return homedir.Expand(pluginDir)
+	dir, err := homedir.Expand(pluginDir)
+	if err != nil {
+		return "", err
+	}
+	if !filepath.IsAbs(dir) {
+		return "", fmt.Errorf("plugin dir %q is not an absolute path", dir)
+	}
+	return dir, nil
 }
 
 // Find a binary in plugin homedir directory or user paths.
 func FindPlugin(pluginame string) (string, error) {
 	cmd := tknPrefix + pluginame
-	dir, _ := getPluginDir()
-	path := filepath.Join(dir, cmd)
-	_, err := os.Stat(path)
-	if err == nil {
-		// Found in dir
-		return path, nil
+	if dir, err := getPluginDir(); err == nil {
+		path := filepath.Join(dir, cmd)
+		if _, err := os.Stat(path); err == nil {
+			return path, nil
+		}
 	}
 
-	path, err = exec.LookPath(cmd)
+	path, err := exec.LookPath(cmd)
 	if err == nil {
 		return path, nil
 	}

pkg/plugins/plugins_test.go

@@ -49,6 +49,42 @@ func TestGetAllTknPluginFromPathPlugindir(t *testing.T) {
 	assert.Equal(t, paths[0], "fromplugindir")
 }
 
+func TestGetPluginDirRelativeTKNPluginsDir(t *testing.T) {
+	t.Setenv(pluginDirEnv, "relative/path")
+	_, err := getPluginDir()
+	assert.ErrorContains(t, err, "not an absolute path")
+}
+
+func TestGetPluginDirRelativeXDGConfigHome(t *testing.T) {
+	t.Setenv(pluginDirEnv, "")
+	t.Setenv("XDG_CONFIG_HOME", "relative/xdg")
+	_, err := getPluginDir()
+	assert.ErrorContains(t, err, "not an absolute path")
+}
+
+func TestFindPluginDoesNotFallBackToCwd(t *testing.T) {
+	nd := fs.NewDir(t, "TestFindPluginCwd")
+	defer nd.Remove()
+	err := os.WriteFile(nd.Join("tkn-evil"), []byte("evil"), 0o700)
+	assert.NilError(t, err)
+
+	// Change into the directory that contains the malicious binary.
+	orig, err := os.Getwd()
+	assert.NilError(t, err)
+	defer os.Chdir(orig) //nolint:errcheck
+	assert.NilError(t, os.Chdir(nd.Path()))
+
+	// Use "." as TKN_PLUGINS_DIR: the old code would resolve filepath.Join(".", "tkn-evil")
+	// against cwd and find the binary; the fixed code rejects "." as non-absolute.
+	// Keep nd off PATH so LookPath cannot find the binary either.
+	t.Setenv(pluginDirEnv, ".")
+	t.Setenv("PATH", "")
+
+	// The binary is only reachable via cwd — FindPlugin must not find it.
+	_, err = FindPlugin("evil")
+	assert.ErrorContains(t, err, "cannot find plugin")
+}
+
 // as well tested differently in root_test.go
 func TestGetAllTknPluginFromPaths(t *testing.T) {
 	nd := fs.NewDir(t, "TestGetAllTknPluginFromPaths1")