validate-theme-id.js.diff

diff --git a/chrome/browser/resources/settings/appearance_page/appearance_page.js b/chrome/browser/resources/settings/appearance_pes
index 5571b2d24992..de068c7715ae 100644
--- a/chrome/browser/resources/settings/appearance_page/appearance_page.js
+++ b/chrome/browser/resources/settings/appearance_page/appearance_page.js
@@ -262,6 +262,7 @@ Polymer({
   themeChanged_: function(themeId, useSystemTheme) {
     if (themeId) {
       assert(!useSystemTheme);
+      assert(isValidExtensionId(themeId));
 
       this.browserProxy_.getThemeInfo(themeId).then(function(info) {
         this.themeSublabel_ = info.name;
diff --git a/ui/webui/resources/js/util.js b/ui/webui/resources/js/util.js
index 28a60d2ce137..22b013369dd9 100644
--- a/ui/webui/resources/js/util.js
+++ b/ui/webui/resources/js/util.js
@@ -46,6 +46,14 @@ function announceAccessibleMessage(msg) {
 }
 
 /**
+ * @param {?} maybeId
+ * @return {boolean} Whether |maybeId| is a valid extension ID.
+ */
+function isValidExtensionId(maybeId) {
+  return /[a-p]{32}/i.test(maybeId);
+}
+
+/**
  * Generates a CSS url string.
  * @param {string} s The URL to generate the CSS url for.
  * @return {string} The CSS url string.