Fix for XSS in branch names

app/assets/javascripts/vue_merge_request_widget/components/states/mr_widget_rebase.vue

 <script>
 import { GlLoadingIcon } from '@gitlab/ui';
+import { escape } from 'lodash';
 import simplePoll from '../../../lib/utils/simple_poll';
 import eventHub from '../../event_hub';
 import statusIcon from '../mr_widget_status_icon.vue';
@@ -44,11 +45,10 @@ export default {
     fastForwardMergeText() {
       return sprintf(
         __(
-          `Fast-forward merge is not possible. Rebase the source branch onto %{startTag}${this.mr.targetBranch}%{endTag} to allow this merge request to be merged.`,
+          'Fast-forward merge is not possible. Rebase the source branch onto %{targetBranch} to allow this merge request to be merged.',
         ),
         {
-          startTag: '<span class="label-branch">',
-          endTag: '</span>',
+          targetBranch: `<span class="label-branch">${escape(this.mr.targetBranch)}</span>`,
         },
         false,
       );

app/views/shared/issuable/form/_branch_chooser.html.haml

@@ -8,7 +8,9 @@
 
 .form-group.row.d-flex.gl-pl-3.gl-pr-3.branch-selector
   .align-self-center
-    %span= s_('From %{source_title} into').html_safe % { source_title: "<code>#{source_title}</code>".html_safe }
+    %span
+      = _('From <code>%{source_title}</code> into').html_safe % { source_title: source_title }
+
     - if issuable.new_record?
       %code= target_title
       &nbsp;

changelogs/unreleased/security-49-xss-branch-names.yml

+---
+title: Fix for XSS in branch names
+merge_request:
+author:
+type: security

locale/gitlab.pot

@@ -8318,6 +8318,9 @@ msgstr ""
 msgid "False positive"
 msgstr ""
 
+msgid "Fast-forward merge is not possible. Rebase the source branch onto %{targetBranch} to allow this merge request to be merged."
+msgstr ""
+
 msgid "Fast-forward merge is not possible. Rebase the source branch onto the target branch or merge target branch into source branch to allow this merge request to be merged."
 msgstr ""
 
@@ -8768,7 +8771,7 @@ msgstr ""
 msgid "From %{providerTitle}"
 msgstr ""
 
-msgid "From %{source_title} into"
+msgid "From <code>%{source_title}</code> into"
 msgstr ""
 
 msgid "From Bitbucket"

spec/features/merge_request/user_creates_merge_request_spec.rb

@@ -5,9 +5,9 @@ require "spec_helper"
 describe "User creates a merge request", :js do
   include ProjectForksHelper
 
+  let_it_be(:project) { create(:project, :repository) }
+  let_it_be(:user) { create(:user) }
   let(:title) { "Some feature" }
-  let(:project) { create(:project, :repository) }
-  let(:user) { create(:user) }
 
   before do
     project.add_maintainer(user)
@@ -38,6 +38,26 @@ describe "User creates a merge request", :js do
     end
   end
 
+  context "XSS branch name exists" do
+    before do
+      project.repository.create_branch("<img/src='x'/onerror=alert('oops')>", "master")
+    end
+
+    it "doesn't execute the dodgy branch name" do
+      visit(project_new_merge_request_path(project))
+
+      find(".js-source-branch").click
+      click_link("<img/src='x'/onerror=alert('oops')>")
+
+      find(".js-target-branch").click
+      click_link("feature")
+
+      click_button("Compare branches")
+
+      expect { page.driver.browser.switch_to.alert }.to raise_error(Selenium::WebDriver::Error::NoSuchAlertError)
+    end
+  end
+
   context "to a forked project" do
     let(:forked_project) { fork_project(project, user, namespace: user.namespace, repository: true) }