Fix XSS in MR source branch name

3 files changed, 17 insertions, 9 deletions

diff --git a/app/presenters/merge_request_presenter.rb b/app/presenters/merge_request_presenter.rb
index 3f565b826dd..1db6c9eff36 100644
--- a/app/presenters/merge_request_presenter.rb
+++ b/app/presenters/merge_request_presenter.rb
@@ -108,16 +108,10 @@ class MergeRequestPresenter < Gitlab::View::Presenter::Delegated
     namespace = source_project_namespace
     branch = source_branch
 
-    if source_branch_exists?
-      namespace = link_to(namespace, project_path(source_project))
-      branch = link_to(branch, project_tree_path(source_project, source_branch))
-    end
+    namespace_link = source_branch_exists? ? link_to(namespace, project_path(source_project)) : ERB::Util.html_escape(namespace)
+    branch_link = source_branch_exists? ? link_to(branch, project_tree_path(source_project, source_branch)) : ERB::Util.html_escape(branch)
 
-    if for_fork?
-      namespace + ":" + branch
-    else
-      branch
-    end
+    for_fork? ? "#{namespace_link}:#{branch_link}" : branch_link
   end
 
   def closing_issues_links
diff --git a/changelogs/unreleased/51527-xss-in-mr-source-branch.yml b/changelogs/unreleased/51527-xss-in-mr-source-branch.yml
new file mode 100644
index 00000000000..dae277b6413
--- /dev/null
+++ b/changelogs/unreleased/51527-xss-in-mr-source-branch.yml
@@ -0,0 +1,5 @@
+---
+title: Fix XSS in merge request source branch name
+merge_request:
+author:
+type: security
diff --git a/spec/presenters/merge_request_presenter_spec.rb b/spec/presenters/merge_request_presenter_spec.rb
index a1b52d8692d..bafcddebbb7 100644
--- a/spec/presenters/merge_request_presenter_spec.rb
+++ b/spec/presenters/merge_request_presenter_spec.rb
@@ -403,6 +403,15 @@ describe MergeRequestPresenter do
       is_expected
         .to eq("<a href=\"/#{resource.source_project.full_path}/tree/#{resource.source_branch}\">#{resource.source_branch}</a>")
     end
+
+    it 'escapes html, when source_branch does not exist' do
+      xss_attempt = "<img src='x' onerror=alert('bad stuff') />"
+
+      allow(resource).to receive(:source_branch) { xss_attempt }
+      allow(resource).to receive(:source_branch_exists?) { false }
+
+      is_expected.to eq(ERB::Util.html_escape(xss_attempt))
+    end
   end
 
   describe '#rebase_path' do