Fix DOS when rendering issue/MR comments

changelogs/unreleased/security-DOS_issue_comments_banzai.yml

+---
+title: Fix Denial of Service for comments when rendering issues/MR comments
+merge_request:
+author:
+type: security

lib/banzai/filter/relative_link_filter.rb

@@ -100,7 +100,7 @@ module Banzai
       end
       def relative_file_path(uri)
-        path = Addressable::URI.unescape(uri.path)
+        path = Addressable::URI.unescape(uri.path).delete("\0")
         request_path = Addressable::URI.unescape(context[:requested_path])
         nested_path = build_relative_path(path, request_path)
         file_exists?(nested_path) ? nested_path : path

spec/lib/banzai/filter/relative_link_filter_spec.rb

@@ -83,6 +83,11 @@ describe Banzai::Filter::RelativeLinkFilter do
     expect { filter(act) }.not_to raise_error
   end
+  it 'does not explode with an escaped null byte' do
+    act = link("/%00")
+    expect { filter(act) }.not_to raise_error
+  end
+
   it 'does not raise an exception with a space in the path' do
     act = link("/uploads/d18213acd3732630991986120e167e3d/Landscape_8.jpg  \nBut here's some more unexpected text :smile:)")
     expect { filter(act) }.not_to raise_error