fix: Namespace confusion when disallowing 'svg' or 'math'

- https://hackerone.com/reports/2519936
- https://hackerone.com/reports/2519941

Also, test that this solution addresses namespace confusion and DOM
mutation with 'math' + 'mtext'.

lib/rails/html/scrubbers.rb

@@ -101,7 +101,11 @@ def keep_node?(node)
         end
 
         def scrub_node(node)
-          node.before(node.children) unless prune # strip
+          # If a node has a namespace, then it's a tag in either a `math` or `svg` foreign context,
+          # and we should always prune it to avoid namespace confusion and mutation XSS vectors.
+          unless prune || node.namespace
+            node.before(node.children)
+          end
           node.remove
         end
 

test/sanitizer_test.rb

@@ -918,7 +918,7 @@ def test_combination_of_svg_and_style_with_script_payload
         # libxml2
         "<svg><style>&lt;script&gt;alert(1)&lt;/script&gt;</style></svg>",
         # libgumbo
-        "<svg><style>alert(1)</style></svg>"
+        "<svg><style></style></svg>",
       ]
 
       assert_includes(acceptable_results, actual)
@@ -976,6 +976,48 @@ def test_combination_of_svg_and_style_with_img_payload_2
       assert_includes(acceptable_results, actual)
     end
 
+    def test_combination_of_style_and_disallowed_svg_with_script_payload
+      # https://hackerone.com/reports/2519936
+      input, tags = "<svg><style><style class='</style><script>alert(1)</script>'>", ["style"]
+      actual = safe_list_sanitize(input, tags: tags)
+      acceptable_results = [
+        # libxml2
+        "<style>&lt;style class='</style>alert(1)'&gt;",
+        # libgumbo
+        "",
+      ]
+
+      assert_includes(acceptable_results, actual)
+    end
+
+    def test_combination_of_style_and_disallowed_math_with_script_payload
+      # https://hackerone.com/reports/2519936
+      input, tags = "<math><style><style class='</style><script>alert(1)</script>'>", ["style"]
+      actual = safe_list_sanitize(input, tags: tags)
+      acceptable_results = [
+        # libxml2
+        "<style>&lt;style class='</style>alert(1)'&gt;",
+        # libgumbo
+        "",
+      ]
+
+      assert_includes(acceptable_results, actual)
+    end
+
+    def test_math_with_disallowed_mtext_and_img_payload
+      # https://hackerone.com/reports/2519941
+      input, tags = "<math><mtext><table><mglyph><style><img src=: onerror=alert(1)>", ["math", "style"]
+      actual = safe_list_sanitize(input, tags: tags)
+      acceptable_results = [
+        # libxml2
+        "<math><style>&lt;img src=: onerror=alert(1)&gt;</style></math>",
+        # libgumbo
+        "<math></math>",
+      ]
+
+      assert_includes(acceptable_results, actual)
+    end
+
     def test_should_sanitize_illegal_style_properties
       raw      = %(display:block; position:absolute; left:0; top:0; width:100%; height:100%; z-index:1; background-color:black; background-image:url(http://www.ragingplatypus.com/i/cam-full.jpg); background-x:center; background-y:center; background-repeat:repeat;)
       expected = %(display:block;width:100%;height:100%;background-color:black;background-x:center;background-y:center;)
@@ -1075,5 +1117,25 @@ class HTML4SafeListSanitizerTest < Minitest::Test
   class HTML5SafeListSanitizerTest < Minitest::Test
     @module_under_test = Rails::HTML5
     include SafeListSanitizerTest
+
+    def test_should_not_be_vulnerable_to_ns_confusion_2519936
+      # https://hackerone.com/reports/2519936
+      input = "<math><style><style class='</style><script>alert(1)</script>'>"
+      result = Rails::HTML5::SafeListSanitizer.new.sanitize(input, tags: ["style"])
+      browser = Nokogiri::HTML5::Document.parse(result)
+      xss = browser.at_xpath("//script")
+
+      assert_nil(xss)
+    end
+
+    def test_should_not_be_vulnerable_to_ns_confusion_2519941
+      # https://hackerone.com/reports/2519941
+      input = "<math><mtext><table><mglyph><style><img src=: onerror=alert(1)>"
+      result = Rails::HTML5::SafeListSanitizer.new.sanitize(input, tags: %w(math style))
+      browser = Nokogiri::HTML5::Document.parse(result)
+      xss = browser.at_xpath("//img/@onerror")
+
+      assert_nil(xss)
+    end
   end if loofah_html5_support?
 end