Don't link URLs with unsafe schemes in Markup::ToHtml (#1832)

`{click}[javascript:alert(1)]` and `[click](javascript:alert(1))` were
rendered as `<a href="javascript:alert(1)">`. The test added for this in
#933 checked the input string instead
of the converted result, so it always passed.

`gen_url` now renders only the link text when the resolved URL has a
`javascript:`, `vbscript:`, `data:` or `file:` scheme, the same schemes
cmark and markdown-it reject:

*
https://github.com/commonmark/cmark/blob/6566d91bdb25898bd910f8bed2a3f18f4a05cf91/src/scanners.re#L322-L332
*
https://github.com/markdown-it/markdown-it/blob/3c51991c32aaa2b002a52c009334ebe5752c84b3/src/markdownit.ts#L60-L61

The check is done after `parse_url` so that `link:javascript:...` is
also caught, and ignores leading C0 controls and spaces and ASCII tabs
and newlines like browsers do:

* https://url.spec.whatwg.org/#concept-basic-url-parser

Co-authored-by: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

Author: kou

lib/rdoc/markup/to_html.rb

@@ -148,6 +148,12 @@ def initialize(pipe: false, output_decoration: true)
       #
       # These methods are used by regexp handling markup added by RDoc::Markup#add_regexp_handling.
 
+      ##
+      # URL schemes that are not linked because they can run scripts or open
+      # local files.  Links to them are rendered as their text only.
+
+      UNSAFE_URL_SCHEME_REGEXP = /\A(?:javascript|vbscript|data|file):/i
+
       # :nodoc:
       URL_CHARACTERS_REGEXP_STR = /[A-Za-z0-9\-._~:\/\?#\[\]@!$&'\(\)*+,;%=]/.source
 
@@ -605,10 +611,15 @@ def convert_string(text)
       # - File references (.rb, .rdoc, .md) are converted to .html paths
       # - Anchor URLs (#foo) pass through unchanged for GitHub-style header linking
       # - Footnote links get wrapped in <sup> tags
+      # - URLs with an unsafe scheme (javascript:, vbscript:, data:, file:) are not
+      #   linked and only +text+ is returned
 
       def gen_url(url, text)
         scheme, url, id = parse_url url
 
+        # Check the parsed URL because link: URLs are used as href without it
+        return text if unsafe_url?(url)
+
         if %w[http https link].include?(scheme) && url =~ /\.(gif|png|jpg|jpeg|bmp)\z/
           "<img src=\"#{url}\" />"
         else
@@ -629,6 +640,19 @@ def gen_url(url, text)
         end
       end
 
+      ##
+      # Does +url+ have a scheme in UNSAFE_URL_SCHEME_REGEXP?
+      #
+      # Browsers ignore leading C0 controls and spaces and all ASCII tabs and
+      # newlines in a URL, so they are removed before checking the scheme.
+      # See https://url.spec.whatwg.org/#concept-basic-url-parser
+      #
+      #: (String) -> bool
+      def unsafe_url?(url)
+        normalized = url.sub(/\A[\x00-\x20]+/, '').delete("\t\n\r")
+        UNSAFE_URL_SCHEME_REGEXP.match?(normalized)
+      end
+
       ##
       # Determines the HTML list element for +list_type+ and +open_tag+
 

test/rdoc/markup/to_html_test.rb

@@ -931,7 +931,8 @@ def test_convert_TIDYLINK_escape_text
   end
 
   def test_convert_TIDYLINK_escape_javascript
-    assert_not_include '{click}[javascript:alert`javascript_scheme`]', '<a href="javascript:'
+    result = @to.convert '{click}[javascript:alert`javascript_scheme`]'
+    assert_equal "\n<p>click</p>\n", result
   end
 
   def test_convert_TIDYLINK_escape_onmouseover
@@ -959,6 +960,32 @@ def test_gen_url
                  @to.gen_url('link:example', 'example')
   end
 
+  def test_gen_url_unsafe_scheme
+    [
+      'javascript:alert(1)',
+      'JavaScript:alert(1)',
+      " \x01javascript:alert(1)",
+      "java\tscript:alert(1)",
+      'vbscript:msgbox(1)',
+      'data:text/html,alert(1)',
+      'file:///etc/passwd',
+    ].each do |url|
+      assert_equal 'example', @to.gen_url(url, 'example'), url.inspect
+    end
+  end
+
+  def test_gen_url_link_unsafe_scheme
+    # The href is "javascript" on Windows because File.split treats
+    # ":alert(1)" as an NTFS alternate data stream
+    result = @to.gen_url('link:javascript:alert(1)', 'example')
+    assert_not_include result, 'href="javascript:'
+  end
+
+  def test_gen_url_scheme_like_path
+    assert_equal '<a href="foo/javascript:alert">example</a>',
+                 @to.gen_url('foo/javascript:alert', 'example')
+  end
+
   def test_gen_url_rdoc_label
     assert_equal '<a href="#foottext-1">example</a>',
                  @to.gen_url('rdoc-label:foottext-1', 'example')