Cross-site scripting vulnerability via `<math>` or `<svg>` element in Sanitize

When HTML is sanitized using Sanitize’s “relaxed” config or a custom config that allows certain
elements, some content in a <math> or <svg> element may not be sanitized correctly even if
math and svg are not in the allowlist.

You are likely to be vulnerable to this issue if you use Sanitize’s relaxed config or a custom
config that allows one or more of the following HTML elements:

• iframe
• math
• noembed
• noframes
• noscript
• plaintext
• script
• style
• svg
• xmp

Impact

Using carefully crafted input, an attacker may be able to sneak arbitrary HTML through Sanitize,
potentially resulting in XSS (cross-site scripting) or other undesired behavior when that HTML is
rendered in a browser.

Releases

This problem has been fixed in Sanitize 5.2.1.

Workarounds

If upgrading is not possible, a workaround is to override the default value of Sanitize’s
:remove_contents config option with the following value, which ensures that the contents of
math and svg elements (among others) are removed entirely when those elements are not in the
allowlist:

%w[iframe math noembed noframes noscript plaintext script style svg xmp]

For example, if you currently use Sanitize’s relaxed config, you can create a custom config
object that overrides the default value of :remove_contents like this:

custom_config = Sanitize::Config.merge(
  Sanitize::Config::RELAXED,
  :remove_contents => %w[iframe math noembed noframes noscript plaintext script style svg xmp]
)

You would then pass this custom config to Sanitize when sanitizing HTML.