CVE-2020-4054 (sanitize): Cross-site scripting vulnerability via `<math>` or `<svg>` element in Sanitize

ADVISORIES

GEM

sanitize

SEVERITY

CVSS v3.x: 7.3 (High)

UNAFFECTED VERSIONS

< 3.0.0

PATCHED VERSIONS

>= 5.2.1

DESCRIPTION

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 =&gt; %w[iframe math noembed noframes noscript plaintext script style svg xmp]
)

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

RubySec

CVE-2020-4054 (sanitize): Cross-site scripting vulnerability via ` $` or ` ` element in Sanitize$