CVSS v3.x: 6.1 (Medium)
- < 5.0.0
- >= 6.0.1
Using carefully crafted input, an attacker may be able to sneak arbitrary HTML through Sanitize
>= 5.0.0, < 6.0.1 when Sanitize is configured with a custom allowlist that allows
noscript elements. This could result in XSS (cross-site scripting) or other undesired behavior when that HTML is rendered in a browser.
Sanitize’s default configs don’t allow
noscript elements and are not vulnerable. This issue only affects users who are using a custom config that adds
noscript to the element allowlist.
>= 6.0.1 always removes
noscript elements and their contents, even when
noscript is in the allowlist.
Users who are unable to upgrade can prevent this issue by using one of Sanitize’s default configs or by ensuring that their custom config does not include
noscript in the element allowlist.
The root cause of this issue is that HTML parsing rules treat the contents of a
noscript element differently depending on whether scripting is enabled in the user agent. Nokogiri (the HTML parser Sanitize uses) doesn’t support scripting so it follows the "scripting disabled" rules, but a web browser with scripting enabled will follow the "scripting enabled" rules. This means that Sanitize can’t reliably make the contents of a
noscript element safe for scripting enabled browsers. The safest thing to do is to remove the element and its contents entirely, which is now what Sanitize does in version 6.0.1 and later.