Sanitize vulnerable to Cross-site Scripting via insufficient neutralization of `style` element content

Impact

Using carefully crafted input, an attacker may be able to sneak
arbitrary HTML and CSS through Sanitize >= 3.0.0, < 6.0.2 when
Sanitize is configured to use the built-in “relaxed” config or
when using a custom config that allows style elements and one
or more CSS at-rules. This could result in XSS (cross-site scripting)
or other undesired behavior when the malicious HTML and CSS are
rendered in a browser.

Patches

Sanitize >= 6.0.2 performs additional escaping of CSS in style
element content, which fixes this issue.

Workarounds

Users who are unable to upgrade can prevent this issue by using a
Sanitize config that doesn’t allow style elements, using a Sanitize
config that doesn’t allow CSS at-rules, or by manually escaping the
character sequence </ as <\/ in style element content.

Credit

This issue was found by @cure53 during an audit of a project that
uses Sanitize and was reported by one of that project’s maintainers.
Thank you!