Cross-site scripting via leaked style elements in github.com/microcosm-cc/bluemonday

The bluemonday HTML sanitizer can leak the contents of a “style” element into HTML output, potentially causing XSS vulnerabilities.

The default bluemonday sanitization policies are not vulnerable. Only user-defined policies allowing “select”, “style”, and “option” elements are affected.

Permitting the “style” element in policies is hazardous, because bluemonday does not contain a CSS sanitizer. Newer versions of bluemonday suppress “style” and “script” elements even when allowed by a policy unless the policy explicitly requests unsafe processing.