The vulnerability in rrrene html_sanitize_ex's CSS scrubber (html_sanitize_ex.Scrubber.CSS.scrub/1) stems from improper neutralization of special elements in output used by downstream components, specifically allowing injection of CSS at-rules. The scrubber uses a Regex.replace to apply an allowlist only to substrings matching property:value declarations, so input not matching this pattern is copied unchanged. This enables injection of CSS at-rules like '@import url(//attacker.example/style.css);' which survive the sanitization. Element boundaries are resolved before the scrubber runs, preventing injected content from escaping the <style> element or executing scripts. The vulnerability affects versions from 0.3.1 before 1.5.4.

An unauthenticated remote attacker can inject CSS at-rules into pages served to other users, potentially causing the inclusion of remote stylesheets. However, the injected content is confined within the <style> element and does not allow script execution. The CVSS 4.0 base score is 2.3, indicating low severity with limited impact.

Patch status is not yet confirmed — check the vendor advisory for current remediation guidance. No official fix or temporary workaround is indicated in the provided data. Users should monitor vendor communications for updates and consider restricting or validating CSS input more strictly until a patch is available.