lxml-html-clean has CSS @import Filter Bypass via Unicode Escapes

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS

6.1 · CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N

EPSS

• CVE-2026-28348: 0.28% estimated exploitation activity in the next 30 days; percentile 20.74 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

Summary The hassneakyjavascript() method strips backslashes before checking for dangerous CSS keywords. This causes CSS Unicode escape sequences to bypass the @import and expression() filters, allowing external CSS loading or XSS in older browsers. Details The root cause is located in clean.py (around line 594): This transformation changes a payload like @\69mport into @69mport. This resulting string does NOT match the blacklist keyword @import. However, all modern browsers' CSS parsers decode \69 as the character 'i' (hex 69) according to CSS spec section 4.3.7, interpreting @\69mport as a valid @import statement. Same root cause bypasses expression() detection: \65xpression(alert(1)) passes through (IE only). PoC If rendered in a browser, the browser loads the external CSS. Variants like @\0069mport, @\69 mport (trailing space), and @\49mport (uppercase I) also work. Impact External CSS loading enables data exfiltration via attribute selectors (e.g., reading CSRF tokens), UI…

Published by the source

2 Mar 2026, 19:19 UTC

Last modified by the source

10 Sep 2026, 03:50 UTC

Source last checked

26 Oct 2026, 13:36 UTC

1. Revision 2 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-hw26-mmpg-fqfg

2. Revision 1 · created · 24 Oct 2026

   Published from osv record GHSA-hw26-mmpg-fqfg

Preview the exact text before you copy it. Commands are read-only suggestions; review them before running anything.

# Patch Next advisory brief: CVE-2026-28348

Advisory: lxml-html-clean has CSS @import Filter Bypass via Unicode Escapes
IDs: CVE-2026-28348, GHSA-hw26-mmpg-fqfg
Record: https://patchnext.com/advisories/cve-2026-28348 (revision 2, source checked 26 Oct 2026, 13:36 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 6.1)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: lxml-html-clean 0.4.4. A vendor fix does not mean a deployment is fixed.

## Affected components
- PyPI lxml-html-clean: affected <= 0.4.3; fixed in 0.4.4

## Suggested inspection (read-only)
- `python -m pip show lxml-html-clean`: prints the installed version. Run with the environment that serves production active.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on lxml-html-clean.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://osv.dev/vulnerability/GHSA-hw26-mmpg-fqfg (OSV record GHSA-hw26-mmpg-fqfg lists affected ranges: PyPI lxml-html-clean >=0 <0.4.4 (fixed))
- Verified: https://github.com/advisories/GHSA-hw26-mmpg-fqfg (GitHub-reviewed advisory GHSA-hw26-mmpg-fqfg lists affected ranges: PyPI lxml-html-clean >=0 <=0.4.3)

Commands are suggestions: review each one before running it.