vulnerability / NVI-00017798 / Revision 1
`lxml_html_clean.Cleaner` does not strip `javascript:` URLs from namespaced URL attributes
GitHub lists this reviewed advisory for lxml_html_clean. Published version ranges and source prerequisites are shown below; this is not an inspection of a visitor’s application.
This report has not inspected your inventory, configuration, deployed build, reachability or evidence. A catalog listing and a severity score do not establish exposure or compromise.
01 / Meaning
Why it matters
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8.2 high
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As of · GitHub Advisory Database
02 / Applicability
The affected context
Affected versions, configuration prerequisites and known exclusions are public. Confirm each one against the deployed service.
lxml_html_clean
source listed< 0.4.5
# `lxml_html_clean.Cleaner` does not strip `javascript:` URLs from namespaced URL attributes (`xlink:href`) **Reporter:** Guillem Lefait · **Date:** 2026-05-10 **Affected:** `lxml` ≤ 6.1.0 and `lxml_html_clean` ≤ 0.4.4 (latest stable) **Confirmed against:** lxml 6.1.0 + lxml_html_clean 0.4.4 on Python 3.13.5, 3.14.4, and 3.15.0a8 (libxml2 2.14.6 / 2.9.14 — bug is in pure-Python sanitizer logic, independent of the libxml2 backend) **Root-cause class:** same as CVE-2021-28957 (`formaction` missing from `link_attrs`) ## Summary `Cleaner` filters URL schemes (`javascript:`, `vbscript:`, …) by walking links via `rewrite_links()`, which delegates to `iterlinks()`, which only yields attributes named in `lxml.html.defs.link_attrs`. That allow-list contains no prefixed names (`xlink:href`) and no `srcset`. As a result, when `Cleaner` is configured with `safe_attrs_only=False` — a documented option for callers that want lenient attribute handling but still expect URL-scheme scrubbing — `` survives sanitization untouched, and any browser that follows the SVG-anchor specification will execute the JavaScript when the rendered link is clicked.
03 / Response
What to do next
- investigate
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04 / Limits
What this report cannot establish
- Customer inventory, deployment, reachability and exploitation have not been inspected.
05 / Provenance
Sources you can inspect
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- GHSA-4jhm-jv67-739f ↗
GitHub Advisory Database · vendor advisory
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- Revision 1 ·
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