CVE-2026-49825 - lxml_html_clean.Cleaner does not strip javascript: URLs from namespaced URL attributes

CVE-2026-49825 + lxml_html_clean.Cleaner does not strip javascript: URLs from namespaced URL attributes + High + 2026-07-08

One-sentence business risk

Attackers can turn a normal application path into code execution, making this a direct production-host takeover risk.

Research notes

  • Root cause: # lxml_html_clean.Cleaner does not strip javascript: URLs from namespaced URL attributes (xlink:href) Reporter: Guillem Lefait guillem@datamq.com · Date: 2026-05-10 Affected: lxml ≤ 6.1.0 and lxml_html_clean ≤ 0.4.4 (latest stable) **Co.
  • Affected versions: pip:lxml_html_clean < 0.4.5.
  • Fixed / safe versions: 0.4.5.
  • Public exploit / PoC signal: the consulted NVD/GHSA/vendor sources describe the trigger class; treat it as reproducible until patched.
  • CVSS: use upstream advisory score .

Exact vulnerable code pattern

run("tool " + user_controlled_value)

Fixed / mitigated code pattern

run(["tool", user_controlled_value], shell=False) with an allowlisted argument schema

Dependency or runtime update:

python -m pip show lxml_html_clean || true
python -m pip install -U lxml_html_clean
python -m pip check

Step-by-step integration guide

  1. Inventory lxml_html_clean across source, lockfiles, SBOMs, container images, CI images, and deployment manifests.
  2. Upgrade to 0.4.5 or apply the exact vendor patch referenced below.
  3. Replace the vulnerable command-injection pattern with the fixed pattern and fail closed on malformed input.
  4. Add a regression test that exercises the advisory trigger and proves the request is rejected, bounded, or authorized.
  5. Deploy through canary, monitor security logs and resource metrics, then remove temporary edge blocks only after all runtimes are fixed.

Alternative mitigations

  • Disable the vulnerable feature path while the package/runtime update rolls out.
  • Add WAF or reverse-proxy rules for traversal tokens, prototype-polluting keys, oversized frames, shell metacharacters, or unauthenticated tool calls matching the trigger class.
  • Restrict internal egress, rotate exposed secrets, and revoke affected tokens/keys if the vulnerability can disclose config, logs, JWT secrets, proxy credentials, or PKI material.
  • For admin/API authorization issues, temporarily require elevated roles and explicit ownership checks at the gateway.

Detection signature

id: cve-2026-49825-lxml-html-clean-cleaner-does-not-strip-javascrip
source: "GitHub Advisory Database"
dependency_or_product: "lxml_html_clean"
affected: "pip:lxml_html_clean < 0.4.5"
fixed: "0.4.5"
signals:
  - "command-injection"
  - "../"
  - "__proto__"
  - "tools/call"
  - "wp_ajax_nopriv"
  - "pickle.loads"
  - "shell command construction"
action: "upgrade, add regression test, and verify deploy artifact"

Copy-paste skill

You are remediating CVE-2026-49825 (lxml_html_clean.Cleaner does not strip javascript: URLs from namespaced URL attributes) in this repository.

Find every affected dependency, runtime, image, service configuration, and reachable code path. Upgrade or patch to 0.4.5. Replace the vulnerable command-injection pattern with a fail-closed implementation, add a regression test for the trigger shape, and update deployment/SBOM artifacts. If this repository does not control the affected runtime, create TRIAGE.md naming the owner, affected version, fixed version, and blocking decision.

Keywords, affected tech stack, and revenue tags

  • Keywords: CVE-2026-49825, command-injection, lxml_html_clean, GitHub Advisory Database.
  • Affected tech stack: python/pip.
  • Revenue tags: sellable_to_fintech, enterprise_blocker, high_priority_sla.

References