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High, CVSS 8.7amended

VTK NULL pointer dereference vulnerability

CVE-2021-42521GHSA-xfhg-9pjg-xg7g

What to do

Vendor fix available. Upgrade each affected package to the fixed version on its release line (vtk 9.0.1), redeploy, then verify what is actually installed.

This page has no inventory attached, so it cannot say whether you are affected. A vendor fix is not proof that your deployment is fixed.

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Affected packages

  • PyPI vtk

    Affected
    < 9.0.1
    Fixed in
    9.0.1

Sources

Severity and exploitation
Exploitation
No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.
CVSS 4.0
8.7 · CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N · score source
EPSS
  • CVE-2021-42521: 1.32% estimated exploitation activity in the next 30 days; percentile 70.81 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.
Source description

As published by the source; long descriptions are shortened.

There is a NULL pointer dereference vulnerability in VTK, and it lies in IO/Infovis/vtkXMLTreeReader.cxx. The vendor didn't check the return value of libxml2 API 'xmlDocGetRootElement', and try to dereference it. It is unsafe as the return value can be NULL and that NULL pointer dereference may crash the application.

Dates and revisions
Published by the source
Last modified by the source
Source last checked
  1. Revision 3 · amended ·

    Amended from ghsa record GHSA-xfhg-9pjg-xg7g

  2. Revision 2 · amended ·

    Reviewed OSV alias/range/CVSS4 correction; package source scopes and original publication preserved. No customer finding resolved.

  3. Revision 1 · created ·

    Published from osv record GHSA-xfhg-9pjg-xg7g

Agent brief

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# Patch Next advisory brief: CVE-2021-42521

Advisory: VTK NULL pointer dereference vulnerability
IDs: CVE-2021-42521, GHSA-xfhg-9pjg-xg7g
Record: https://patchnext.com/advisories/cve-2021-42521 (revision 3, source checked 26 Oct 2026, 13:27 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: High (CVSS 8.7)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: vtk 9.0.1. A vendor fix does not mean a deployment is fixed.

## Affected components
- PyPI vtk: affected < 9.0.1; fixed in 9.0.1

## Suggested inspection (read-only)
- `python -m pip show vtk`: 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 vtk.
- 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-xfhg-9pjg-xg7g (OSV record GHSA-xfhg-9pjg-xg7g lists affected ranges: PyPI vtk >=0 <9.0.1 (fixed))
- Verified: https://github.com/advisories/GHSA-xfhg-9pjg-xg7g (GitHub-reviewed advisory GHSA-xfhg-9pjg-xg7g lists affected ranges: PyPI vtk >=0 <9.0.1)

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