NLTK: Downloader.download follows hardlinks and overwrites outside-root files

Exploitation

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

CVSS 4.0

6.9 · CVSS:4.0/AV:L/AC:L/AT:N/PR:L/UI:N/VC:N/VI:H/VA:H/SC:N/SI:N/SA:N · score source

EPSS

• CVE-2026-81727: 0.19% estimated exploitation activity in the next 30 days; percentile 8.41 (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 NLTK's downloader now blocks symlink escapes during ZIP extraction, but it still treats pre-existing hardlinks inside the install tree as ordinary in-root files. A normal package install can therefore overwrite an outside-root inode through that hardlink. Details - Vulnerability type: Filesystem containment bypass - Affected component: nltk.downloader.Downloader.download, nltk.downloader.Downloader.incrdownload - Affected versions: Published 3.9.4 and current source v3.10.0-rc2 both reproduced for the extraction-stage overwrite. - Patched versions: 3.10.3 - Root cause: The downloader validates traversal and symlink conditions but does not reject pre-existing hardlink aliases inside the install tree. The install flow correctly rejects a pre-existing symlink at an extraction target, yet it accepts a pre-existing hardlink at the same path. When the package is installed, extracted member data is written through the hardlink and mutates the outside inode. PoC Preconditions - The…

Published by the source

2 Sep 2026, 14:35 UTC

Last modified by the source

2 Sep 2026, 14:35 UTC

Source last checked

26 Oct 2026, 13:51 UTC

1. Revision 1 · created · 26 Oct 2026

   Published from ghsa record GHSA-f794-5jv7-7672

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# Patch Next advisory brief: CVE-2026-81727

Advisory: NLTK: Downloader.download follows hardlinks and overwrites outside-root files
IDs: CVE-2026-81727, GHSA-f794-5jv7-7672
Record: https://patchnext.com/advisories/cve-2026-81727 (revision 1, source checked 26 Oct 2026, 13:51 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 6.9)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: nltk 3.10.3. A vendor fix does not mean a deployment is fixed.

## Affected components
- PyPI nltk: affected <= 3.10.2; fixed in 3.10.3

## Suggested inspection (read-only)
- `python -m pip show nltk`: 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 nltk.
- 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://github.com/advisories/GHSA-f794-5jv7-7672 (GitHub-reviewed advisory GHSA-f794-5jv7-7672 lists affected ranges: PyPI nltk >=0 <=3.10.2)

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