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High, CVSS 8.6amended

NLTK: Symlink escape in CorpusReader allows arbitrary local file read outside the corpus root

CVE-2026-70626GHSA-r6gq-whwq-mvg9

What to do

Vendor fix available. Upgrade each affected package to the fixed version on its release line (nltk 3.9.4), 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 nltk

    Affected
    <= 3.9.3
    Fixed in
    3.9.4

Sources

Severity and exploitation
Exploitation
No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.
CVSS 4.0
8.6 · CVSS:4.0/AV:L/AC:L/AT:N/PR:N/UI:N/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N · score source
EPSS
  • CVE-2026-70626: 0.20% estimated exploitation activity in the next 30 days; percentile 9.86 (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.

Summary nltk.corpus.reader.api.CorpusReader.open() can be used to read files outside the intended corpus root via a symlink placed inside that root. Although NLTK blocks absolute paths and .. traversal, the current boundary check is only lexical and does not account for symlink resolution. This leads to an arbitrary local file read / filesystem sandbox bypass for applications that rely on CorpusReader or FileSystemPathPointer to restrict file access. Details The vulnerable flow is: - nltk/corpus/reader/api.py:222 - CorpusReader.open() blocks absolute paths and .., then calls self.root.join(file).open() - nltk/data.py:398 - FileSystemPathPointer.join() joins the requested file ID and checks whether the resulting path still appears to remain under the configured root The problem is that the check is based on the lexical path after os.path.normpath(), not on the resolved path after following symlinks. Current behavior: 1. CorpusReader.open() rejects: - absolute paths - .. path traversal…

Dates and revisions
Published by the source
Last modified by the source
Source last checked
  1. Revision 4 · amended ·

    Amended from ghsa record GHSA-r6gq-whwq-mvg9

  2. Revision 3 · amended ·

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

  3. Revision 2 · amended ·

    Amended from osv record GHSA-r6gq-whwq-mvg9

  4. Revision 1 · created ·

    Published from ghsa record GHSA-r6gq-whwq-mvg9

Agent brief

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# Patch Next advisory brief: CVE-2026-70626

Advisory: NLTK: Symlink escape in CorpusReader allows arbitrary local file read outside the corpus root
IDs: CVE-2026-70626, GHSA-r6gq-whwq-mvg9
Record: https://patchnext.com/advisories/cve-2026-70626 (revision 4, 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: High (CVSS 8.6)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: nltk 3.9.4. A vendor fix does not mean a deployment is fixed.

## Affected components
- PyPI nltk: affected <= 3.9.3; fixed in 3.9.4

## 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-r6gq-whwq-mvg9 (GitHub-reviewed advisory GHSA-r6gq-whwq-mvg9 lists affected ranges: PyPI nltk >=0 <=3.9.3)
- Verified: https://osv.dev/vulnerability/GHSA-r6gq-whwq-mvg9 (OSV record GHSA-r6gq-whwq-mvg9 lists affected ranges: PyPI nltk >=0 <3.9.4 (fixed))

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