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NltkApplication

CVE-2026-33236

HIGH · 8.1 CVSS v3.1 Published 2026-03-20
Fix available
A fix is available. Upgrade to after 3.9.3 or later.
See remediation →
88/100
Remediation priority · High
Public exploit Remotely reachable No privileges Patch available

Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.

NVD · unedited
NLTK (Natural Language Toolkit) is a suite of open source Python modules, data sets, and tutorials supporting research and development in Natural Language Processing. In versions 3.9.3 and prior, the NLTK downloader does not validate the `subdir` and `id` attributes when processing remote XML index files. Attackers can control a remote XML index server to provide malicious values containing path traversal sequences (such as `../`), which can lead to arbitrary directory creation, arbitrary file creation, and arbitrary file overwrite. Commit 89fe2ec2c6bae6e2e7a46dad65cc34231976ed8a patches the issue.

Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.

dbcve analysis · high confidence

The NLTK downloader in versions 3.9.3 and prior fails to validate `subdir` and `id` attributes when parsing remote XML index files. An attacker controlling a malicious XML index server can embed path traversal sequences (`../`) in these attributes, allowing arbitrary directory creation, file creation, and file overwrite on the victim's filesystem.

MitigationUpgrade NLTK to a version newer than 3.9.3. Until patched, avoid downloading data from untrusted or non-official NLTK data sources and consider network segmentation to block unauthorized XML index server connections.

Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.

Affected products & versions What the vendor confirmedThe version ranges the vendor confirmed as vulnerable. If your version sits inside a range here, treat yourself as exposed until you have upgraded.

NVD · CPE data
NltkApplication
Affected:<= 3.9.3

CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.

From the vector
Attack vector
Network
Complexity
Low
Privileges
None
User interaction
Required
Scope
Unchanged
Confidentiality
None
Integrity
High
Availability
High

CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:H/A:H

Am I affected? How to checkSteps we derive from the advisory and the affected-version data, so you can decide whether this CVE reaches your setup. They are a guide, not a scan — your own configuration is the authority.

dbcve checks

Work through these to decide whether this CVE applies to you.

  1. Check installed NLTK version
    Run 'python -c "import nltk; print(nltk.__version__)"' or 'pip show nltk' to display the installed version
    Affected if The version is 3.9.3 or lower
  2. Identify the configured NLTK data server
    Run 'python -c "from nltk.downloader import Downloader; d = Downloader(); print(d._INDEX_URL)"' to see which server URL the downloader is configured to use
    Affected if The URL is not the official NLTK data server (https://raw.githubusercontent.com/nltk/nltk_data/gh-pages/index.xml) or is a custom/untrusted source
  3. Check for custom downloader configuration files
    Inspect the NLTK data directory (typically ~/.nltk) for any custom configuration files or server redirect files that may point to untrusted sources
    Affected if Non-standard or custom server configurations are present in the NLTK data directory
  4. Verify XML parser behavior with path traversal
    Inspect the nltk.downloader module source code to confirm whether the 'subdir' and 'id' attributes are validated before use in file operations (look for path traversal protection)
    Affected if The code lacks validation for '../' sequences in the XML parsing logic for index files

You are affected if your NLTK version is 3.9.3 or lower AND the downloader is configured to use a non-official or untrusted data server, allowing a remote attacker to supply path traversal sequences via crafted XML index responses.

Generated from the published advisory. Verify against your own configuration.

Check your environment

Paste your version and any relevant configuration and it will be compared against the affected criteria above. Do not include secrets or credentials.

AI-assisted, checked against the advisory. Informational, not a guarantee.

Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.

dbcve · scoped
Upgrade available Upgrade to a release after 3.9.3
Vendor patch github.com →
Interim mitigation

Upgrade NLTK to a version newer than 3.9.3. Until patched, avoid downloading data from untrusted or non-official NLTK data sources and consider network segmentation to block unauthorized XML index server connections.

Recommended fix Moderate confidence

nltk > 3.9.3 (latest stable release)

  1. Run `pip install --upgrade nltk` to install the latest version of NLTK which includes the security patch for the path traversal vulnerability in the downloader
  2. Alternatively, if you need a specific version, verify that the version you install is newer than 3.9.3 and includes commit 89fe2ec2c6bae6e2e7a46dad65cc34231976ed8a
  3. After upgrading, verify the fix by checking that the NLTK downloader now validates the `subdir` and `id` attributes from remote XML index files
Caveat Review any code that relies on specific NLTK behavior, as minor behavior changes may occur between versions

Generated from the published advisory — verify against the referenced sources before acting.

Fix this in Nltk Scoped from the published advisory
  • Consultation4.0 h
  • Implementation8.0 h
  • Testing6.0 h
  • Review / QA4.0 h
22.0 hours of engineering $3,860
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References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.

Primary sources

Practitioner notes

Contributed

Peer-ranked notes from engineers who’ve handled CVE-2026-33236 in production — separate from our analysis above.

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What this is

A place for practitioners to share what actually worked: a mitigation you’ve tested, a configuration change, a version- or environment-specific caveat, or a link to a verified patch. The most useful notes rise to the top as peers upvote them, so the signal stays high.

What belongs here
  • Verified mitigations, workarounds, and config changes
  • Version or environment caveats, and links to real fixes
  • No weaponised exploit code, or anything meant to cause harm
  • No spam, self-promotion, credentials, or personal data