CVE-2026-12243: nltk Remote Code Execution Vulnerability

HIGH
Advisory Identifier: GHSA-m42h-3232-vpv3
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CVSS 7.5
CWE-22
Published
Advisory Summary

CVE-2026-12243 is a HIGH (CVSS 7.5) remote code execution vulnerability (CWE-22: Improper Limitation of a Pathname to a Restricted Directory ('Path Traversal')) affecting nltk libraries in the PyPI ecosystem. The issue involves arbitrary file read via path traversal in nltk.data.load() through percent-encoded sequences. Remediation is available by upgrading affected packages to version 3.10.0.

CVE-2026-12243 at a Glance

CVE Identifier CVE-2026-12243
GHSA Identifier GHSA-m42h-3232-vpv3
Severity HIGH
CVSS v3.1 Base Score 7.5
CWE Classification CWE-22: Improper Limitation of a Pathname to a Restricted Directory ('Path Traversal')
Affected Product nltk
Package Ecosystem PyPI
Published Date
Last Updated

Security Impact

CVSS: 7.5

The security impact metrics below are parsed directly from the CVSS v3.1 vector string (CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N).

Attack Vector Network (AV:N)
Attack Complexity Low (AC:L)
Privileges Required None (PR:N)
User Interaction None (UI:N)
Scope Unchanged (S:U)
Confidentiality High (C:H)
Integrity None (I:N)
Availability None (A:N)

How to Fix CVE-2026-12243

To remediate this vulnerability, upgrade all affected nltk dependencies to a patched release. Both direct and transitive dependencies should be checked, as frameworks or security libraries may pull in vulnerable versions transitively.

nltk fixed: 3.10.0 
pip install nltk>=3.10.0
Verify Direct and Transitive Dependencies

Run the following command in your project directory to verify that all resolved dependencies use the patched release: 

pip show nltk

How to Check if You Are Affected

To determine whether your Python environment is affected by CVE-2026-12243, inspect your installed packages using pip:

pip show nltk

Analyze the terminal output to locate any version of nltk in your dependency tree. If any resolved version matches the vulnerable ranges listed in the table below (for example, versions earlier than 3.10.0), your application is affected.

Note: The pip show nltk command checks currently installed packages in your active Python environment.

Technical Details

Vulnerability Mechanism & Technical Analysis

This vulnerability is classified under CWE-22: Improper Limitation of a Pathname to a Restricted Directory ('Path Traversal'), where the product uses external input to construct a pathname that is intended to identify a file or directory that is located underneath a restricted parent directory, but the product does not properly neutralize special elements within the pathname that can cause the pathname to resolve to a location that is outside of the restricted directory. In nltk, software state validation or input boundaries are circumvented during processing. Review the official source advisory statement below for specific affected components, classes, and reproduction details.

Source Advisory Statement

# Summary nltk.data.load() and nltk.data.find() resolve user-supplied resource names to filesystem paths using url2pathname(), which decodes percent-encoded sequences (e.g. %2e%2e to ..). Path safety checks are performed on the raw, still-encoded string before decoding occurs. An attacker supplying %2e%2e instead of .. bypasses all path validation and reads arbitrary files outside the NLTK data directory. # Vulnerable Code nltk/data.py - find() function: url2pathname() decodes %2e%2e -> .. AFTER any safety check p = os.path.join(path_, url2pathname(resource_name)) if os.path.exists(p): return FileSystemPathPointer(p) # Proof of Concept import nltk.data nltk.data.path = ["/home/user/nltk_data"] %2e%2e decodes to .. via url2pathname(), escaping the data dir data = nltk.data.load("%2e%2e/SECRET_credentials.txt", format="raw") print(data) b'AWS_SECRET_KEY=AKIAIOSFODNN7EXAMPLE\nDATABASE_PASS=hunter2\n' All of these bypass path checks and decode identically: # Payload After url2pathname() %2e%2e/secret ../secret .%2e/secret ../secret %2e./secret ../secret %2E%2E/secret ../secret Root Cause url2pathname() is called after path safety checks, not before. Encoding .. as %2e%2e passes every check, then decodes to a traversal sequence at filesystem access time. # Fix Decode before checking: from urllib.parse import unquote resource_name = unquote(resource_name) # decode first, then validate # Impact An attacker who controls the resource name passed to nltk.data.load() can read any file the process has permission to access - credentials, environment files, SSH private keys, /etc/passwd, /proc/self/environ, application config files, etc. This affects any application that passes user-controlled input to nltk.data.load() or nltk.data.find().

CVE-2026-12243 Affected nltk Versions

1 component(s)
Fixed Versions:
3.10.0
Product / Package Ecosystem Affected Versions Fixed Version
nltk
pypi/nltk
PyPI
< 3.10.0
3.10.0

References

GitHub Advisory / Wiki ADVISORY
https://github.com/nltk/nltk/security/advisories/GHSA-m42h-3232-vpv3
NVD — CVE-2026-12243 NVD
https://nvd.nist.gov/vuln/detail/CVE-2026-12243
nltk Source Repository REPOSITORY
https://github.com/nltk/nltk/issues/3504
nltk Source Repository REPOSITORY
https://github.com/nltk/nltk/pull/3522
Security Fix Commit FIX COMMIT
https://github.com/nltk/nltk/commit/aec4fce1b84ad725b8975f7365b23a4f626572a9
nltk Source Repository REPOSITORY
https://github.com/nltk/nltk
nltk Source Repository REPOSITORY
https://github.com/pypa/advisory-database/tree/main/vulns/nltk/PYSEC-2026-597.yaml
huntr.com/bounties/39aa9354-5... WEB
https://huntr.com/bounties/39aa9354-54ca-4e77-96da-580eb1fe6ed1
nltk Security Information SECURITY
https://securityinfinity.com/research/path-traversal-in-nltks-nltk-data-load-via-percent-encoded-sequences

Frequently Asked Questions

What is CVE-2026-12243?
CVE-2026-12243 is a security vulnerability in nltk involving Arbitrary File Read via Path Traversal in nltk.data.load() through Percent-Encoded Sequences under CWE-22: Improper Limitation of a Pathname to a Restricted Directory ('Path Traversal').
Which nltk versions are affected?
Affected versions include releases prior to 3.10.0 across affected packages: nltk.
What version fixes CVE-2026-12243?
Upgrading to 3.10.0 (or the latest patched release for your respective branch) resolves the vulnerability.
Is nltk affected?
Yes, nltk is affected in releases prior to 3.10.0. Upgrade your dependency to 3.10.0 or later.
What is the CVSS score of CVE-2026-12243?
CVE-2026-12243 has a CVSS v3.1 base score of 7.5 (HIGH) with vector string CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N.
What CWE is associated with CVE-2026-12243?
CVE-2026-12243 is classified under CWE-22: Improper Limitation of a Pathname to a Restricted Directory ('Path Traversal') (The product uses external input to construct a pathname that is intended to identify a file or directory that is located underneath a restricted parent directory, but the product does not properly neutralize special elements within the pathname that can cause the pathname to resolve to a location that is outside of the restricted directory.).
How can I check PyPI dependencies for CVE-2026-12243?
Run the dependency tree command: 'pip show nltk'. This command lists direct and transitive dependencies in your build configuration so you can verify if a vulnerable version is resolved.
Machine-readable OSV JSON OSV v1.5 Schema
Normalized schema output
View Raw JSON 
{
  "schema_version" : "1.4.0",
  "id" : "GHSA-m42h-3232-vpv3",
  "modified" : "2026-08-13T20:45:05Z",
  "published" : "2026-08-13T20:45:05Z",
  "aliases" : [ "CVE-2026-12243" ],
  "summary" : "nltk: Arbitrary File Read via Path Traversal in nltk.data.load() through Percent-Encoded Sequences",
  "details" : "# Summary\nnltk.data.load() and nltk.data.find() resolve user-supplied resource names to filesystem paths using url2pathname(), which decodes percent-encoded sequences (e.g. %2e%2e to ..). Path safety checks are performed on the raw, still-encoded string before decoding occurs. An attacker supplying %2e%2e instead of .. bypasses all path validation and reads arbitrary files outside the NLTK data directory.\n\n# Vulnerable Code\nnltk/data.py - find() function:\n url2pathname() decodes %2e%2e -> .. AFTER any safety check\np = os.path.join(path_, url2pathname(resource_name))\nif os.path.exists(p):\n    return FileSystemPathPointer(p)\n\n# Proof of Concept\nimport nltk.data\nnltk.data.path = [\"/home/user/nltk_data\"]\n%2e%2e decodes to .. via url2pathname(), escaping the data dir\ndata = nltk.data.load(\"%2e%2e/SECRET_credentials.txt\", format=\"raw\")\nprint(data)\n b'AWS_SECRET_KEY=AKIAIOSFODNN7EXAMPLE\\nDATABASE_PASS=hunter2\\n'\nAll of these bypass path checks and decode identically:\n\n# Payload\tAfter url2pathname()\n%2e%2e/secret\t../secret\n.%2e/secret\t../secret\n%2e./secret\t../secret\n%2E%2E/secret\t../secret\nRoot Cause\nurl2pathname() is called after path safety checks, not before. Encoding .. as %2e%2e passes every check, then decodes to a traversal sequence at filesystem access time.\n\n# Fix\nDecode before checking:\n\nfrom urllib.parse import unquote\nresource_name = unquote(resource_name)  # decode first, then validate\n\n# Impact\nAn attacker who controls the resource name passed to nltk.data.load() can read any file the process has permission to access - credentials, environment files, SSH private keys, /etc/passwd, /proc/self/environ, application config files, etc. This affects any application that passes user-controlled input to nltk.data.load() or nltk.data.find().",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "PyPI",
      "name" : "nltk"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "3.10.0"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/nltk/nltk/security/advisories/GHSA-m42h-3232-vpv3"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-12243"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/nltk/nltk/issues/3504"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/nltk/nltk/pull/3522"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/nltk/nltk/commit/aec4fce1b84ad725b8975f7365b23a4f626572a9"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/nltk/nltk"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/pypa/advisory-database/tree/main/vulns/nltk/PYSEC-2026-597.yaml"
  }, {
    "type" : "WEB",
    "url" : "https://huntr.com/bounties/39aa9354-54ca-4e77-96da-580eb1fe6ed1"
  }, {
    "type" : "WEB",
    "url" : "https://securityinfinity.com/research/path-traversal-in-nltks-nltk-data-load-via-percent-encoded-sequences"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-22" ],
    "severity" : "HIGH",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-08-13T20:45:05Z",
    "nvd_published_at" : null
  }
}