CVE-2026-71513: Nltk Remote Code Execution Vulnerability

HIGH
Advisory Identifier: GHSA-5gh2-94qg-qppq
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CVSS 8.8
CWE-502
Published
Advisory Summary

CVE-2026-71513 is a HIGH (CVSS 8.8) remote code execution vulnerability (CWE-502: Deserialization of Untrusted Data) affecting Nltk libraries in the PyPI ecosystem. The issue involves nltk allowlistunpickler dotted-name validation bypass allows remote code execution. Remediation is available by upgrading affected packages to version 3.10.3.

CVE-2026-71513 at a Glance

CVE Identifier CVE-2026-71513
GHSA Identifier GHSA-5gh2-94qg-qppq
Severity HIGH
CVSS v3.1 Base Score 8.8
CWE Classification CWE-502: Deserialization of Untrusted Data
Affected Product Nltk
Package Ecosystem PyPI
Published Date
Last Updated

Security Impact

CVSS: 8.8

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:R/S:U/C:H/I:H/A:H).

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

How to Fix CVE-2026-71513

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: 3.10.3 
pip install nltk>=3.10.3
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-2026-71513, 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 3.10.3), 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-502: Deserialization of Untrusted Data, where the product deserializes untrusted data without sufficiently ensuring that the resulting data will be valid. 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

NLTK before 3.10.3 contains a remote code execution vulnerability in AllowlistUnpickler that validates only the pickle module string and not the global name, allowing attackers to resolve dotted names by attribute traversal to callables outside the allowlisted namespace. Attackers can craft untrusted transition-parser models that execute arbitrary commands when TransitionParser.parse loads the model through allowlisted_pickle_load.

CVE-2026-71513 Affected Nltk Versions

1 component(s)
Fixed Versions:
3.10.3
Product / Package Ecosystem Affected Versions Fixed Version
nltk
pypi/nltk
PyPI
≥ 3.10.0
< 3.10.3
3.10.3

References

NVD — CVE-2026-71513 NVD
https://nvd.nist.gov/vuln/detail/CVE-2026-71513
Security Fix Commit FIX COMMIT
https://github.com/nltk/nltk/commit/c3e37113742a1ebeeb4f2ca58941f320f98805ea
Nltk Source Repository REPOSITORY
https://github.com/nltk/nltk
Nltk Source Repository REPOSITORY
https://github.com/nltk/nltk/blob/v3.10.2/nltk/picklesec.py#L119-L124
www.vulncheck.com/advisories/nltk-thr... WEB
https://www.vulncheck.com/advisories/nltk-through-remote-code-execution-via-allowlistunpickler-dotted-name-bypass

Frequently Asked Questions

What is CVE-2026-71513?
CVE-2026-71513 is a security vulnerability in Nltk involving NLTK AllowlistUnpickler dotted-name validation bypass allows remote code execution under CWE-502: Deserialization of Untrusted Data.
Which Nltk versions are affected?
Affected versions include releases prior to 3.10.3 across affected packages: nltk.
What version fixes CVE-2026-71513?
Upgrading to 3.10.3 (or the latest patched release for your respective branch) resolves the vulnerability.
Is nltk affected?
Yes, nltk is affected in releases prior to 3.10.3. Upgrade your dependency to 3.10.3 or later.
What is the CVSS score of CVE-2026-71513?
CVE-2026-71513 has a CVSS v3.1 base score of 8.8 (HIGH) with vector string CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H.
What CWE is associated with CVE-2026-71513?
CVE-2026-71513 is classified under CWE-502: Deserialization of Untrusted Data (The product deserializes untrusted data without sufficiently ensuring that the resulting data will be valid.).
How can I check PyPI dependencies for CVE-2026-71513?
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-5gh2-94qg-qppq",
  "modified" : "2026-09-02T15:18:39Z",
  "published" : "2026-08-22T15:31:03Z",
  "aliases" : [ "CVE-2026-71513" ],
  "summary" : "NLTK AllowlistUnpickler dotted-name validation bypass allows remote code execution",
  "details" : "NLTK before 3.10.3 contains a remote code execution vulnerability in AllowlistUnpickler that validates only the pickle module string and not the global name, allowing attackers to resolve dotted names by attribute traversal to callables outside the allowlisted namespace. Attackers can craft untrusted transition-parser models that execute arbitrary commands when TransitionParser.parse loads the model through allowlisted_pickle_load.",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H"
  }, {
    "type" : "CVSS_V4",
    "score" : "CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:P/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "PyPI",
      "name" : "nltk"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "3.10.0"
      }, {
        "fixed" : "3.10.3"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-71513"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/nltk/nltk/commit/c3e37113742a1ebeeb4f2ca58941f320f98805ea"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/nltk/nltk"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/nltk/nltk/blob/v3.10.2/nltk/picklesec.py#L119-L124"
  }, {
    "type" : "WEB",
    "url" : "https://www.vulncheck.com/advisories/nltk-through-remote-code-execution-via-allowlistunpickler-dotted-name-bypass"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-502" ],
    "severity" : "HIGH",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-09-02T15:18:39Z",
    "nvd_published_at" : "2026-08-22T14:16:33Z"
  }
}