CVE-2026-12372: Nltk NLTK Network URL Validation Permits Vulnerability

LOW
Advisory Identifier: GHSA-2jhm-w3mp-jcwr
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CVSS 3.7
CWE-918
Published
Advisory Summary

CVE-2026-12372 is a LOW (CVSS 3.7) nltk network url validation permits vulnerability (CWE-918: Server-Side Request Forgery (SSRF)) affecting Nltk libraries in the PyPI ecosystem. The issue involves nltk network url validation permits ssrf to rfc 6598 shared-address-space hosts. Remediation is available by upgrading affected packages to version 3.10.0.

CVE-2026-12372 at a Glance

CVE Identifier CVE-2026-12372
GHSA Identifier GHSA-2jhm-w3mp-jcwr
Severity LOW
CVSS v3.1 Base Score 3.7
CWE Classification CWE-918: Server-Side Request Forgery (SSRF)
Affected Product Nltk
Package Ecosystem PyPI
Published Date
Last Updated

Security Impact

CVSS: 3.7

The security impact metrics below are parsed directly from the CVSS v3.1 vector string (CVSS:3.0/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:N/A:N).

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

How to Fix CVE-2026-12372

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-2026-12372, 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-918: Server-Side Request Forgery (SSRF), where the web server receives a url or similar request from an upstream component and retrieves the contents of this url, but it does not sufficiently ensure that the request is being sent to the expected destination. 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

A Server-Side Request Forgery (SSRF) vulnerability exists in nltk/nltk versions 3.9.4 and the current develop branch. The `nltk.pathsec.validate_network_url()` function, intended to prevent SSRF by rejecting internal network addresses, fails to reject IPs in the RFC 6598 shared address space (`100.64.0.0/10`). This occurs because Python's `ipaddress` module does not classify such addresses as `is_private` or `is_global`, and the current guard only checks `is_private` and a few explicit categories. An attacker who can influence a URL passed to NLTK's network-loading helpers can exploit this vulnerability to make a strict-mode application send requests to shared-address-space hosts, potentially exposing non-public infrastructure reachable from the application host. The impact is limited to SSRF-style confidentiality exposure, with no code execution claimed.

CVE-2026-12372 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

NVD — CVE-2026-12372 NVD
https://nvd.nist.gov/vuln/detail/CVE-2026-12372
Security Fix Commit FIX COMMIT
https://github.com/nltk/nltk/commit/4a820afa58810cd05049b6c6eae306694d6cfe65
Nltk Source Repository REPOSITORY
https://github.com/nltk/nltk
Nltk Release RELEASE
https://github.com/nltk/nltk/releases/tag/v3.10.0
Nltk Source Repository REPOSITORY
https://github.com/pypa/advisory-database/tree/main/vulns/nltk/PYSEC-2026-3955.yaml
huntr.com/bounties/2911b643-5... WEB
https://huntr.com/bounties/2911b643-571c-42e4-b0c2-9a1fa6f491db

Frequently Asked Questions

What is CVE-2026-12372?
CVE-2026-12372 is a security vulnerability in Nltk involving NLTK network URL validation permits SSRF to RFC 6598 shared-address-space hosts under CWE-918: Server-Side Request Forgery (SSRF).
Which Nltk versions are affected?
Affected versions include releases prior to 3.10.0 across affected packages: nltk.
What version fixes CVE-2026-12372?
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-2026-12372?
CVE-2026-12372 has a CVSS v3.1 base score of 3.7 (LOW) with vector string CVSS:3.0/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:N/A:N.
What CWE is associated with CVE-2026-12372?
CVE-2026-12372 is classified under CWE-918: Server-Side Request Forgery (SSRF) (The web server receives a URL or similar request from an upstream component and retrieves the contents of this URL, but it does not sufficiently ensure that the request is being sent to the expected destination.).
How can I check PyPI dependencies for CVE-2026-12372?
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-2jhm-w3mp-jcwr",
  "modified" : "2026-10-02T23:11:23Z",
  "published" : "2026-08-10T00:31:42Z",
  "aliases" : [ "CVE-2026-12372" ],
  "summary" : "NLTK network URL validation permits SSRF to RFC 6598 shared-address-space hosts",
  "details" : "A Server-Side Request Forgery (SSRF) vulnerability exists in nltk/nltk versions 3.9.4 and the current develop branch. The `nltk.pathsec.validate_network_url()` function, intended to prevent SSRF by rejecting internal network addresses, fails to reject IPs in the RFC 6598 shared address space (`100.64.0.0/10`). This occurs because Python's `ipaddress` module does not classify such addresses as `is_private` or `is_global`, and the current guard only checks `is_private` and a few explicit categories. An attacker who can influence a URL passed to NLTK's network-loading helpers can exploit this vulnerability to make a strict-mode application send requests to shared-address-space hosts, potentially exposing non-public infrastructure reachable from the application host. The impact is limited to SSRF-style confidentiality exposure, with no code execution claimed.",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.0/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:N/A:N"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "PyPI",
      "name" : "nltk"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "3.10.0"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-12372"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/nltk/nltk/commit/4a820afa58810cd05049b6c6eae306694d6cfe65"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/nltk/nltk"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/nltk/nltk/releases/tag/v3.10.0"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/pypa/advisory-database/tree/main/vulns/nltk/PYSEC-2026-3955.yaml"
  }, {
    "type" : "WEB",
    "url" : "https://huntr.com/bounties/2911b643-571c-42e4-b0c2-9a1fa6f491db"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-918" ],
    "severity" : "LOW",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-10-02T23:11:23Z",
    "nvd_published_at" : "2026-08-09T23:16:35Z"
  }
}