CVE-2026-66393: Nltk Denial of Service Vulnerability

MODERATE
Advisory Identifier: GHSA-rf74-v2fm-23pw
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CWE-674
Published
Advisory Summary

CVE-2026-66393 is a MODERATE denial of service vulnerability (CWE-674: Uncontrolled Recursion) affecting Nltk libraries in the PyPI ecosystem. The issue involves natural language toolkit (nltk) has unbounded recursion in jsontaggeddecoder.decode_obj() may cause dos. Remediation is available by upgrading affected packages to version 3.9.4.

CVE-2026-66393 at a Glance

CVE Identifier CVE-2026-66393
GHSA Identifier GHSA-rf74-v2fm-23pw
Severity MODERATE
CVSS v3.1 Base Score N/A
CWE Classification CWE-674: Uncontrolled Recursion
Affected Product Nltk
Package Ecosystem PyPI
Published Date
Last Updated

How to Fix CVE-2026-66393

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: 3.9.4 
pip install nltk>=3.9.4
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-2026-66393, 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 3.9.4), 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-674: Uncontrolled Recursion, where the product does not properly control the amount of recursion that takes place, consuming excessive resources, such as allocated memory or the program stack. 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 `JSONTaggedDecoder.decode_obj()` in `nltk/jsontags.py` calls itself recursively without any depth limit. A deeply nested JSON structure exceeding `sys.getrecursionlimit()` (default: 1000) will raise an unhandled `RecursionError`, crashing the Python process. ### Affected code File: `nltk/jsontags.py`, lines 47–52 ```python @classmethod def decode_obj(cls, obj): if isinstance(obj, dict): obj = {key: cls.decode_obj(val) for (key, val) in obj.items()} elif isinstance(obj, list): obj = list(cls.decode_obj(val) for val in obj) ``` ### Proof of Concept ```python import sys, json from nltk.jsontags import JSONTaggedDecoder depth = sys.getrecursionlimit() + 50 # e.g. 1050 payload = '{"x":' * depth + "null" + "}" * depth # Raises RecursionError, crashing the process json.loads(payload, cls=JSONTaggedDecoder) ``` ### Impact Any code path that passes externally-supplied JSON to `JSONTaggedDecoder` is vulnerable to denial of service. The severity depends on whether such a path exists in the calling code (e.g. `nltk/data.py`). ### Suggested Fix Add a depth parameter with a hard limit: ```python @classmethod def decode_obj(cls, obj, _depth=0): if _depth > 100: raise ValueError("JSON nesting too deep") if isinstance(obj, dict): obj = {key: cls.decode_obj(val, _depth + 1) for (key, val) in obj.items()} elif isinstance(obj, list): obj = list(cls.decode_obj(val, _depth + 1) for val in obj) ```

CVE-2026-66393 Affected Nltk Versions

1 component(s)
Fixed Versions:
3.9.4
Product / Package Ecosystem Affected Versions Fixed Version
nltk
pypi/nltk
PyPI
< 3.9.4
3.9.4

References

GitHub Advisory / Wiki ADVISORY
https://github.com/nltk/nltk/security/advisories/GHSA-rf74-v2fm-23pw
NVD — CVE-2026-66393 NVD
https://nvd.nist.gov/vuln/detail/CVE-2026-66393
Security Fix Commit FIX COMMIT
https://github.com/nltk/nltk/commit/00cdcd392142e6c745e7120c8d50a24127df5fad
Nltk Source Repository REPOSITORY
https://github.com/nltk/nltk
Nltk Source Repository REPOSITORY
https://github.com/pypa/advisory-database/tree/main/vulns/nltk/PYSEC-2026-3724.yaml
www.vulncheck.com/advisories/nltk-bef... WEB
https://www.vulncheck.com/advisories/nltk-before-denial-of-service-via-jsontaggeddecoder

Frequently Asked Questions

What is CVE-2026-66393?
CVE-2026-66393 is a security vulnerability in Nltk involving Natural Language Toolkit (NLTK) has unbounded recursion in JSONTaggedDecoder.decode_obj() may cause DoS under CWE-674: Uncontrolled Recursion.
Which Nltk versions are affected?
Affected versions include releases prior to 3.9.4 across affected packages: nltk.
What version fixes CVE-2026-66393?
Upgrading to 3.9.4 (or the latest patched release for your respective branch) resolves the vulnerability.
Is nltk affected?
Yes, nltk is affected in releases prior to 3.9.4. Upgrade your dependency to 3.9.4 or later.
What CWE is associated with CVE-2026-66393?
CVE-2026-66393 is classified under CWE-674: Uncontrolled Recursion (The product does not properly control the amount of recursion that takes place, consuming excessive resources, such as allocated memory or the program stack.).
How can I check PyPI dependencies for CVE-2026-66393?
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-rf74-v2fm-23pw",
  "modified" : "2026-09-02T15:33:51Z",
  "published" : "2026-03-18T20:17:43Z",
  "aliases" : [ "CVE-2026-66393" ],
  "summary" : "Natural Language Toolkit (NLTK) has unbounded recursion in JSONTaggedDecoder.decode_obj() may cause DoS",
  "details" : "### Summary\n`JSONTaggedDecoder.decode_obj()` in `nltk/jsontags.py` calls itself \nrecursively without any depth limit. A deeply nested JSON structure \nexceeding `sys.getrecursionlimit()` (default: 1000) will raise an \nunhandled `RecursionError`, crashing the Python process.\n\n### Affected code\nFile: `nltk/jsontags.py`, lines 47–52\n```python\n@classmethod\ndef decode_obj(cls, obj):\n    if isinstance(obj, dict):\n        obj = {key: cls.decode_obj(val) for (key, val) in obj.items()}\n    elif isinstance(obj, list):\n        obj = list(cls.decode_obj(val) for val in obj)\n```\n\n### Proof of Concept\n```python\nimport sys, json\nfrom nltk.jsontags import JSONTaggedDecoder\n\ndepth = sys.getrecursionlimit() + 50  # e.g. 1050\npayload = '{\"x\":' * depth + \"null\" + \"}\" * depth\n\n# Raises RecursionError, crashing the process\njson.loads(payload, cls=JSONTaggedDecoder)\n```\n\n### Impact\nAny code path that passes externally-supplied JSON to \n`JSONTaggedDecoder` is vulnerable to denial of service.\nThe severity depends on whether such a path exists in the \ncalling code (e.g. `nltk/data.py`).\n\n### Suggested Fix\nAdd a depth parameter with a hard limit:\n```python\n@classmethod\ndef decode_obj(cls, obj, _depth=0):\n    if _depth > 100:\n        raise ValueError(\"JSON nesting too deep\")\n    if isinstance(obj, dict):\n        obj = {key: cls.decode_obj(val, _depth + 1) \n               for (key, val) in obj.items()}\n    elif isinstance(obj, list):\n        obj = list(cls.decode_obj(val, _depth + 1) for val in obj)\n```",
  "severity" : [ {
    "type" : "CVSS_V4",
    "score" : "CVSS:4.0/AV:L/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:L/SC:N/SI:N/SA:N"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "PyPI",
      "name" : "nltk"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "3.9.4"
      } ]
    } ],
    "database_specific" : {
      "last_known_affected_version_range" : "<= 3.9.3"
    }
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/nltk/nltk/security/advisories/GHSA-rf74-v2fm-23pw"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-66393"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/nltk/nltk/commit/00cdcd392142e6c745e7120c8d50a24127df5fad"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/nltk/nltk"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/pypa/advisory-database/tree/main/vulns/nltk/PYSEC-2026-3724.yaml"
  }, {
    "type" : "WEB",
    "url" : "https://www.vulncheck.com/advisories/nltk-before-denial-of-service-via-jsontaggeddecoder"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-674" ],
    "severity" : "MODERATE",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-03-18T20:17:43Z",
    "nvd_published_at" : null
  }
}