---
title: "CVE-2026-66393\n    | Ubuntu"
description: Ubuntu is an open source software operating system that runs from the
  desktop, to the cloud, to all your internet connected things.
url: https://ubuntu.com/security/CVE-2026-66393?format=md
keywords: index, follow
---

# CVE-2026-66393

Publication date 22 August 2026

Last updated 2 September 2026

---

Ubuntu priority

**Medium**

[Why this priority?](https://ubuntu.com/security/cves/about#priority )

## Cvss 3 Severity Score

**7.5 · High**

[Score breakdown](https://ubuntu.com/security/CVE-2026-66393?format=md#impact-score)

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## Description

NLTK versions before 3.9.4 contain an unbounded recursion vulnerability in
JSONTaggedDecoder.decode\_obj() that allows attackers to cause denial of
service by supplying deeply nested JSON structures. Attackers can craft
JSON payloads exceeding the recursion limit to trigger an unhandled
RecursionError that crashes the Python process.

## Status

Show unmaintained releases

| Package | Ubuntu Release | Status |
| --- | --- | --- |
| nltk | 26.04 LTS resolute | Needs evaluation |
| 24.04 LTS noble | Needs evaluation |
| 22.04 LTS jammy | Needs evaluation |
| 20.04 LTS focal | Needs evaluation |
| 18.04 LTS bionic | Needs evaluation |
| 16.04 LTS xenial | Needs evaluation |
| 14.04 LTS trusty | Needs evaluation |

---

* [How can I get the fixes?](https://ubuntu.com/security/cves/about#security)
* [What do statuses mean?](https://ubuntu.com/security/cves/about#statuses)

## Severity score breakdown

CVSS version:

CVSS v4.0

[CVSS v4.0](https://ubuntu.com/security/CVE-2026-66393?format=md)
[CVSS v3.0](https://ubuntu.com/security/CVE-2026-66393?format=md)

**Base score**

8.7 · High

* Base metrics

  | Parameter | Value |
  | --- | --- |
  | Attack vector | Network |
  | Attack complexity | Low |
  | Attack requirements | None |
  | Privileges required | None |
  | User interaction | None |
  | Vulnerable system - Confidentiality impact | None |
  | Vulnerable system - Integrity impact | None |
  | Vulnerable system - Availability impact | High |
  | Subsequent system - Confidentiality impact | None |
  | Subsequent system - Integrity impact | None |
  | Subsequent system - Availability impact | None |
* Scores

  | Parameter | Value |
  | --- | --- |
  | Base score | 8.7 · High |
  | Base + Threat score | - |
  | Base + Environmental score | - |
  | Base + Threat + Environmental score | - |

**Vector:** CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N

## References

* [MITRE](https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2026-66393)
* [NVD](https://nvd.nist.gov/vuln/detail/CVE-2026-66393)
* [Launchpad](https://launchpad.net/bugs/cve/CVE-2026-66393)
* [Debian](https://security-tracker.debian.org/tracker/CVE-2026-66393)

### Other references

* <https://www.cve.org/CVERecord?id=CVE-2026-66393>
* <https://github.com/nltk/nltk/security/advisories/GHSA-rf74-v2fm-23pw>