# CVE-2026-66393: NLTK before 3.9.4 Denial of Service via JSONTaggedDecoder

> Attackers can crash the application by sending deeply nested JSON data to NLTK's decoder, causing a denial of service.

- **Severity:** High 8.7 (`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`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 22 Aug 2026
- **Updated:** 26 Aug 2026
- **Also known as:** DEBIAN-CVE-2026-66393, ECHO-1e3a-1e96-134c, GHSA-rf74-v2fm-23pw, MINI-9v5c-fgp7-c634, PYSEC-2026-3724, UBUNTU-CVE-2026-66393

## What is CVE-2026-66393?

CVE-2026-66393 is a high-severity denial of service vulnerability in nltk, published on 22 Aug 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official 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.

Weakness: CWE-674 (Uncontrolled Recursion)

## Which versions are affected by CVE-2026-66393?

3 open source packages ship an affected version, including nltk. It affects 1 product.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `nltk` | PyPI | All versions | 3.9.4 |
| `nltk` | PyPI | All versions | 3.9.4 |
| `nltk` | PyPI | All versions | 3.9.4 |

Products: Nltk Nltk.

## How do I fix CVE-2026-66393?

Upgrade nltk to 3.9.4 or later.

## Is CVE-2026-66393 being exploited?

It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-66393 exists.

## Other vulnerabilities in nltk

- [CVE-2026-0848](https://insights.sbomify.com/CVE-2026-0848/): Arbitrary Code Execution in NLTK StanfordSegmenter via Untrusted JAR Loading (critical 10.0)
- [CVE-2025-14009](https://insights.sbomify.com/CVE-2025-14009/): Zip Slip Vulnerability in nltk/nltk Leading to Remote Code Execution (critical 10.0)
- [CVE-2026-78683](https://insights.sbomify.com/CVE-2026-78683/): NLTK before 3.10.0 Remote Code Execution via Unsafe Pickle Deserialization (critical 9.4)
- [CVE-2026-79657](https://insights.sbomify.com/CVE-2026-79657/): NLTK before 3.10.3 Remote Code Execution via Unsafe Pickle Deserialization (critical 9.3)
- [CVE-2026-79675](https://insights.sbomify.com/CVE-2026-79675/): NLTK before 3.10.3 JVM Argument Injection via Per-Call Options (critical 9.3)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-66393 and fix it if it is affected.

What it is: CVE-2026-66393 is a high-severity denial of service vulnerability in nltk, published on 22 Aug 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-66393 exists.

Affected packages and fixed versions:
- nltk (PyPI): affected all versions; fixed in 3.9.4
- nltk (PyPI): affected all versions; fixed in 3.9.4
- nltk (PyPI): affected all versions; fixed in 3.9.4

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-66393/ (Markdown: https://insights.sbomify.com/CVE-2026-66393/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-66393
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-66393
- OSV: https://osv.dev/vulnerability/CVE-2026-66393

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-66393/
Summaries are generated and may contain mistakes; the official description above is the record.