Natural Language Toolkit (NLTK): DNS-rebinding SSRF filter bypass in nltk.pathsec.urlopen (nltk.download / nltk.data.load) defeats ENFORCE mode

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS

8.6 · CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:N/A:N

EPSS

No dated EPSS signal available.

As published by the source; long descriptions are shortened.

Summary nltk.pathsec provides an SSRF filter that NLTK documents as a security control, blocking loopback, private, link-local, and multicast ranges (including obfuscated forms) and recommending strict ENFORCE mode for security-sensitive environments. The filter is bypassable by DNS rebinding: validatenetworkurl() resolves the hostname and checks the resulting IP, but the actual HTTP connection re-resolves the hostname independently at connect time and connects to that second result. The validated IP is never the one connected to. An attacker controlling DNS for a hostname (a TTL-0 rebinding record) returns a public IP for the validation lookup and an internal/loopback IP for the connection lookup, defeating the filter even under nltk.pathsec.ENFORCE = True. Details urlopen() validates, then hands the raw hostname to urllib, which performs a second name resolution deep in the connection layer (http.client.HTTPConnection.connect → socket.createconnection → socket.getaddrinfo). The…

Published by the source

31 Jul 2026, 16:51 UTC

Last modified by the source

31 Jul 2026, 16:51 UTC

Source last checked

26 Oct 2026, 13:50 UTC

1. Revision 1 · created · 26 Oct 2026

   Published from ghsa record GHSA-qvv7-cg9c-w4x3

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# Patch Next advisory brief: CVE-2026-12075

Advisory: Natural Language Toolkit (NLTK): DNS-rebinding SSRF filter bypass in nltk.pathsec.urlopen (nltk.download / nltk.data.load) defeats ENFORCE mode
IDs: CVE-2026-12075, GHSA-qvv7-cg9c-w4x3
Record: https://patchnext.com/advisories/cve-2026-12075 (revision 1, source checked 26 Oct 2026, 13:50 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: High (CVSS 8.6)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: nltk 3.10.0. A vendor fix does not mean a deployment is fixed.

## Affected components
- PyPI nltk: affected <= 3.9.4; fixed in 3.10.0

## Suggested inspection (read-only)
- `python -m pip show nltk`: prints the installed version. Run with the environment that serves production active.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on nltk.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://github.com/advisories/GHSA-qvv7-cg9c-w4x3 (GitHub-reviewed advisory GHSA-qvv7-cg9c-w4x3 lists affected ranges: PyPI nltk >=0 <=3.9.4)

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