NLTK: BllipParser passes caller-supplied model paths to the bllipparser C extension with no pathsec check, enabling arbitrary read

Exploitation

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

CVSS

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

EPSS

• CVE-2026-126295: 0.29% estimated exploitation activity in the next 30 days; percentile 21.87 (FIRST EPSS, 2026-11-14). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

Summary `BllipParser` in `nltk/parse/bllip.py` hands caller-supplied filesystem paths to the native `bllipparser` C extension without routing them through NLTK's `pathsec` validation. The constructor parameters `parser_model`, `reranker_features`, and `reranker_weights`, and the alternative `from_unified_model_dir()` entry point, flow directly to the C extension's `load_parser_model()` and `load_reranker_model()` calls, which open and read the given path at the OS level. Any application that passes an attacker-influenced path to this API reads a file of the attacker's choosing from anywhere on the filesystem, outside the NLTK data roots that every other NLTK tool wrapper confines access to.

Root Cause As of NLTK 3.10.0, `SECURITY.md` documents a centralized I/O security module (`nltk.pathsec`) with strict enforcement enabled by default, under which file-backed resources must resolve inside allowed NLTK data directories. For paths handed to external tools and native extensions…

Published by the source

13 Nov 2026, 23:20 UTC

Last modified by the source

13 Nov 2026, 23:30 UTC

Source last checked

14 Nov 2026, 07:50 UTC

1. Revision 2 · amended · 14 Nov 2026

   Amended from osv record GHSA-fg9w-wfp5-j933

2. Revision 1 · created · 13 Nov 2026

   Published from ghsa record GHSA-fg9w-wfp5-j933

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# Patch Next advisory brief: CVE-2026-126295

Advisory: NLTK: BllipParser passes caller-supplied model paths to the bllipparser C extension with no pathsec check, enabling arbitrary read
IDs: CVE-2026-126295, GHSA-fg9w-wfp5-j933
Record: https://patchnext.com/advisories/cve-2026-126295 (revision 2, source checked 14 Nov 2026, 07:50 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 6.8)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: nltk 3.10.4. A vendor fix does not mean a deployment is fixed.

## Affected components
- PyPI nltk: affected >= 3.10.0, < 3.10.4; fixed in 3.10.4

## 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-fg9w-wfp5-j933 (GitHub-reviewed advisory GHSA-fg9w-wfp5-j933 lists affected ranges: PyPI nltk >=3.10.0 <=3.10.3)
- Verified: https://osv.dev/vulnerability/GHSA-fg9w-wfp5-j933 (OSV record GHSA-fg9w-wfp5-j933 lists affected ranges: PyPI nltk >=3.10.0 <3.10.4 (fixed))

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