---
id: CVE-2026-79676
aliases:
  - GHSA-p4rw-rvv2-7xwr
  - PYSEC-2026-3737
title: >-
  NLTK: Corpus readers follow symlinks outside trusted roots despite pathsec
  enforcement
summary: >-
  NLTK: Corpus readers follow symlinks outside trusted roots despite pathsec
  enforcement
severity: high
vendor: nltk
product: nltk
ecosystem: pip
affected:
  - nltk < 3.10.3
patched:
  - nltk 3.10.3
published: '2026-09-08'
updated: '2026-09-08'
sourceUpdated: '2026-09-08T17:00:04.101709807Z'
source: OSV
sourceUrl: 'https://osv.dev/vulnerability/GHSA-p4rw-rvv2-7xwr'
references:
  - url: 'https://github.com/nltk/nltk/security/advisories/GHSA-p4rw-rvv2-7xwr'
  - url: 'https://nvd.nist.gov/vuln/detail/CVE-2026-79676'
  - url: >-
      https://github.com/nltk/nltk/commit/10d34b3f4fe3fec74b76527a409eb0acbac2e8ab
  - url: 'https://github.com/nltk/nltk'
  - url: 'https://github.com/nltk/nltk/releases/tag/v3.10.3'
  - url: >-
      https://github.com/pypa/advisory-database/tree/main/vulns/nltk/PYSEC-2026-3737.yaml
  - url: >-
      https://www.vulncheck.com/advisories/nltk-before-path-traversal-via-symlink-bypass
  - url: 'https://github.com/advisories/GHSA-p4rw-rvv2-7xwr'
tags:
  - osv
  - pip
  - ghsa
epss: 0.00447
epssPercentile: 0.36217
cwe:
  - CWE-22
  - CWE-59
ingestedAt: '2026-09-02T19:31:24.475Z'
---

## Overview

### Summary

Several corpus readers still step outside NLTK's symlink-aware trusted-root model. They derive in-root paths from trusted corpus state, convert those paths back into plain strings, and reopen them with built-in `open()` rather than `nltk.pathsec.open()`.

### Details

- **Vulnerability type:** Path traversal and symlink boundary bypass
- **Affected component:** `nltk.corpus.reader.ipipan`, `nltk.corpus.reader.crubadan`, `nltk.corpus.reader.lin`
- **Affected versions:** Published `3.9.4` and current source `v3.10.0-rc2` both reproduced.
- **Patched versions:** Not yet patched
- **Root cause:** Root-derived paths are reopened with raw `open()` without preserving the trusted-root boundary.

`IPIPANCorpusReader` opens `header.xml` derived from `morph.xml`, `CrubadanCorpusReader` opens `table.txt` directly, and `LinThesaurusCorpusReader` opens `simN.lsp` paths returned from its own root helpers. Under `pathsec.ENFORCE=True`, a symlink placed inside the trusted corpus root can point outside the root and still be parsed successfully. It was confirmed parsed outside-root content is returned through public methods such as `channels()`, `domains()`, `categories()`, `langs()`, `crubadan_to_iso()`, `synonyms()`, and `scored_synonyms()`.

### PoC

**Preconditions**
- The application processes attacker-influenced corpora inside a trusted NLTK data root or trusted corpus directory.

**Steps**
1. Create a trusted corpus root and keep `pathsec.ENFORCE=True` with that root allowlisted.
2. Place symlinked reader inputs such as `header.xml`, `table.txt`, or `simN.lsp` inside the root and point them to external files.
3. Instantiate the corresponding corpus reader and call its normal public methods.
4. Observe that parsed outside-root values are returned even though `pathsec.open()` blocks the same symlink targets.

**Minimal reproducible excerpt**

```text
{'ipipan': ['LEAK', 'TOPSECRET', 'CLASSIFIED'], 'crubadan': ['LEAK'], 'lin': [('LEAK', 9.5)]}
```

### Impact

An attacker who can stage corpus files or symlinks under a trusted data root can disclose outside-root content through normal corpus-reader results, defeating the boundary NLTK documents for shared and untrusted-input environments.

### Remediation

Preserve `PathPointer` and `required_root` semantics end to end. Replace direct `open()` calls with `nltk.pathsec.open()` or a reader helper that keeps the trusted-root boundary intact.

### References

- https://github.com/nltk/nltk/blob/3.9.4/nltk/corpus/reader/ipipan.py#L162-L192
- https://github.com/nltk/nltk/blob/3.9.4/nltk/corpus/reader/crubadan.py#L74-L98
- https://github.com/nltk/nltk/blob/3.9.4/nltk/corpus/reader/lin.py#L40-L43
- https://github.com/nltk/nltk/blob/v3.10.0-rc2/nltk/pathsec.py#L521-L545

---

## Fix + full-codebase audit (verified)


I swept every raw file open in the corpus readers, not just the three the umbrella named:

| Reader | Site | Advisory | Root scoping |
|---|---|---|---|
| crubadan | table.txt + `<code>-3grams.txt` | p4rw / j5pw | `required_root=self.root` |
| lin | simN.lsp | p4rw | `required_root=self.root` |
| xmldocs | XMLCorpusView bare-string fileid | 934p (base reader) | global fallback (view has no root) |
| pl196x | textids index | **found by audit** | `required_root=self._root` |
| mte | MTEFileReader | mvf5 | `required_root` threaded through 8 call sites |
| toolbox | StandardFormat.open codecs.open | cr8c | global sandbox (low-level parser) |
| named_entity | load_ace_file ann/text | 7qj2 | global sandbox |
| nkjp | XML_Tool source file | p4rw class | `required_root=self._root` |

`ipipan` already validates via the earlier #3727 fix — unchanged.

## Fix
Each site now calls `nltk.pathsec.validate_path(path, required_root=…)` before opening. Where the reader has a concrete corpus root, the check is **scoped** with `required_root` (rejects any escape outside that root). `XMLCorpusView` carries no root, so it falls back to the global data-root sandbox via `getattr(self, "_root", None)` — which also avoids an AttributeError on the bare-string path.

## Reproduced (captured)
```
raw open(symlink) reads: 'TOPSECRET_OUTSIDE_ROOT'          <- the bypass
validate_path(symlink, required_root): ValueError -> BLOCKS the escape
validate_path(legit in-root): PASSED                       <- loads normally
```

## Honest residual
The global-sandbox fallback (toolbox, named_entity, xmldocs-view) is only as tight as the allowed-roots list, which currently includes the **system temp dir**. Scoping every reader with `required_root` and removing the temp dir from the allowed roots would harden it further (separate advisory / task).

## Tests
`test_corpus_reader_pathsec.py` — symlink escape rejected, in-root file allowed, XMLCorpusView string-fileid no AttributeError, MTEFileReader out-of-root rejected. 46 existing corpus/toolbox tests pass; all edited modules import (no circular import). pre-commit (black/isort/ruff) clean.

---

## Scope caveat
`validate_path` blocks every symlink escape variant (verified) and equals `pathsec.open()`'s guarantee, but does NOT block **hardlinks** (no symlink to resolve; tracked separately as GHSA-f794-5jv7-7672) or the validate-then-open TOCTOU race (shared by `pathsec.open`; needs O_NOFOLLOW/openat).

## Affected packages

- `nltk < 3.10.3`

## Remediation

Upgrade to a patched release:

- `nltk 3.10.3`