NLTK: Corpus readers follow symlinks outside trusted roots despite pathsec enforcement

### 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

CVSS 4.0

CVSS:4.0/AV:N/AC:H/AT:N/PR:N/UI:N/VC:H/VI:N/VA:N/SC:N/SI:N/SA:N

Severity from

GitHub (reviewed advisory)

Weakness

CWE-22, CWE-59

Also known as

CVE-2026-79676, PYSEC-2026-3737

• github.com/nltk/nltk/security/advisories/GHSA-p4rw-rvv2-7xwr
• nvd.nist.gov/vuln/detail/CVE-2026-79676
• github.com/nltk/nltk/commit/10d34b3f4fe3fec74b76527a409eb0acbac2e8ab
• github.com/nltk/nltk
• github.com/nltk/nltk/releases/tag/v3.10.3
• github.com/pypa/advisory-database/tree/main/vulns/nltk/PYSEC-2026-3737.yaml
• www.vulncheck.com/advisories/nltk-before-path-traversal-via-symlink-bypass