Path Traversal via Percent-Encoding in nltk.data.find() and nltk.data.load()_CVE-2026-12243

7.5 / 10

HIGH

CVSS:3.0/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N

Description

NLTK version 3.9.4 is vulnerable to a path traversal attack due to an incomplete fix for GitHub Issue #3504. The `_UNSAFE_NO_PROTOCOL_RE` regex in `nltk/data.py` checks for literal `../` sequences but fails to account for percent-encoded traversal sequences such as `..%2f`. The `url2pathname()` function decodes these sequences after the validation step, allowing an attacker to bypass the protection. This vulnerability enables an attacker to read arbitrary files accessible to the Python process by controlling the resource name parameter passed to `nltk.data.load()` or `nltk.data.find()`. The issue affects applications that rely on NLTK for resource loading, including NLP web applications, Jupyter notebooks, and CLI tools. The default `pathsec.ENFORCE=False` setting exacerbates the impact by not blocking the file read at the `open()` stage.

Basic Information

ID CVE-2026-12243

Source @huntr_ai

Published Jun 30, 2026 at 00:14

Affected Product

Vendor nltk

Product nltk/nltk

Version unspecified

Affected Versions nltk nltk/nltk unspecified

CWE Classification

CWE-22

References

huntr.com /bounties/39aa9354-54ca-4e77-96da-580eb1fe6ed1

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    "published": "2026-06-30T00:14:35.370Z",
    "modified": "2026-06-30T00:14:35.370Z",
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