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

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Summary

CVECVE-2026-12243
StatePUBLISHED
Assigner@huntr_ai
Source PriorityCVE Program / NVD first with legacy fallback
Published2026-06-30 01:16:29 UTC
Updated2026-06-30 20:10:25 UTC
DescriptionNLTK 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.

Risk And Classification

Primary CVSS: v3.0 7.5 HIGH from [email protected]

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

EPSS: 0.005100000 probability, percentile 0.397570000 (date 2026-07-04)

Problem Types: CWE-22 | CWE-22 CWE-22 Improper Limitation of a Pathname to a Restricted Directory ('Path Traversal')

VersionSourceTypeScoreSeverityVector
3.0[email protected]Secondary7.5HIGHCVSS:3.0/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N
3.0CNADECLARED7.5HIGHCVSS:3.0/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N

CVSS v3.0 Breakdown

Attack Vector
Network
Attack Complexity
Low
Privileges Required
None
User Interaction
None
Scope
Unchanged
Confidentiality
High
Integrity
None
Availability
None

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

NVD Known Affected Configurations (CPE 2.3)

TypeVendorProductVersionUpdateEditionLanguage
Application Nltk Nltk 3.9.4 All All All

Vendor Declared Affected Products

SourceVendorProductVersionPlatforms
CNA Nltk Nltk/nltk affected unspecified latest custom Not specified

References

ReferenceSourceLinkTags
huntr.com/bounties/39aa9354-54ca-4e77-96da-580eb1fe6ed1 134c704f-9b21-4f2e-91b3-4a467353bcc0 huntr.com Exploit, Mitigation, Third Party Advisory
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