Bug 2494748 (CVE-2026-12243)

Summary: CVE-2026-12243 nltk: NLTK: Information disclosure via path traversal vulnerability
Product: [Other] Security Response Reporter: OSIDB Bzimport <bzimport>
Component: vulnerabilityAssignee: Product Security DevOps Team <prodsec-dev>
Status: NEW --- QA Contact:
Severity: high Docs Contact:
Priority: high    
Version: unspecifiedCC: anpicker, dschmidt, hasun, jfula, jkoehler, jlanda, jowilson, kshier, lphiri, nyancey, ometelka, ptisnovs, simaishi, stcannon, syedriko, teagle, xdharmai, yguenane
Target Milestone: ---Keywords: Security
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Hardware: All   
OS: Linux   
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A flaw was found in NLTK. An attacker can exploit a path traversal vulnerability by providing specially crafted input to `nltk.data.load()` or `nltk.data.find()`. This allows the attacker to read arbitrary files accessible to the Python process, leading to information disclosure. The vulnerability arises from an incomplete fix that fails to account for percent-encoded traversal sequences.
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oVirt Team: --- RHEL 7.3 requirements from Atomic Host:
Cloudforms Team: --- Target Upstream Version:
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Bug Depends On: 2494932, 2494933    
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Description OSIDB Bzimport 2026-06-30 02:02:00 UTC 
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.