CVE-2026-12259
Received Received - Intake

Arbitrary File Write via Unverified Package Download in NLTK

Publication date: 2026-08-03

Last updated on: 2026-08-03

Assigner: huntr.dev

Description
In nltk version 3.9.4, the `nltk.downloader.Downloader._download_package()` function writes downloaded package bytes to disk and may extract them before enforcing SHA-256 or MD5 checksum validation. This allows an attacker to tamper with the package response body for `info.url` through a compromised mirror, malicious proxy, or other source-substitution condition, leading to the installation of attacker-controlled package bytes. The vulnerability can result in malicious corpus or model content being trusted by downstream users or applications.

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Meta Information

Published
2026-08-03
Last Modified
2026-08-03
Generated
2026-08-03
AI Q&A
2026-08-03
EPSS Evaluated
N/A
NVD
CVE-2026-12259
EUVD
EUVD-2026-52226

Affected Vendors & Products

Showing 1 associated CPE
Vendor Product Version / Range
nltk nltk 3.9.4

Helpful Resources

Exploitability

CWE
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KEV
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CWE ID Description
CWE-494 The product downloads source code or an executable from a remote location and executes the code without sufficiently verifying the origin and integrity of the code.

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Executive Summary

In nltk version 3.9.4, the Downloader._download_package() function downloads package bytes to disk and may extract them before verifying SHA-256 or MD5 checksums. This allows an attacker to replace the package content via a compromised mirror, proxy, or similar method, leading to installation of malicious package bytes.

Detection Guidance

To detect this vulnerability, check if your system uses nltk version 3.9.4 or earlier. Run the command: pip show nltk. If installed, verify the version. Additionally, monitor network traffic for unexpected downloads from NLTK package sources or unusual package modifications.

Impact Analysis

An attacker could trick you into installing malicious corpus or model content through a compromised download source. This could lead to execution of arbitrary code or compromise of applications relying on the downloaded packages.

Compliance Impact

This vulnerability could lead to the installation of malicious corpus or model content, which may result in unauthorized data access or processing. This could violate GDPR's data integrity and security requirements or HIPAA's safeguards for protected health information if exploited.

Mitigation Strategies

Upgrade NLTK to a version where SHA-256 or MD5 checksum validation is enforced before writing package bytes to disk. Avoid using untrusted mirrors or proxies for package downloads.

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