Attacker-controlled package installation via late checksum validation
Published Aug 3, 2026 · Updated Aug 3, 2026
Integrity-check bypass in NLTK 3.9.4 allows man-in-the-middle attackers to install attacker-controlled data packages. The Downloader._download_package() method writes the response from info.url to disk and can extract ZIP contents without comparing SHA-256 or MD5 values to trusted metadata. Exploitation requires a compromised mirror, malicious proxy, or equivalent source substitution; installed corpus or model content can then be trusted by downstream code.
Summary
What happened
Integrity-check bypass in NLTK 3.9.4 allows man-in-the-middle attackers to install attacker-controlled data packages. The Downloader._download_package() method writes the response from info.url to disk and can extract ZIP contents without comparing SHA-256 or MD5 values to trusted metadata. Exploitation requires a compromised mirror, malicious proxy, or equivalent source substitution; installed corpus or model content can then be trusted by downstream code.
The record
- CVE
- CVE-2026-12259
- Published
- Aug 3, 2026
- Updated
- Aug 3, 2026
- Vendor
- NLTK Project
- Product
- NLTK
- Classifications
- CWE-494, T1557
- Attack vector
- network
- Privileges
- unauthenticated
Timeline
How it unfolded
- Aug 3, 2026CVE publishedPublication date reported by the CVE source.
- Aug 3, 2026Record updatedLatest update available in the CVE record.
Exploitability
Present is not the same as exploitable
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Is a vulnerable build present?
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- Affected versionversion=unspecified <=latest
What conditions does exploitation require?
What is affected?
Published CVSS scores
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Attacks
What attackers are doing with it
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Weakness, pattern, technique
Public exploit references
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Technologies
Your stack
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Your stack
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