Bug 2510340 (CVE-2026-12259)

Summary: CVE-2026-12259 nltk: nltk: Installation of attacker-controlled packages due to improper checksum validation
Product: [Other] Security Response Reporter: OSIDB Bzimport <bzimport>
Component: vulnerabilityAssignee: Product Security <prodsec-ir-bot>
Status: NEW --- QA Contact:
Severity: medium Docs Contact:
Priority: medium    
Version: unspecifiedCC: anpicker, dschmidt, ebourniv, hasun, jfula, jlanda, jowilson, kshier, lgallett, nyancey, ometelka, ptisnovs, sbunciak, simaishi, stcannon, syedriko, teagle, xdharmai, yguenane
Target Milestone: ---Keywords: Security
Target Release: ---   
Hardware: All   
OS: Linux   
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A flaw was found in nltk. The `nltk.downloader.Downloader._download_package()` function does not enforce SHA-256 or MD5 checksum validation before writing and extracting downloaded package bytes to disk. A remote attacker can exploit this by tampering with the package response body through a compromised mirror, malicious proxy, or other source-substitution condition. This can lead to the installation of attacker-controlled package bytes, resulting in malicious content being trusted by downstream users or applications.
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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: 2511386, 2511387    
Bug Blocks:    

Description OSIDB Bzimport 2026-08-03 09:02:48 UTC 
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.