Staging Environment: Content and features may be unstable or change without notice.
Advisory details: pypa / nltk/PYSEC-2026-4318
Advisory ID pypa/nltk/PYSEC-2026-4318
Aliases CVE-2026-12259
PYSEC-2026-4318
Summary 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.
Status Published
Exploitability 0.5
Weighted Severity 5.9
Risk 3.0
Affected and Fixed Packages Package Details
Introduced and Fixed Package Commit Patches Package Commit Patches Details
Weaknesses (0)
There are no known CWE.
System Score Found at
cvssv3.1 6.5 https://github.com/pypa/advisory-database/blob/main/vulns/nltk/PYSEC-2026-4318.yaml
Reference id Reference type URL
659ccf6d-12d4-4d4a-84c0-078633c35a5d Other https://huntr.com/bounties/659ccf6d-12d4-4d4a-84c0-078633c35a5d
No exploits are available.

No EPSS data available for this advisory.

Patch URL
There are no known patches.
Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:H/A:N Found at https://github.com/pypa/advisory-database/blob/main/vulns/nltk/PYSEC-2026-4318.yaml
Attack Vector (AV) Attack Complexity (AC) Privileges Required (PR) User Interaction (UI) Scope (S) Confidentiality Impact (C) Integrity Impact (I) Availability Impact (A)

network

adjacent_network

local

physical

low

high

none

low

high

none

required

unchanged

changed

high

low

none

high

low

none

high

low

none

There are no SSVC decisions available.