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CVE-2026-81727— NLTK before 3.10.3 Hardlink File Overwrite via downloader

Quick assessment

Affected
nltk nltk
Exploitation
No confirmed in-the-wild exploitation; assess based on exposure
Recommended action
Check the vendor advisory and references for a fixed version. If immediate upgrade is impossible, restrict exposure and increase monitoring.

在 NLTK 3.10.3 之前的版本中， 和 方法中存在一个文件系统包含（filesystem containment）绕过漏洞，允许攻击者通过预先存在的硬链接（hardlinks）覆盖安装根目录之外的文件。拥有共享下载器目录写权限的攻击者可以创建指向根目录外文件的硬链接，从而在正常的包解压过程中覆盖这些文件，导致预期安装目录树之外的文件被修改。

CVSS 7.1 · High

Possible ATT&CK Techniques 1 AI

T1195 · Supply Chain Compromise

Affected Version Matrix 2

VendorProduct Version RangeStatus
nltk nltk < 3.10.3 affected
3.10.3 unaffected
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I. Basic Information for CVE-2026-81727

Vulnerability Information
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Vulnerability Title
NLTK before 3.10.3 Hardlink File Overwrite via downloader
Source: CVE Program / CVE List V5
Vulnerability Description
NLTK versions before 3.10.3 contain a filesystem containment bypass vulnerability in the Downloader.download and Downloader.incr_download methods that allows attackers to overwrite files outside the install root through pre-existing hardlinks. Attackers with write access to a shared downloader directory can create hardlinks pointing to outside-root files that are then overwritten during normal package extraction, mutating files outside the intended install tree.
Source: CVE Program / CVE List V5
CVSS Information
CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:H/A:H
Source: CVE Program / CVE List V5
Vulnerability Type
在文件访问前对链接解析不恰当(链接跟随)
Source: CVE Program / CVE List V5

Affected Products

Vendor Product Affected Versions CPE Subscribe
nltk nltk 0 ~ 3.10.3 -

II. Public POCs for CVE-2026-81727

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III. Intelligence Information for CVE-2026-81727

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Vendor Advisories for CVE-2026-81727 (2)

Same Patch Batch · nltk · 2026-08-27 · 6 CVEs total

CVE-2026-81722 7.5 HIGH nltk PorterStemmer before 3.10.3 Quadratic-time DoS
CVE-2026-81726 7.0 HIGH NLTK through 3.10.3 Path Traversal via Model-Artifact APIs
CVE-2026-81724 5.3 MEDIUM NLTK before 3.10.3 Denial of Service via Uncontrolled Recursion
CVE-2026-81723 3.7 LOW NLTK before 3.10.3 Quadratic CPU Exhaustion via XMLCorpusView
CVE-2026-81725 3.7 LOW NLTK before 3.10.3 Regular Expression Denial of Service via Pl196xCorpusReader

IV. Related Vulnerabilities

Same product: nltk
Same vendor: nltk
Same weakness: CWE-59

V. Comments for CVE-2026-81727

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