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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.

Natural Language Toolkit是Natural Language Toolkit组织开源的一个自然语言处理工具包。 Natural Language Toolkit 3.10.3之前版本存在后置链接漏洞，该漏洞源于Downloader.download和Downloader.incr_download方法存在文件系统包含绕过问题，攻击者可借助预先存在的硬链接覆盖安装根目录之外的文件。

CVSS 7.1 · High EPSS 0.19% · P8

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
Vulnerability Title
Natural Language Toolkit 后置链接漏洞
Source: CNNVD (China National Vulnerability Database)
Vulnerability Description
Natural Language Toolkit是Natural Language Toolkit组织开源的一个自然语言处理工具包。 Natural Language Toolkit 3.10.3之前版本存在后置链接漏洞，该漏洞源于Downloader.download和Downloader.incr_download方法存在文件系统包含绕过问题，攻击者可借助预先存在的硬链接覆盖安装根目录之外的文件。
Source: CNNVD (China National Vulnerability Database)
CVSS Information
N/A
Source: CNNVD (China National Vulnerability Database)
Vulnerability Type
N/A
Source: CNNVD (China National Vulnerability Database)

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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