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CVE-2026-81722— nltk PorterStemmer before 3.10.3 Quadratic-time DoS

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 3.10.2及之前版本存在资源管理错误漏洞，该漏洞源于PorterStemmer.stem()算法复杂度低效，_is_consonant()辅助函数每次调用均向后遍历所有尾部'y'字符，_measure()对每个词干位置调用它，导致O(n^2)行为，单个约20-50KB的不可信令牌可能占用CPU核心数秒至数分钟，造成拒绝服务

CVSS 7.5 · High EPSS 0.52% · P43

Possible ATT&CK Techniques 1 AI

T1499.001 · OS Exhaustion Flood

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-2026-81722

Vulnerability Information
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Vulnerability Title
nltk PorterStemmer before 3.10.3 Quadratic-time DoS
Source: CVE Program / CVE List V5
Vulnerability Description
nltk PorterStemmer in versions <= 3.10.2 (fixed in 3.10.3) contains an inefficient-algorithmic-complexity denial of service in PorterStemmer.stem(). The _is_consonant() helper walks backward over the entire run of trailing 'y' characters on every call, and _measure() invokes it for each stem position, causing O(n^2) behavior. A single ~20-50 KB untrusted token consisting of a long run of the letter 'y' followed by a matching suffix (e.g., 'ness') can pin a CPU core for seconds to minutes, causing availability impact.
Source: CVE Program / CVE List V5
CVSS Information
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
Source: CVE Program / CVE List V5
Vulnerability Type
算法复杂性
Source: CVE Program / CVE List V5
Vulnerability Title
Natural Language Toolkit 资源管理错误漏洞
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 3.10.2及之前版本存在资源管理错误漏洞，该漏洞源于PorterStemmer.stem()算法复杂度低效，_is_consonant()辅助函数每次调用均向后遍历所有尾部'y'字符，_measure()对每个词干位置调用它，导致O(n^2)行为，单个约20-50KB的不可信令牌可能占用CPU核心数秒至数分钟，造成拒绝服务
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-2026-81722

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III. Intelligence Information for CVE-2026-81722

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Vendor Advisories for CVE-2026-81722 (2)

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

CVE-2026-81727 7.1 HIGH NLTK before 3.10.3 Hardlink File Overwrite via downloader
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-407

V. Comments for CVE-2026-81722

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