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CVE-2026-78681— NLTK before 3.10.3 Entity Expansion DoS via ElementTree

Quick assessment

Affected
nltk nltk
Exploitation
Public or AI PoC available; prioritize validation
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之前版本存在资源管理错误漏洞，该漏洞源于使用xml.etree.ElementTree解析XML时认可文档DTD中的实体声明，攻击者可构造嵌套实体声明的XML有效载荷，导致内存膨胀并造成拒绝服务。

CVSS 7.5 · High EPSS 0.52% · P42

Possible ATT&CK Techniques 1 AI

T1496 · Resource Hijacking

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-2026-78681

Vulnerability Information
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Vulnerability Title
NLTK before 3.10.3 Entity Expansion DoS via ElementTree
Source: CVE Program / CVE List V5
Vulnerability Description
NLTK versions before 3.10.3 use xml.etree.ElementTree to parse XML in multiple modules, which honors entity declarations in document DTDs. Attackers can craft XML payloads with nested entity declarations that expand from hundreds of bytes to megabytes in memory, causing denial of service.
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
DTD中递归实体索引的不恰当限制(XML实体扩展)
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 3.10.3之前版本存在资源管理错误漏洞，该漏洞源于使用xml.etree.ElementTree解析XML时认可文档DTD中的实体声明，攻击者可构造嵌套实体声明的XML有效载荷，导致内存膨胀并造成拒绝服务。
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-2026-78681

# POC Description Source Link Shenlong Link
AI-Generated POC Verified env Premium
Reproduced successfully in a real sandbox · Below is the actual recording of building the environment and exploiting the vulnerability.
Reproduction recording is a Pro+ exclusive
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claude_code · 6859 chars
GitHub 复现环境（可 clone 直接跑）
Pro+ exclusive includes:
Vulnerability reproduction recording (real sandbox build + trigger, exclusive)
In-depth vulnerability mechanism
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Full executable POC code
Exploit chain & mitigation
POC zip download
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III. Intelligence Information for CVE-2026-78681

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Vendor Advisories for CVE-2026-78681 (2)

Same Patch Batch · nltk · 2026-08-25 · 8 CVEs total

CVE-2026-79657 9.8 CRITICAL NLTK before 3.10.3 Remote Code Execution via Unsafe Pickle Deserialization
CVE-2026-79675 9.8 CRITICAL NLTK before 3.10.3 JVM Argument Injection via Per-Call Options
CVE-2026-78683 9.6 CRITICAL NLTK before 3.10.0 Remote Code Execution via Unsafe Pickle Deserialization
CVE-2026-79674 8.2 HIGH NLTK 3.10.2 Path Traversal via corpus-reader constructors
CVE-2026-78680 7.8 HIGH NLTK before 3.10.3 Arbitrary Code Execution via Graphviz dot Binary
CVE-2026-78682 7.5 HIGH NLTK before 3.10.3 SSRF Protection Bypass via Proxy
CVE-2026-79676 5.9 MEDIUM NLTK before 3.10.3 Path Traversal via Symlink Bypass

IV. Related Vulnerabilities

Same product: nltk
Same vendor: nltk
Same weakness: CWE-776

V. Comments for CVE-2026-78681

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