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CVE-2026-86434— commonmark 2.0.0 through 2.8.3 Denial of Service via Slug Collision

Quick assessment

Affected
thephpleague commonmark
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.

league/commonmark 版本 >= 2.0.0 且 < 2.8.4（已在 2.9.0 中修复）存在一个服务拒绝（DoS）漏洞，位于 函数中。当多个标题的 slug 发生冲突时，该函数每次都会将数字后缀的搜索从 1 重新开始，导致对于折叠到同一基础 slug 的 K 个标题，时间复杂度达到 O(K²)。该漏洞路径在注册了 、 或 时被触发。未经身份验证的攻击者可以在一个小型 Markdown 文档中，通过构造许多标题折叠到同一个基础 slug（例如使用空的 ATX 标题、相同标题文本或仅包含标点的标题），

CVSS 7.5 · High

Affected Version Matrix 2

VendorProduct Version RangeStatus
thephpleague commonmark 2.0.0< 2.8.4 affected
2.8.4 unaffected
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I. Basic Information for CVE-2026-86434

Vulnerability Information
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Vulnerability Title
commonmark 2.0.0 through 2.8.3 Denial of Service via Slug Collision
Source: CVE Program / CVE List V5
Vulnerability Description
league/commonmark versions >= 2.0.0 and < 2.8.4 (patched in 2.9.0) contain a denial of service vulnerability in UniqueSlugNormalizer::normalize(), which restarts its numeric-suffix search from 1 on every slug collision, resulting in O(K^2) time complexity for K headings that collapse to the same base slug. The vulnerable path is reached when HeadingPermalinkExtension, FootnoteExtension, or TableOfContentsExtension is registered. An unauthenticated attacker can force many headings onto a single base slug (e.g., via empty ATX headings, identical heading text, or punctuation-only headings) in a small Markdown document, consuming excessive CPU and denying 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
算法复杂性
Source: CVE Program / CVE List V5

Affected Products

Vendor Product Affected Versions CPE Subscribe
thephpleague commonmark 2.0.0 ~ 2.8.4 -

II. Public POCs for CVE-2026-86434

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III. Intelligence Information for CVE-2026-86434

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Vendor Advisories for CVE-2026-86434 (2)

Same Patch Batch · thephpleague · 2026-09-07 · 8 CVEs total

CVE-2026-86430 7.5 HIGH league/commonmark before 2.9.1 Denial of Service via parsing
CVE-2026-86429 7.5 HIGH commonmark before 2.9.1 Denial of Service via SmartPunct and Attributes
CVE-2026-86435 7.5 HIGH commonmark 1.5.0 before 2.8.4 Denial of Service via Footnote
CVE-2026-86433 7.5 HIGH commonmark 1.5.0 before 2.8.4 Denial of Service via Attributes
CVE-2026-86428 7.5 HIGH commonmark 1.5.0 before 2.10.0 Denial of Service via Attributes
CVE-2026-86431 7.2 HIGH commonmark before 2.9.1 XSS via AttributesExtension form feed bypass
CVE-2026-86432 5.3 MEDIUM commonmark 2.0.0 before 2.8.4 Denial of Service via XML

IV. Related Vulnerabilities

Same product: commonmark
Same vendor: thephpleague
Same weakness: CWE-407

V. Comments for CVE-2026-86434

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