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CVE-2026-86431— commonmark before 2.9.1 XSS via AttributesExtension form feed bypass

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.

的 版本中，版本号在 2.7.0 及以上且低于 2.9.1 的版本存在一个跨站脚本（XSS）漏洞，该漏洞位于 中。 在属性名称前添加单个 U+000C 换页符（Form Feed）字节（例如： 形式），可以绕过 中对 事件处理器的过滤机制。原因是 PHP 的 函数默认不会去除 U+000C 字符，导致该属性被原样写入输出 HTML，浏览器会将其解析为有效的事件处理程序。 此外，这种前缀还能绕过 检查，即使设置了 为 ，也能使 或 属性中的 URI 通过过滤。 利用条件：需要处理不可信的 Markdown 内容，且必

CVSS 7.2 · High

Possible ATT&CK Techniques 1 AI

T1190 · Exploit Public-Facing Application

Affected Version Matrix 2

VendorProduct Version RangeStatus
thephpleague commonmark 2.7.0< 2.9.1 affected
2.9.1 unaffected
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I. Basic Information for CVE-2026-86431

Vulnerability Information
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Vulnerability Title
commonmark before 2.9.1 XSS via AttributesExtension form feed bypass
Source: CVE Program / CVE List V5
Vulnerability Description
league/commonmark (thephpleague/commonmark) versions >= 2.7.0 and < 2.9.1 contain a cross-site scripting vulnerability in the AttributesExtension. Prefixing an attribute name with a single U+000C form feed byte (e.g. {\x0Conclick="alert(1)"}) bypasses the AttributesHelper::filterAttributes() 'on*' event-handler filter because PHP's trim() does not strip U+000C, causing the attribute to be written verbatim into the output where browsers parse it as a genuine event handler. The same prefix also defeats the allow_unsafe_links check, allowing javascript: URIs through href/src attributes even when allow_unsafe_links is false. Exploitation requires processing untrusted Markdown with the AttributesExtension enabled; the injected script executes when the rendered HTML is viewed. Fixed in 2.9.1.
Source: CVE Program / CVE List V5
CVSS Information
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:L/I:L/A:N
Source: CVE Program / CVE List V5
Vulnerability Type
在Web页面生成时对输入的转义处理不恰当(跨站脚本)
Source: CVE Program / CVE List V5

Affected Products

Vendor Product Affected Versions CPE Subscribe
thephpleague commonmark 2.7.0 ~ 2.9.1 -

II. Public POCs for CVE-2026-86431

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III. Intelligence Information for CVE-2026-86431

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Patches & Fixes for CVE-2026-86431 (1)

Vendor Advisories for CVE-2026-86431 (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-2026-86434 7.5 HIGH commonmark 2.0.0 through 2.8.3 Denial of Service via Slug Collision
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-2026-86432 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-79

V. Comments for CVE-2026-86431

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