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CVE-2026-81900— Concrete CMS before 9.5.3 is vulnerable to Stored XSS in the YouTube block (vWidth/vHeight)

Quick assessment

Affected
Concrete CMS Concrete CMS
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.

Concrete CMS 9.5.3 之前的版本仅对 YouTube 模块中存储的宽度和高度值应用了 操作，并将这些值直接打印到 iframe 的 HTML 属性中，而未进行转义或强制转换为整数，从而导致存储型跨站脚本（Stored XSS）漏洞。 拥有 权限的用户可以注入事件处理器，使得在渲染页面的访客执行脚本；当受害者是管理员时，该脚本将以管理员权限运行。 Concrete CMS 安全团队为该漏洞分配了 CVSS v4.0 分数 7.3，向量值为 。 感谢 sh4d0byss 报告此漏洞。

CVSS 7.3 · High

Possible ATT&CK Techniques 1 AI

T1195 · Supply Chain Compromise
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I. Basic Information for CVE-2026-81900

Vulnerability Information
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Vulnerability Title
Concrete CMS before 9.5.3 is vulnerable to Stored XSS in the YouTube block (vWidth/vHeight)
Source: CVE Program / CVE List V5
Vulnerability Description
Concrete CMS before 9.5.3 applied only trim() to the YouTube block's stored width and height values and printed them into iframe HTML attributes without escaping or integer casting, resulting in stored cross-site scripting. A user with edit_block permission could inject an event handler that executed script for visitors rendering the page, acting with administrative privileges where the victim was an administrator. The Concrete CMS security team gave this vulnerability a CVSS v4.0 score of 7.3 with vector CVSS:4.0/AV:N/AC:L/AT:P/PR:H/UI:P/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N. Thanks sh4d0byss for reporting.
Source: CVE Program / CVE List V5
CVSS Information
CVSS:4.0/AV:N/AC:L/AT:P/PR:H/UI:P/VC:H/VI:H/VA:H/SC:N/SI:N/SA: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
Concrete CMS Concrete CMS 5.0.0 ~ 9.5.2 -

II. Public POCs for CVE-2026-81900

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III. Intelligence Information for CVE-2026-81900

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Vendor Pages for CVE-2026-81900 (1)

Same Patch Batch · Concrete CMS · 2026-09-14 · 7 CVEs total

CVE-2026-18117 7.3 HIGH Concrete CMS 9.0.0 to 9.5.2 is vulnerable to Stored XSS in Custom Page Alias Name
CVE-2026-18116 7.3 HIGH Concrete CMS 8.3.0 to 9.5.2 is vulnerable to Stored XSS in Calendar Event Name via Workflo
CVE-2026-81901 7.2 HIGH Concrete CMS 9.2.0 to 9.5.2 is vulnerable to stored XSS due to missing authorization in th
CVE-2026-81902 7.1 HIGH Concrete CMS 9.0.0 to 9.5.2 is vulnerable to CSRF on Orphan Block Cleanup
CVE-2026-18119 7.0 HIGH Concrete CMS below 9.5.3 is vulnerable to Stored XSS via unsanitized inline block custom s
CVE-2026-81903 7.0 HIGH Concrete CMS 9.0.0 to 9.5.2 is vulnerable to Stored XSS via Page Container icon

IV. Related Vulnerabilities

Same product: Concrete CMS
Same vendor: Concrete CMS
Same weakness: CWE-79

V. Comments for CVE-2026-81900

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