# CVE-2026-81900 — Concrete CMS

> As of 2026-09-15, CVE-2026-81900 is a High-severity (band derived from the CVSS v4.0 score, not stated by the CNA) vulnerability in Concrete CMS, CVSS v4.0 7.3, EPSS 0.2% (16.5th percentile). No Threadlinqs-tracked threat campaign has been attributed to CVE-2026-81900 as of 2026-09-15; the identifier is re-checked against the Threadlinqs threat corpus on every daily ingest.

**Last updated:** 2026-09-15

## What is CVE-2026-81900?

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.

The record classifies CVE-2026-81900 under weakness class [CWE-79](https://intel.threadlinqs.com/cwe/CWE-79). 1 affected-product entry is recorded, across 1 vendor, listed below. The identifier was first published 21 days ago.

## Severity and exploitation probability

- **CVSS v4.0 base score:** 7.3 — High (derived from score) (`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`)
- **EPSS (FIRST):** 0.2% probability of exploitation in the next 30 days, higher than 16.5% of all scored CVEs
- **CISA KEV:** Not listed in the CISA Known Exploited Vulnerabilities catalog
- **Threadlinqs priority:** 4.3/10 — a Threadlinqs composite of the CVSS base score, the EPSS percentile and public exploit availability
- **Published:** 2026-09-14, last modified 2026-09-15

## Is CVE-2026-81900 being exploited?

It currently carries a trending score of 34 in the Threadlinqs vulnerability feed.

## Affected products and versions

- **Concrete CMS**: Concrete CMS

## How to fix CVE-2026-81900

No vendor patch reference has been recorded for CVE-2026-81900 in the tracked sources. Follow the references below for a fix, and treat the products listed above as exposed until the vendor states otherwise.

## Threat activity tracking CVE-2026-81900

No threat campaign in the Threadlinqs corpus currently references CVE-2026-81900, in its CVE list or as an indicator. The linkage is recomputed from the live corpus on every daily ingest, so this section fills in as soon as a tracked campaign cites the identifier.

## Sources

Enriched from CVE.org, NVD, FIRST EPSS. Last verified by Threadlinqs on 2026-09-15. This product uses the NVD API but is not endorsed or certified by the NVD.

**Vendor advisory and patch**

- [documentation.concretecms.org — release notes](https://documentation.concretecms.org/developers/introduction/version-history/953-release-notes)

Canonical: https://intel.threadlinqs.com/cve/CVE-2026-81900
Full detection coverage and IOCs for threats exploiting CVE-2026-81900 via the Threadlinqs MCP server (Purple tier): https://intel.threadlinqs.com/mcp