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

> Stored XSS in Concrete CMS YouTube block allows attackers to execute scripts in visitors' browsers, potentially leading to account takeover.

- **Severity:** High 7.3 (`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`)
- **Exploited in the wild:** Not known
- **Login needed:** An administrator
- **Attackable:** Over the internet
- **Victim action needed:** A user has to click or open something
- **Fix available:** No
- **Published:** 14 Sep 2026
- **Updated:** 15 Sep 2026
- **Also known as:** GHSA-c87m-38hc-rc4g

## What is CVE-2026-81900?

CVE-2026-81900 is a high-severity cross-site scripting vulnerability in Concretecms Concrete Cms, published on 14 Sep 2026. An attacker needs administrator access to exploit it, and only if a user opens a file or follows a link.

### Official 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.

Weakness: CWE-79 (Improper Neutralization of Input During Web Page Generation)

## Which versions are affected by CVE-2026-81900?

It affects 1 product.


Products: Concretecms Concrete Cms.

## How do I fix CVE-2026-81900?

No fix has been published for CVE-2026-81900 yet. Limit exposure of the affected software until one is available.

## Is CVE-2026-81900 being exploited?

There is no known exploitation of CVE-2026-81900 in the wild and no public exploit code.

## Other vulnerabilities in Concretecms Concrete Cms

- [CVE-2023-48648](https://insights.sbomify.com/CVE-2023-48648/): Concrete CMS allows unauthorized access because directories can be created with insecure permissions (critical 9.8)
- [CVE-2022-21829](https://insights.sbomify.com/CVE-2022-21829/): Code injection in concrete CMS (critical 9.8)
- [CVE-2021-22958](https://insights.sbomify.com/CVE-2021-22958/): Server-Side Request Forgery vulnerability in concrete5 (critical 9.8)
- [CVE-2021-40098](https://insights.sbomify.com/CVE-2021-40098/): Path traversal in n/a (critical 9.8)
- [CVE-2026-8134](https://insights.sbomify.com/CVE-2026-8134/): Concrete CMS Vulnerable to Relative Path Traversal (critical 9.4)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-81900 and fix it if it is affected.

What it is: CVE-2026-81900 is a high-severity cross-site scripting vulnerability in Concretecms Concrete Cms, published on 14 Sep 2026. An attacker needs administrator access to exploit it, and only if a user opens a file or follows a link.
Exploitation: There is no known exploitation of CVE-2026-81900 in the wild and no public exploit code.

Affected product: Concretecms Concrete Cms. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-81900/ (Markdown: https://insights.sbomify.com/CVE-2026-81900/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-81900
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-81900
- OSV: https://osv.dev/vulnerability/CVE-2026-81900

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-81900/
Summaries are generated and may contain mistakes; the official description above is the record.