# CVE-2026-18111: Cross-site scripting in Concrete CMS

> Stored XSS in Concrete CMS allows page editors to inject JavaScript, leading to session hijacking and administrative privilege escalation for viewers.

- **Severity:** High 8.5 (`CVSS:4.0/AV:N/AC:L/AT:N/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:** 15 Sep 2026
- **Updated:** 22 Sep 2026
- **Also known as:** GHSA-vgg6-gg65-r39p

## What is CVE-2026-18111?

CVE-2026-18111 is a high-severity cross-site scripting vulnerability in Concretecms Concrete Cms, published on 15 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 9 before 9.5.3 was vulnerable to stored cross-site scripting (XSS) in the Feature, Feature Link, Hero Image, and Image blocks and before Concrete 8.5.21 in the feature and Image blocks because the external link URL was insufficiently validated by the link filter and was rendered without output escaping. A user with page-editing permissions (such as Add Block combined with Edit Contents on a single page) could store a crafted external link value that broke out of the link markup and injected arbitrary JavaScript. The script executed in the browser session of any user who subsequently viewed, previewed, or edited the affected page, which could lead to session hijacking and escalation of privileges up to full administrative takeover. The Concrete CMS security team gave this vulnerability a CVSS v4.0 score of 8.5 with vector CVSS:4.0/AV:N/AC:L/AT:N/PR:H/UI:P/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N. Thanks to KhanMarshai for reporting this issue.

Weakness: CWE-306 (Missing Authentication for Critical Functionality)

## Which versions are affected by CVE-2026-18111?

It affects 1 product.


Products: Concretecms Concrete Cms.

## How do I fix CVE-2026-18111?

No fix has been published for CVE-2026-18111 yet. Limit exposure of the affected software until one is available.

## Is CVE-2026-18111 being exploited?

There is no known exploitation of CVE-2026-18111 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-2026-18111 and fix it if it is affected.

What it is: CVE-2026-18111 is a high-severity cross-site scripting vulnerability in Concretecms Concrete Cms, published on 15 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-2026-18111 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-2026-18111/ (Markdown: https://insights.sbomify.com/CVE-2026-18111/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-18111
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-18111
- OSV: https://osv.dev/vulnerability/CVE-2026-18111

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-18111/
Summaries are generated and may contain mistakes; the official description above is the record.