CVE-2026-81911: Concrete CMS versions 9.0.0 to 9.5.2 is vulnerable to Stored XSS in Board Custom Slot dialog

Routine

MEDIUM 5.4UPGRADE

CVE-2026-81911 is a medium-severity flaw (CVSS 5.4) affecting concrete cms. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

Concrete CMS versions 9.0.0 to 9.5.2 is vulnerable to Stored XSS in Board Custom Slot dialog. The custom_slot save_template endpoint authorizes the request only against the target board instance (canEditBoardContents()) and then persists the client-supplied selectedTemplateOption[collection] verbatim, rather than rebuilding the content object collection server-side and verifying that each item belongs to the authorized board's data pool. A user w

Why this priority

No elevated risk signals yet

A fixed release or distro patch exists.

Risk facts

CVSS score	5.4 (Medium)
Attack vector	Network (remote)
Attack complexity	Low
Privileges needed	Low
User interaction	Required
EPSS exploit chance	0.2% (top 85%)
CISA exploited list	No
Weakness	CWE-79
Published	2026-09-11
Last modified	2026-09-18

Publisher (CNA): ConcreteCMS · Vendor: concretecms

Last updated 2026-09-18 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2026-09-11 20:19 UTCPublished CVE record published
2026-09-18 19:00 UTCUpdated Record last modified

concretecms concrete cms UPGRADE

Affected: >= 9.0.0 and < 9.5.3

Fixed in: 9.5.3

Safe to downgrade to: a release before 9.0.0

1. Check installed version

apt-cache policy concrete-cms || rpm -q concrete-cms || concrete-cms --version

2. UPGRADE to 9.5.3 (Debian / Ubuntu)

sudo apt update && sudo apt install --only-upgrade concrete-cms

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade concrete-cms

Note

Distros often backport fixes without changing the version number. Confirm with the distro advisory or changelog.

3. If no update is available yet, DOWNGRADE to a release before 9.0.0 (Debian / Ubuntu)

apt-cache madison concrete-cms sudo apt install concrete-cms=<VERSION_BEFORE_9.0.0> sudo apt-mark hold concrete-cms

3. If no update is available yet, DOWNGRADE (RHEL / Amazon Linux / Fedora)

dnf --showduplicates list concrete-cms sudo dnf downgrade concrete-cms-<VERSION_BEFORE_9.0.0> sudo dnf versionlock add concrete-cms

Downgrade risk

Downgrading can break dependent software and may reopen older bugs. Read the distro advisory first and test on a non-production host.

After downgrade

Restart the service, and keep the package held until a fixed release ships.

Verify at the official source NVD · CVE Program · Report a problem with this page
Scores and fixes here are computed automatically from those and other public sources. Confirm with the vendor advisory before changing production systems.

Quick answers

How do I fix CVE-2026-81911?

concrete cms: upgrade to 9.5.3. Exact commands for each OS are on this page.

Is CVE-2026-81911 being exploited?

CVE-2026-81911 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

What does CVE-2026-81911 affect?

CVE-2026-81911 affects concrete cms. Severity is medium (CVSS 5.4). Concrete CMS versions 9.0.0 to 9.5.2 is vulnerable to Stored XSS in Board Custom Slot dialog. The custom_slot save_template endpoint authorizes the request only against the target board instance (canEditBoardContents()) and then persists th

How urgent is CVE-2026-81911?

Priority 23/100. No elevated risk signals yet.

Related CVEs

CVE-2026-18111 MEDIUM 6.1 UPGRADE Concrete CMS 9 before 9.5.3 was vulnerable to stored cross-site scripting (XSS) in the Feature, Feature Link,
CVE-2026-18117 MEDIUM 5.4 UPGRADE Concrete CMS 9.0.0 through 9.5.3 is vulnerable to stored XSS via the custom page alias name (customAliasName)
CVE-2026-68526 MEDIUM 4.3 UPGRADE Concrete CMS before 9.5.3 did not validate an anti-CSRF token in the Calendar event duplicate dialog controlle
CVE-2026-81912 MEDIUM 5.7 UPGRADE Concrete CMS before 9.5.3 is vulnerable to Cross-Site Request Forgery in the Move Multiple Groups feature
CVE-2026-81913 MEDIUM 6.1 UPGRADE Concrete CMS versions 9.5.0 through 9.5.2 are vulnerable to Open Redirect via the rcURL parameter

References

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