CVE-2026-81921: Concrete CMS 8.5.3 through 9.5.2 enabled the OAuth 2.0 refresh-token grant using the unmodified upstream Leagu

Routine

MEDIUM 5.4UPGRADE

CVE-2026-81921 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 8.5.3 through 9.5.2 enabled the OAuth 2.0 refresh-token grant using the unmodified upstream League grant, which issued new access tokens from a valid refresh token without re-checking the associated account's active status. A user who obtained a refresh token while active could therefore continue to mint valid access tokens after being deactivated or suspended (uIsActive=0), so deactivating an account did not revoke its API access. T

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	None
EPSS exploit chance	0.2% (top 85%)
CISA exploited list	No
Weakness	CWE-862
Published	2026-09-15
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

2026-09-15 19:17 UTCPublished CVE record published
2026-09-18 15:29 UTCUpdated Record last modified

concretecms concrete cms UPGRADE

Affected: >= 8.5.3 and < 9.5.3

Fixed in: 9.5.3

Safe to downgrade to: a release before 8.5.3

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 8.5.3 (Debian / Ubuntu)

apt-cache madison concrete-cms sudo apt install concrete-cms=<VERSION_BEFORE_8.5.3> 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_8.5.3> 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-2026-81921?

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

Is CVE-2026-81921 being exploited?

CVE-2026-81921 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-2026-81921 affect?

CVE-2026-81921 affects concrete cms. Severity is medium (CVSS 5.4). Concrete CMS 8.5.3 through 9.5.2 enabled the OAuth 2.0 refresh-token grant using the unmodified upstream League grant, which issued new access tokens from a valid refresh token without re-checking the associated account's active status. A u

How urgent is CVE-2026-81921?

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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