CVE-2026-81921

Received Received - Intake

OAuth 2.0 Token Refresh Bypass in Concrete CMS

Publication date: 2026-09-15

Last updated on: 2026-09-18

Assigner: ConcreteCMS

Description

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. The Concrete CMS security team gave this vulnerability a CVSS v4.0 score of 2.3 with vector CVSS:4.0/AV:N/AC:L/AT:P/PR:L/UI:N/VC:L/VI:L/VA:L/SC:L/SI:L/SA:L. Thanks sh4d0byss for reporting.

CVSS Scores

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Meta Information

Published

2026-09-15

Last Modified

2026-09-18

Generated

2026-10-06

AI Q&A

2026-09-16

EPSS Evaluated

2026-10-04

NVD

CVE-2026-81921

EUVD

EUVD-2026-79287

Affected Vendors & Products

Vendor	Product	Version / Range
concretecms	concrete_cms	From 8.5.3 (inc) to 9.5.3 (exc)

Helpful Resources

Exploitability

CWE

KEV

CWE ID	Description
CWE-862	The product does not perform an authorization check when an actor attempts to access a resource or perform an action.

Attack-Flow Graph

Executive Summary

Concrete CMS versions 8.5.3 through 9.5.2 had a flaw in OAuth 2.0 token handling. When a user was deactivated, their refresh token should have been invalidated, but the system continued issuing new access tokens without verifying if the account was still active. This allowed suspended users to retain API access indefinitely.

Detection Guidance

This vulnerability requires checking OAuth 2.0 refresh token behavior in Concrete CMS. Review access logs for refresh token usage after account deactivation. Check if deactivated accounts can still generate new access tokens via API calls. No specific commands are provided in the context.

Impact Analysis

If you use Concrete CMS with OAuth 2.0 authentication, deactivated or suspended accounts could still access APIs and perform actions. This may lead to unauthorized data access, privilege escalation, or continued use of services despite administrative actions to revoke access.

Compliance Impact

This vulnerability could violate compliance requirements that mandate immediate revocation of access for deactivated accounts, such as GDPR's data protection principles or HIPAA's access control rules. Failure to properly deactivate accounts may result in non-compliance penalties.

Mitigation Strategies

Upgrade Concrete CMS to a version that patches this vulnerability. Check the official Concrete CMS security advisories for the latest updates. Review and revoke any active refresh tokens for deactivated accounts to prevent unauthorized API access.

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