Concrete CMS up to 9.5.2 OAuth 2.0 Refresh Token Grant improper authentication

CVSS Meta Temp Score
CVSS is a standardized scoring system to determine possibilities of attacks. The Temp Score considers temporal factors like disclosure, exploit and countermeasures. The unique Meta Score calculates the average score of different sources to provide a normalized scoring system.
Current Exploit Price (≈)
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CTI Interest Score
Our Cyber Threat Intelligence team is monitoring different web sites, mailing lists, exploit markets and social media networks. The CTI Interest Score identifies the interest of attackers and the security community for this specific vulnerability in real-time. A high score indicates an elevated risk to be targeted for this vulnerability.
6.3$0-$5k0.45-

Summaryinfo

A vulnerability identified as critical has been detected in Concrete CMS up to 9.5.2. This impacts an unknown function of the component OAuth 2.0 Refresh Token Grant. The manipulation leads to improper authentication. This vulnerability is uniquely identified as CVE-2026-81921. The attack is possible to be carried out remotely. No exploit exists.

Detailsinfo

A vulnerability was found in Concrete CMS up to 9.5.2. It has been rated as critical. This issue affects some unknown functionality of the component OAuth 2.0 Refresh Token Grant. The manipulation with an unknown input leads to a improper authentication vulnerability. Using CWE to declare the problem leads to CWE-287. When an actor claims to have a given identity, the product does not prove or insufficiently proves that the claim is correct. Impacted is confidentiality, integrity, and availability. The summary by CVE is:

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.

It is possible to read the advisory at documentation.concretecms.org. The identification of this vulnerability is CVE-2026-81921 since 08/27/2026. The exploitation is known to be easy. The attack may be initiated remotely. The technical details are unknown and an exploit is not publicly available. The pricing for an exploit might be around USD $0-$5k at the moment (estimation calculated on 09/15/2026).

There is no information about possible countermeasures known. It may be suggested to replace the affected object with an alternative product.

Be aware that VulDB is the high quality source for vulnerability data.

Productinfo

Type

Name

Version

License

CPE 2.3info

CPE 2.2info

CVSSv4info

VulDB Vector: 🔒
VulDB Reliability: 🔍

CNA CVSS-B Score: 🔒
CNA CVSS-BT Score: 🔒
CNA Vector: 🔒

CVSSv3info

VulDB Meta Base Score: 6.3
VulDB Meta Temp Score: 6.3

VulDB Base Score: 6.3
VulDB Temp Score: 6.3
VulDB Vector: 🔒
VulDB Reliability: 🔍

CVSSv2info

AVACAuCIA
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VectorComplexityAuthenticationConfidentialityIntegrityAvailability
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VulDB Base Score: 🔒
VulDB Temp Score: 🔒
VulDB Reliability: 🔍

Exploitinginfo

Class: Improper authentication
CWE: CWE-287
CAPEC: 🔒
ATT&CK: 🔒

Physical: No
Local: No
Remote: Yes

Availability: 🔒
Status: Not defined
Price Prediction: 🔍
Current Price Estimation: 🔒

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Threat Intelligenceinfo

Interest: 🔍
Active Actors: 🔍
Active APT Groups: 🔍

Countermeasuresinfo

Recommended: no mitigation known
Status: 🔍

0-Day Time: 🔒

Timelineinfo

08/27/2026 CVE reserved
09/15/2026 +19 days Advisory disclosed
09/15/2026 +0 days VulDB entry created
09/15/2026 +0 days VulDB entry last update

Sourcesinfo

Advisory: documentation.concretecms.org
Status: Not defined

CVE: CVE-2026-81921 (🔒)
GCVE (CVE): GCVE-0-2026-81921
GCVE (VulDB): GCVE-100-404567

Entryinfo

Created: 09/15/2026 21:31
Changes: 09/15/2026 21:31 (64)
Complete: 🔍
Cache ID: 216::103

Be aware that VulDB is the high quality source for vulnerability data.

Discussion

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