Mattermost up to 10.11.11/11.2.3/11.3.1/11.4.0/11.4.x IsSameUser incorrect implementation of authentication algorithm

CVSS Meta Temp Score
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CTI Interest Score
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5.2$0-$5k0.09

Summaryinfo

A vulnerability was found in Mattermost up to 10.11.11/11.2.3/11.3.1/11.4.0/11.4.x. It has been declared as problematic. This affects the function IsSameUser. The manipulation results in incorrect implementation of authentication algorithm. This vulnerability is cataloged as CVE-2026-27656. The attack may be launched remotely. There is no exploit available. It is recommended to upgrade the affected component.

Detailsinfo

A vulnerability classified as problematic has been found in Mattermost up to 10.11.11/11.2.3/11.3.1/11.4.0/11.4.x. Affected is the function IsSameUser. The manipulation with an unknown input leads to a incorrect implementation of authentication algorithm vulnerability. CWE is classifying the issue as CWE-303. The requirements for the product dictate the use of an established authentication algorithm, but the implementation of the algorithm is incorrect. This is going to have an impact on confidentiality, integrity, and availability. CVE summarizes:

Mattermost versions 11.4.x <= 11.4.0, 11.3.x <= 11.3.1, 11.2.x <= 11.2.3, 10.11.x <= 10.11.11 fail to properly validate user identity in the OpenID {{IsSameUser()}} comparison logic, which allows an attacker to take over arbitrary user accounts via an overly permissive substring matching flaw in the user discovery flow.. Mattermost Advisory ID: MMSA-2026-00590

The weakness was shared by Christopher Poile. The advisory is available at mattermost.com. This vulnerability is traded as CVE-2026-27656 since 02/23/2026. The exploitability is told to be difficult. It is possible to launch the attack remotely. Additional levels of successful authentication are needed for exploitation. Technical details are known, but there is no available exploit.

Upgrading to version 10.11.12, 11.2.4, 11.3.2, 11.4.1 or 11.5.0 eliminates this vulnerability.

If you want to get best quality of vulnerability data, you may have to visit VulDB.

Productinfo

Type

Name

Version

License

CPE 2.3info

CPE 2.2info

CVSSv4info

VulDB Vector: 🔒
VulDB Reliability: 🔍

CVSSv3info

VulDB Meta Base Score: 5.3
VulDB Meta Temp Score: 5.2

VulDB Base Score: 4.1
VulDB Temp Score: 3.9
VulDB Vector: 🔒
VulDB Reliability: 🔍

NVD Base Score: 6.1
NVD Vector: 🔒

CNA Base Score: 5.7
CNA Vector (Mattermost): 🔒

CVSSv2info

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

Exploitinginfo

Class: Incorrect implementation of authentication algorithm
CWE: CWE-303
CAPEC: 🔒
ATT&CK: 🔒

Physical: No
Local: No
Remote: Yes

Availability: 🔒
Status: Not defined

EPSS Score: 🔒
EPSS Percentile: 🔒

Price Prediction: 🔍
Current Price Estimation: 🔒

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Threat Intelligenceinfo

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

Countermeasuresinfo

Recommended: Upgrade
Status: 🔍

0-Day Time: 🔒

Upgrade: Mattermost 10.11.12/11.2.4/11.3.2/11.4.1/11.5.0

Timelineinfo

02/23/2026 CVE reserved
03/25/2026 +30 days Advisory disclosed
03/25/2026 +0 days VulDB entry created
04/01/2026 +7 days VulDB entry last update

Sourcesinfo

Advisory: mattermost.com
Researcher: Christopher Poile
Status: Confirmed

CVE: CVE-2026-27656 (🔒)
GCVE (CVE): GCVE-0-2026-27656
GCVE (VulDB): GCVE-100-353394

Entryinfo

Created: 03/25/2026 20:01
Updated: 04/01/2026 04:48
Changes: 03/25/2026 20:01 (64), 04/01/2026 04:48 (12)
Complete: 🔍
Cache ID: 216:F22:103

If you want to get best quality of vulnerability data, you may have to visit VulDB.

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