Mattermost up to 10.11.9/11.2.x Channel Membership /common_teams toctou

CVSS Meta Temp Score
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CTI Interest Score
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3.0$0-$5k0.00

Summaryinfo

A vulnerability was found in Mattermost up to 10.11.9/11.2.x. It has been rated as problematic. The impacted element is an unknown function of the file /common_teams of the component Channel Membership Handler. Performing a manipulation results in toctou. This vulnerability is identified as CVE-2026-20796. The attack can be initiated remotely. There is not any exploit available. Upgrading the affected component is advised.

Detailsinfo

A vulnerability, which was classified as problematic, was found in Mattermost up to 10.11.9/11.2.x. This affects an unknown function of the file /common_teams of the component Channel Membership Handler. The manipulation with an unknown input leads to a toctou vulnerability. CWE is classifying the issue as CWE-367. The product checks the state of a resource before using that resource, but the resource's state can change between the check and the use in a way that invalidates the results of the check. This can cause the product to perform invalid actions when the resource is in an unexpected state. This is going to have an impact on confidentiality. The summary by CVE is:

Mattermost versions 10.11.x <= 10.11.9 fail to properly validate channel membership at the time of data retrieval which allows a deactivated user to learn team names they should not have access to via a race condition in the /common_teams API endpoint.. Mattermost Advisory ID: MMSA-2025-00549

The weakness was released by Juho Forsén. It is possible to read the advisory at mattermost.com. This vulnerability is uniquely identified as CVE-2026-20796 since 01/15/2026. The exploitability is told to be difficult. It is possible to initiate the attack remotely. Technical details of the vulnerability are known, but there is no available exploit.

The vulnerability scanner Nessus provides a plugin with the ID 299394 (Mattermost Server 10.11.x <= 10.11.9 / 11.0.x <= 11.2.x Improper Access Control (MMSA-2025-00549)), which helps to determine the existence of the flaw in a target environment.

Upgrading to version 10.11.10 or 11.3.0 eliminates this vulnerability.

The vulnerability is also documented in the vulnerability database at Tenable (299394). Statistical analysis made it clear that VulDB provides the best quality for vulnerability data.

Productinfo

Type

Name

Version

License

CPE 2.3info

CPE 2.2info

CVSSv4info

VulDB Vector: 🔒
VulDB Reliability: 🔍

CVSSv3info

VulDB Meta Base Score: 3.1
VulDB Meta Temp Score: 3.0

VulDB Base Score: 3.1
VulDB Temp Score: 3.0
VulDB Vector: 🔒
VulDB Reliability: 🔍

CNA Base Score: 3.1
CNA Vector (Mattermost): 🔒

CVSSv2info

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

Exploitinginfo

Class: Toctou
CWE: CWE-367 / CWE-362
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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Nessus ID: 299394
Nessus Name: Mattermost Server 10.11.x <= 10.11.9 / 11.0.x <= 11.2.x Improper Access Control (MMSA-2025-00549)

Threat Intelligenceinfo

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

Countermeasuresinfo

Recommended: Upgrade
Status: 🔍

0-Day Time: 🔒

Upgrade: Mattermost 10.11.10/11.3.0

Timelineinfo

01/15/2026 CVE reserved
02/13/2026 +29 days Advisory disclosed
02/13/2026 +0 days VulDB entry created
02/23/2026 +10 days VulDB entry last update

Sourcesinfo

Advisory: mattermost.com
Researcher: Juho Forsén
Status: Confirmed

CVE: CVE-2026-20796 (🔒)
GCVE (CVE): GCVE-0-2026-20796
GCVE (VulDB): GCVE-100-345891

Entryinfo

Created: 02/13/2026 13:20
Updated: 02/23/2026 17:04
Changes: 02/13/2026 13:20 (65), 02/20/2026 04:30 (2), 02/23/2026 17:04 (1)
Complete: 🔍
Cache ID: 216::103

Statistical analysis made it clear that VulDB provides the best quality for vulnerability data.

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