GHSA-c28q-m4gf-vg4q CVE-2026-6689 GO-2026-6283

Mattermost doesn't enforce PermissionInviteUser when setting AllowOpenInvite or AllowedDomains during team creation

Medium Go

Severity: 4.3 / 10; Medium · CVSS 3.1
Exploit chance (EPSS): 0.15%; Chance of exploitation in the next 30 days. Higher than 4.35% of scored vulnerabilities.
Known exploited: No; Not in CISA's catalog
Weakness: CWE-862 Missing Authorization
Fix: Available; 2 packages have a fixed version

Description

Mattermost versions 11.6.x <= 11.6.1, 11.5.x <= 11.5.4, 10.11.x <= 10.11.15, 10.11.x <= 10.11.16 Fail to enforce PermissionInviteUser when setting AllowOpenInvite or AllowedDomains during team creation (the check was only applied on update/patch), which allows an authenticated user holding PermissionCreateTeam but not PermissionInviteUser on the resulting team to configure invite-controlled team settings (make the team publicly joinable via open invite and/or constrain membership via allowed domains) that they are not permitted to set on an existing team via POST /api/v4/teams with allow_open_invite: true and/or a non-empty allowed_domains in the request body.. Mattermost Advisory ID: MMSA-2026-00655

Affected versions

Go

Package	Vulnerable	Fixed in
github.com/mattermost/mattermost-server	`>= 11.6.0, < 11.6.1`	`11.6.1`
github.com/mattermost/mattermost-server	`>= 11.5.0, < 11.5.5`	`11.5.5`
github.com/mattermost/mattermost-server	`>= 10.11.0, < 10.11.17`	`10.11.17`
github.com/mattermost/mattermost-server	`>= 10.11.0+incompatible, < 10.11.17+incompatible` `>= 11.5.0+incompatible, < 11.5.5+incompatible` `>= 11.6.0+incompatible, < 11.6.1+incompatible`	`10.11.17+incompatible` `11.5.5+incompatible` `11.6.1+incompatible`
github.com/mattermost/mattermost-server/v5	`all versions`	No fix yet
github.com/mattermost/mattermost-server/v6	`all versions`	No fix yet
github.com/mattermost/mattermost/server/v8	`>= 8.0.0-20250731163400-5b955468ea1e, < 8.0.0-20260501144115-7d6816abdfd1`	`8.0.0-20260501144115-7d6816abdfd1`

How to fix

Move to the fixed version or later with your package manager, update the lockfile, and run your tests. If the package only arrives through another dependency, upgrade that dependency first. If none of its releases pulls a fixed version yet, force the version with your package manager's override.

github.com/mattermost/mattermost-server Go → `11.6.1`

go

go get github.com/mattermost/mattermost-server@v11.6.1 && go mod tidy

github.com/mattermost/mattermost/server/v8 Go → `8.0.0-20260501144115-7d6816abdfd1`

go

go get github.com/mattermost/mattermost/server/v8@v8.0.0-20260501144115-7d6816abdfd1 && go mod tidy

Severity breakdown

CVSS 3.1 4.3

CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:L/A:N

Attack vector: Network
Attack complexity: Low
Privileges required: Low
User interaction: None
Scope: Unchanged
Confidentiality: None
Integrity: Low
Availability: None

Weakness

CWE-862: Missing Authorization

The product does not perform an authorization check when an actor attempts to access a resource or perform an action.

How to prevent it

Architecture and Design: Divide the product into anonymous, normal, privileged, and administrative areas. Reduce the attack surface by carefully mapping roles with data and functionality. Use role-based access control (RBAC) [REF-229] to enforce the roles at the appropriate boundaries. Note that this approach may not protect against horizontal authorization, i.e., it will not protect a user from attacking others with the same role.
Architecture and Design: Ensure that access control checks are performed related to the business logic. These checks may be different than the access control checks that are applied to more generic resources such as files, connections, processes, memory, and database records. For example, a database may restrict access for medical records to a specific database user, but each record might only be intended to be accessible to the patient and the patient's doctor [REF-7].
Architecture and Design: Use a vetted library or framework that does not allow this weakness to occur or provides constructs that make this weakness easier to avoid. For example, consider using authorization frameworks such as the JAAS Authorization Framework [REF-233] and the OWASP ESAPI Access Control feature [REF-45].
Architecture and Design: For web applications, make sure that the access control mechanism is enforced correctly at the server side on every page. Users should not be able to access any unauthorized functionality or information by simply requesting direct access to that page. One way to do this is to ensure that all pages containing sensitive information are not cached, and that all such pages restrict access to requests that are accompanied by an active and authenticated session token associated with a user who has the required permissions to access that page.
System Configuration, Installation: Use the access control capabilities of your operating system and server environment and define your access control lists accordingly. Use a "default deny" policy when defining these ACLs.

References

Advisories

Packages

github.com/mattermost/mattermost

Timeline

Jun 12, 2026 Published
Sep 10, 2026 Last updated by the source
Oct 31, 2026 Latest EPSS score

Advisory	Severity	Why it's related
GHSA-xmfh-3ccg-c9fx Mattermost has an Incorrect Authorization issue	Low	Also affects github.com/mattermost/mattermost-server
GHSA-h998-hxxj-8q83 Mattermost doesn't invalidate cached authentication state for active WebSocket connections during global session revocation	Medium	Also affects github.com/mattermost/mattermost-server
GHSA-g5vr-6pgg-74qv Mattermost doesn't enforce bot-specific permission checks on the user active status endpoint	Low	Also affects github.com/mattermost/mattermost-server
GHSA-jqgv-39mg-7c2r Mattermost doesn't authenticate Atlassian Connect installed callbacks, allowing a remote unauthenticated attacker to inject a rogue sharedSecret	Medium	Also affects github.com/mattermost/mattermost-server
GHSA-mxq2-5jpg-7474 Mattermost doesn't validate channel ownership of an existing subscription before applying edits	Medium	Also affects github.com/mattermost/mattermost-server
GHSA-2g8v-grq3-hq2g Mattermost doesn't enforce administrator authorization on the {{setDefaultInstance}} call within the {{/gitlab connect}} command handler	Medium	Also affects github.com/mattermost/mattermost-server
GHSA-6hxm-w4hv-vgvw Mattermost doesn't require role-management authorization when setting the scheme_admin flag on group syncable link and patch endpoints	High	Also affects github.com/mattermost/mattermost-server
GHSA-9p44-r552-4wp9 Mattermost doesn't sanitize the Remote Cluster API response on PATCH operations	Medium	Also affects github.com/mattermost/mattermost-server

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