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CVE-2026-9820

Mattermost schemes teams endpoint exposes private team invite IDs

Mattermost versions 11.7.x <= 11.7.2, 10.11.x <= 10.11.19 fail to sanitize team objects returned by the scheme teams endpoint, which allows a user with the User Manager role to obtain invite links for private teams and use them to join or share access to those teams via the scheme teams API endpoint.. Mattermost Advisory ID: MMSA-2026-00671

CVSS 3.1 · 3.8 Low EPSS < 1% KEV · Not listed
Vendors
Mattermost
Products
Mattermost Mattermost Server
CVE-2026-6541

Unscoped updates to other playbooks' metric configuration

Mattermost versions 11.7.x <= 11.7.1, 11.6.x <= 11.6.4, 10.11.x <= 10.11.19 fail to restrict metric configuration changes to the playbook being saved, which allows an authenticated user with team access to alter another user’s playbook metric settings via a crafted import or update request with a foreign metric ID. Mattermost Advisory ID: MMSA-2026-00653

CVSS 3.1 · 4.3 Medium EPSS < 1% KEV · Not listed
Vendors
Mattermost
Products
Mattermost Mattermost Server
CVE-2026-10085

Ordinary group/direct message member can enable group_constrained and remove all channel participants

Mattermost versions 11.7.x <= 11.7.2, 11.6.x <= 11.6.4, 10.11.x <= 10.11.19 fail to restrict the group_constrained channel flag to public and private channels that support group synchronization, which allows an ordinary group or direct message member to remove all participants from the conversation via the channel patch API.. Mattermost Advisory ID: MMSA-2026-00688

CVSS 3.1 · 5.4 Medium EPSS < 1% KEV · Not listed
Vendors
Mattermost
Products
Mattermost Mattermost Server
CVE-2026-10106

Unauthorized users can trigger interactive post actions in private channels via action cookie channel mismatch in Mattermost

Mattermost versions 11.7.x <= 11.7.2, 11.6.x <= 11.6.4, 10.11.x <= 10.11.19 fail to verify that the channel referenced in an action cookie matches the channel of the target post, which allows an authenticated user without access to a private channel to trigger interactive post actions on posts in that channel via a cookie obtained from any accessible channel.. Mattermost Advisory ID: MMSA-2026-00690

CVSS 3.1 · 6.5 Medium EPSS < 1% KEV · Not listed
Vendors
Mattermost
Products
Mattermost Mattermost Server
CVE-2026-6850

Crafted message attachment causes client-side denial of service via markdown parser regex backtracking in Mattermost

Mattermost versions 11.7.x <= 11.7.2, 11.6.x <= 11.6.4, 10.11.x <= 10.11.19 fail to validate the length and content of message attachment field values, which allows an authenticated attacker to cause a denial of service for all users in a channel via a post containing a specially crafted payload that triggers catastrophic backtracking in the client-side markdown parser.. Mattermost Advisory ID: MMSA-2026-00658

CVSS 3.1 · 6.5 Medium EPSS < 1% KEV · Not listed
Vendors
Mattermost
Products
Mattermost Mattermost Server
CVE-2026-9597

Deactivated guest accounts can authenticate via magic-link token in Mattermost REST API login endpoint

Mattermost versions 11.7.x <= 11.7.2, 11.6.x <= 11.6.4 fail to verify whether a guest account is deactivated before creating a session in the magic-link token login path, which allows a deactivated guest user to obtain a fully functional session via a magic-link token issued prior to deactivation.. Mattermost Advisory ID: MMSA-2026-00681

CVSS 3.1 · 5.4 Medium EPSS < 1% KEV · Not listed
Vendors
Mattermost
Products
Mattermost Mattermost Server
CVE-2026-13426

Client4 fails to validate path parameters

The Mattermost Go module github.com/mattermost/mattermost/server/public versions < v0.1.22 fail to validate path parameters when constructing API route paths which allows an attacker to redirect API calls to unintended endpoints via crafted IDs containing path traversal components. Mattermost Advisory ID: MMSA-2025-00532

CVSS 3.1 · 5.4 Medium EPSS < 1% KEV · Not listed
Vendors
Mattermost
Products
Mattermost Server
CVE-2026-3472

Markdown image rendering bypass in AI bot tool result posts in Mattermost

Mattermost versions 10.11.x <= 10.11.18, 11.6.x <= 11.6.3, 11.5.x <= 11.5.6 fail to properly apply markdown image rendering restrictions to AI bot tool result posts, which allows an authenticated attacker to exfiltrate data to an attacker-controlled server via injecting markdown image syntax into tool result content rendered by a victim's client.. Mattermost Advisory ID: MMSA-2026-00619

CVSS 3.1 · 3.5 Low EPSS < 1% KEV · Not listed
Vendors
Mattermost
Products
Mattermost Mattermost Server
CVE-2026-4339

SSRF via unvalidated attachment URLs in Mattermost Agents plugin MCP server

Mattermost versions 10.11.x <= 10.11.18, 11.6.x <= 11.6.3, 11.5.x <= 11.5.6 fail to validate attachment URLs against internal or private IP ranges in the Mattermost Agents plugin MCP server which allows an attacker with access to the MCP server in stdio mode to perform server-side request forgery (SSRF) and exfiltrate data from internal network services via supplying internal URLs as file attachments in post creation requests.. Mattermost Advisory ID: MMSA-2026-00635

CVSS 3.1 · 6.5 Medium EPSS < 1% KEV · Not listed
Vendors
Mattermost
Products
Mattermost Mattermost Server
CVE-2026-8823

User Manager can demote bot accounts to guest without bot-management permission

Mattermost versions 11.7.x <= 11.7.0, 10.11.x <= 10.11.17 fail to validate bot targets when demoting users to guests which allows a lower-privileged administrator to degrade arbitrary bot accounts via the standard demote-user API.. Mattermost Advisory ID: MMSA-2026-00669

CVSS 3.1 · 3.8 Low EPSS < 1% KEV · Not listed
Vendors
Mattermost
Products
Mattermost Mattermost Server
CVE-2026-8074

Improper Permission Check Allows User Manager to Deactivate Bot Accounts

Mattermost versions 11.7.x <= 11.7.0, 10.11.x <= 10.11.17 fail to enforce bot-specific permission checks on the user active status endpoint, which allows a User Manager with user management write access but no Integrations access to deactivate bot accounts via the PUT /api/v4/users/{id}/active API endpoint.. Mattermost Advisory ID: MMSA-2026-00667

CVSS 3.1 · 3.8 Low EPSS < 1% KEV · Not listed
Vendors
Mattermost
Products
Mattermost Mattermost Server
CVE-2026-9162

Global session revocation does not invalidate active WebSocket connections

Mattermost versions 11.7.x <= 11.7.0, 11.6.x <= 11.6.2, 11.5.x <= 11.5.5, 10.11.x <= 10.11.17 fail to invalidate cached authentication state for active WebSocket connections during global session revocation, which allows a user with an existing WebSocket connection to remain authenticated and continue receiving real-time events until the cached session expires or the client reconnects.. Mattermost Advisory ID: MMSA-2026-00664

CVSS 3.1 · 4.3 Medium EPSS < 1% KEV · Not listed
Vendors
Mattermost
Products
Mattermost Mattermost Server
CVE-2026-5139

GitLab Plugin Allows Non-Admin Users to Modify Default Instance Configuration

Mattermost versions 11.7.x <= 11.7.0, 11.6.x <= 11.6.2, 11.5.x <= 11.5.5, 10.11.x <= 10.11.17 fail to enforce administrator authorization on the {{setDefaultInstance}} call within the {{/gitlab connect}} command handler, which allows any authenticated user to overwrite the global default GitLab instance configuration via the {{/gitlab connect <instance-name>}} slash command.. Mattermost Advisory ID: MMSA-2026-00644

CVSS 3.1 · 5.4 Medium EPSS < 1% KEV · Not listed
Vendors
Mattermost
Products
Mattermost Mattermost Server
CVE-2026-6673

Mattermost Jira plugin had unauthenticated {{/ac/installed}} lifecycle callback during pending Jira Cloud install

Mattermost versions 11.7.x <= 11.7.0, 11.6.x <= 11.6.2, 11.5.x <= 11.5.5, 10.11.x <= 10.11.17 fail to authenticate Atlassian Connect installed callbacks, allowing a remote unauthenticated attacker to inject a rogue sharedSecret and disrupt the Jira integration via POST to /ac/installed during the pending-install window.. Mattermost Advisory ID: MMSA-2026-00654

CVSS 3.1 · 6.4 Medium EPSS < 1% KEV · Not listed
Vendors
Mattermost
Products
Mattermost Mattermost Server
CVE-2026-6062

IDOR in Jira plugin subscription edit endpoint

Mattermost versions 11.7.x <= 11.7.0, 11.6.x <= 11.6.2, 11.5.x <= 11.5.5, 10.11.x <= 10.11.17 Fail to validate channel ownership of an existing subscription before applying edits which allows an authenticated attacker to hijack subscriptions from channels they have no access to via a crafted PUT request to the subscription edit endpoint.. Mattermost Advisory ID: MMSA-2026-00650

CVSS 3.1 · 6.4 Medium EPSS < 1% KEV · Not listed
Vendors
Mattermost
Products
Mattermost Mattermost Server
CVE-2026-4053

post edit time limit is not enforced on some post update operations

Mattermost versions 11.5.x <= 11.5.1, 10.11.x <= 10.11.13 fail to enforce the PostEditTimeLimit on non-message post fields which allows an authenticated user to modify post file attachments, props, and pin status after the edit window has expired via the post patch and update API endpoints.. Mattermost Advisory ID: MMSA-2026-00631

CVSS 3.1 · 3.1 Low EPSS < 1% KEV · Not listed
Vendors
Mattermost
Products
Mattermost Mattermost Server
CVE-2025-31363

Data exfiltration via AI plugin Jira tool

Mattermost versions 10.4.x <= 10.4.2, 10.5.x <= 10.5.0, 9.11.x <= 9.11.9 fail to restrict domains the LLM can request to contact upstream which allows an authenticated user to exfiltrate data from an arbitrary server accessible to the victim via performing a prompt injection in the AI plugin's Jira tool.

CVSS 3.1 · 3 Low EPSS < 1% KEV · Not listed
Vendors
Mattermost
Products
Mattermost Server
CVE-2026-6961

CVE-2026-6961: Path traversal via unsanitized FileInfo.Name in Mattermost federation sync

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 Mattermost fails to sanitize FileInfo.Name received from federated peers during shared channel file sync, which allows an attacker who controls a federated server to write files to arbitrary locations within the target server's filestore via path traversal sequences in the filename field.. Mattermost Advisory ID: MMSA-2026-00661

CVSS 3.1 · 7.6 High EPSS < 1% KEV · Not listed
Vendors
Mattermost
Products
Mattermost Mattermost Server
CVE-2026-7184

Mattermost Remote Cluster PATCH API Leaks Authentication Tokens

Mattermost versions 11.6.x <= 11.6.1, 11.5.x <= 11.5.4, 10.11.x <= 10.11.15 fail to sanitize the Remote Cluster API response on PATCH operations, which allows authenticated users with the {{manage_secure_connections}} permission to obtain remote cluster authentication tokens via a PATCH request to the remote cluster endpoint.. Mattermost Advisory ID: MMSA-2026-00662

CVSS 3.1 · 6.5 Medium EPSS < 1% KEV · Not listed
Vendors
Mattermost
Products
Mattermost Mattermost Server
CVE-2026-6739

Mattermost: Delegated admins could patch protected default system roles

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 require system-level permission when patching protected default system roles, which allows authenticated users with delegated user-management permissions to escalate privileges by altering built-in role permissions via the role patch API.. Mattermost Advisory ID: MMSA-2026-00656

CVSS 3.1 · 6.7 Medium EPSS < 1% KEV · Not listed
Vendors
Mattermost
Products
Mattermost Mattermost Server