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CVE-2026-3495

Unescaped variables during error page composition

Mattermost versions 11.5.x <= 11.5.1, 10.11.x <= 10.11.13 fail to escape some variables that could contain malicious content during error page composition which allows an attacker with access to edit some site configuration to execute some malicious code via injecting some JS as part of those values.. Mattermost Advisory ID: MMSA-2026-00622

CVSS 3.1 · 3.8 Low EPSS < 1% KEV · Not listed
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Mattermost Mattermost Server
CVE-2026-3637

Mattermost fails to enforce create_post permission when editing posts

Mattermost versions 11.5.x <= 11.5.1, 10.11.x <= 10.11.13, 11.4.x <= 11.4.3 fail to check the create_post channel permission during post edit operations which allows an authenticated attacker with revoked posting privileges to modify their existing posts via direct API requests to the post update and patch endpoints.. Mattermost Advisory ID: MMSA-2026-00627

CVSS 3.1 · 4.3 Medium EPSS < 1% KEV · Not listed
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Mattermost Mattermost Server
CVE-2026-4273

Insufficient token rotation validation in remote cluster invite confirmation

Mattermost versions 11.5.x <= 11.5.1, 10.11.x <= 10.11.13 fail to validate that the RefreshedToken differs from the original invite token during remote cluster invite confirmation which allows an authenticated attacker to bypass token rotation and reuse the original invite token via sending a crafted invite confirmation with a RefreshedToken matching the original token. Mattermost Advisory ID: MMSA-2026-00575

CVSS 3.1 · 3.7 Low EPSS < 1% KEV · Not listed
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Mattermost Mattermost Server
CVE-2026-6340

Memory Exhaustion via Malicious 7zip File Upload

Mattermost versions 11.5.x <= 11.5.1, 10.11.x <= 10.11.13, 11.4.x <= 11.4.3 fail to validate 7zip archive structure before processing which allows an authenticated attacker to cause server memory exhaustion and denial of service via uploading a specially crafted 7zip file with excessive folder declarations.. Mattermost Advisory ID: MMSA-2026-00573

CVSS 3.1 · 4.3 Medium EPSS < 1% KEV · Not listed
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Mattermost
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Mattermost Mattermost Server
CVE-2026-28732

Slash command trigger-word update allowed command hijacking

Mattermost versions 11.5.x <= 11.5.1, 10.11.x <= 10.11.13, 11.4.x <= 11.4.3 Fail to enforce slash command trigger-word uniqueness during command updates which allows an authenticated team member with Manage Own Slash Commands permission to hijack and impersonate existing system or custom slash commands via editing their own slash command trigger to an already-registered trigger through the command update API. Mattermost Advisory ID: MMSA-2026-00597

CVSS 3.1 · 4.3 Medium EPSS < 1% KEV · Not listed
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Mattermost
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Mattermost Mattermost Server
CVE-2026-28759

Insufficient authorization in shared channel membership sync allows remote cluster to remove users from arbitrary channels

Mattermost versions 11.5.x <= 11.5.1, 10.11.x <= 10.11.13, 11.4.x <= 11.4.3 fail to validate that a remote cluster has access to a channel before processing membership removal requests during shared channel membership sync, which allows a malicious remote cluster to remove any user from any channel, including private channels, via crafted membership sync messages targeting channels the remote cluster is not authorized to access. Mattermost Advisory ID: MMSA-2026-00576

CVSS 3.1 · 4.3 Medium EPSS < 1% KEV · Not listed
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Mattermost
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Mattermost Mattermost Server
CVE-2026-2325

Improper Input Validation in MS Teams Meetings API Handler

Mattermost versions 11.5.x <= 11.5.1, 10.11.x <= 10.11.13, 11.4.x <= 11.4.3 fail to limit the size of the request body on the start meeting API endpoint, which allows an authenticated attacker to cause resource exhaustion or denial of service via a crafted oversized HTTP POST request to {{/api/v1/meetings}}.. Mattermost Advisory ID: MMSA-2026-00608

CVSS 3.1 · 4.3 Medium EPSS < 1% KEV · Not listed
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Mattermost
Products
Mattermost Mattermost Server
CVE-2026-4286

Playbooks Plugin fails to validate team transfers, allowing unauthorized removal of member access via playbook update

Mattermost versions 11.5.x <= 11.5.1, 10.11.x <= 10.11.13 fail to check if {{team_id}} was being changed when updating playbooks, allowing users with only {{Manage Playbook Configurations}} permission to change a playbook's team, bypassing manage members restriction via PUT api. Mattermost Advisory ID: MMSA-2025-00552

CVSS 3.1 · 3.1 Low EPSS < 1% KEV · Not listed
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Mattermost
Products
Mattermost Mattermost Server
CVE-2026-5163

Missing authorization check in AI message rewrite endpoint allows access to private thread content

Mattermost versions 11.5.x <= 11.5.1 fail to verify channel membership when processing AI-assisted message rewrites which allows an authenticated attacker to read the content of threads in private channels and direct messages they do not have access to via a crafted request to the post rewrite endpoint.. Mattermost Advisory ID: MMSA-2026-00645

CVSS 3.1 · 6.5 Medium EPSS < 1% KEV · Not listed
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Mattermost
Products
Mattermost Mattermost Server
CVE-2026-6339

Missing request origin validation on burn-on-read reveal endpoint

Mattermost versions 11.5.x <= 11.5.1, 11.4.x <= 11.4.3 fail to validate the X-Requested-With header on the burn-on-read reveal endpoint which allows an authenticated channel member to force the reveal of a burn-on-read message without recipient consent via a crafted Markdown image tag.. Mattermost Advisory ID: MMSA-2026-00636

CVSS 3.1 · 4.3 Medium EPSS < 1% KEV · Not listed
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Mattermost
Products
Mattermost Mattermost Server
CVE-2026-6343

Mattermost Playbooks Plugin fails to enforce view permissions in list endpoints, allowing unauthorized access to public playbooks

Mattermost versions 11.5.x <= 11.5.1, 10.11.x <= 10.11.13, 11.4.x <= 11.4.3 fail to check public/private permissions which allows members without these permissions to access public playbooks via /get.. Mattermost Advisory ID: MMSA-2026-00591

CVSS 3.1 · 4.3 Medium EPSS < 1% KEV · Not listed
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Mattermost
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Mattermost Mattermost Server
CVE-2026-6345

Prevent password disclosure and force reset during Slack import

Mattermost versions 11.5.x <= 11.5.1, 10.11.x <= 10.11.13, 11.4.x <= 11.4.3 fail prevent disclosure of created user password which allows a malicious attacker to impersonate a user via the use of some of those passwords.. Mattermost Advisory ID: MMSA-2026-00614

CVSS 3.1 · 6.5 Medium EPSS < 1% KEV · Not listed
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Mattermost
Products
Mattermost Mattermost Server
CVE-2026-6346

Sensitive credentials exposed in plaintext in Mattermost support packets

Mattermost versions 11.5.x <= 11.5.1, 10.11.x <= 10.11.13, 11.4.x <= 11.4.3 fail to sanitize sensitive configuration fields before including them in support packet generation, which allows a Mattermost System Admin or any party with access to a support packet to obtain sensitive credentials in plaintext via downloading a support packet from the System Console.. Mattermost Advisory ID: MMSA-2026-00607

CVSS 3.1 · 8.7 High EPSS < 1% KEV · Not listed
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Mattermost
Products
Mattermost Mattermost Server
CVE-2026-6347

Mattermost Calls plugin exposes TURN server credentials in plaintext in support packets

Mattermost versions 11.5.x <= 11.5.1, 10.11.x <= 10.11.13, 11.4.x <= 11.4.3 fail to sanitize sensitive configuration fields in the Mattermost Calls plugin which allows an attacker with access to a support packet to obtain TURN server credentials via the plaintext values present in the exported plugin configuration.. Mattermost Advisory ID: MMSA-2026-00605

CVSS 3.1 · 7.6 High EPSS < 1% KEV · Not listed
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Mattermost
Products
Mattermost Mattermost Server
CVE-2026-4054

SVG content served through Mattermost image proxy despite Content-Type restrictions causes client-side denial of service

Mattermost versions 11.5.x <= 11.5.1, 10.11.x <= 10.11.13, 11.4.x <= 11.4.3 Fail to validate the response body of proxied images, which allows a remote attacker to enact client-side DoS via an SVG file served from an attacker-controlled origin under a non-SVG Content-Type header (e.g. image/png) embedded in an og:image meta tag or Markdown image link.. Mattermost Advisory ID: MMSA-2026-00630

CVSS 3.1 · 4.3 Medium EPSS < 1% KEV · Not listed
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Mattermost
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Mattermost Mattermost Server
CVE-2026-28736

Focalboard IDOR in file content endpoint allows cross-user file access (unsupported product, no fix)

** UNSUPPORTED WHEN ASSIGNED ** Focalboard version 8.0 fails to validate file ownership when serving uploaded files. This allows an authenticated attacker who knows a victim's fileID to read the content of the file. NOTE: Focalboard as a standalone product is not maintained and no fix will be issued.

CVSS 3.1 · 4.3 Medium EPSS < 1% KEV · Not listed
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Mattermost
Products
Focalboard
CVE-2026-25773

Focalboard Second-Order SQL Injection in category reorder endpoint allows data exfiltration (unsupported product, no fix)

** UNSUPPORTED WHEN ASSIGNED ** Focalboard version 8.0 fails to sanitize category IDs before incorporating them into dynamic SQL statements when reordering categories. An attacker can inject a malicious SQL payload into the category id field, which is stored in the database and later executed unsanitized when the category reorder API processes the stored value. This Second-Order SQL Injection (Time-Based Blind) allows an authenticated attacker to exfiltrate sensitive data including password hashes of other users. NOTE: Focalboard as a standalone product is not maintained and no fix will be issued.

CVSS 3.1 · 8.1 High EPSS < 1% KEV · Not listed
Vendors
Mattermost
Products
Focalboard
CVE-2026-21388

Unbounded Request Body Read in MS Teams Plugin {{/lifecycle}} Webhook Endpoint

Mattermost Plugins versions <=2.3.1 fail to limit the request body size on the {{/lifecycle}} webhook endpoint which allows an authenticated attacker to cause memory exhaustion and denial of service via sending an oversized JSON payload. Mattermost Advisory ID: MMSA-2026-00610

CVSS 3.1 · 3.7 Low EPSS < 1% KEV · Not listed
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Mattermost
Products
Mattermost Mattermost Server
CVE-2026-27769

Connected Workspaces: Malicious remote server can manipulate arbitrary user's status

Mattermost versions 10.11.x <= 10.11.12 fail to validate whether users were correctly owned by the correct Connected Workspace which allows a malicious remote server connected using the Conntexted Workspaces feature to change the displayed status of local users via the Connected Workspaces API.. Mattermost Advisory ID: MMSA-2026-00603

CVSS 3.1 · 2.7 Low EPSS < 1% KEV · Not listed
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Mattermost
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Mattermost Mattermost Server
CVE-2026-28741

CSRF Protection Bypass Allows Updating a User's Authentication Method

Mattermost versions 10.11.x <= 10.11.12, 11.5.x <= 11.5.0, 11.4.x <= 11.4.2, 11.3.x <= 11.3.2 fail to validate CSRF tokens on an authentication endpoint which allows an attacker to update a user's authentication method via a CSRF attack by tricking a user into visiting a malicious page. Mattermost Advisory ID: MMSA-2026-00625

CVSS 3.1 · 6.8 Medium EPSS < 1% KEV · Not listed
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Mattermost
Products
Mattermost Mattermost Server