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CVE-2026-16049

_GitLab Plugin allows cross-channel post injection and phishing via missing channel permission checks in issue API endpoints_

Mattermost Plugins versions <=11.8 10.20.11 11.5.7.0 _The Mattermost GitLab plugin fails to verify channel permissions when processing API requests with a caller-supplied_ {{post_id}}_, and fails to validate the_ {{web_url}} _parameter against the configured GitLab instance, which allows an authenticated attacker to inject bot-authored messages containing arbitrary URLs into channels they do not have access to via the_ {{createIssue}} _and_ {{attachCommentToIssue}} _API endpoints._ Mattermost Advisory ID: MMSA-2026-00673

CVSS 3.1 · 3.9 Low EPSS < 1% KEV · Not listed
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CVE-2026-2299

Improper Access Control in Mattermost Google Drive Plugin File Creation Endpoint

The Mattermost Google Drive plugin before version 1.1.0 fails to validate channel membership in the file creation endpoint, allowing authenticated users with a connected Google account to share Google Drive files to unauthorized private channels and disclose private channel membership.

CVSS 3.1 · 4.2 Medium EPSS < 1% KEV · Not listed
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Google Drive Mattermost Google Drive Plugin
CVE-2026-7521

SAML certificate deletion allows path traversal to delete arbitrary files outside the config directory

Mattermost versions 11.8.x <= 11.8.0, 11.7.x <= 11.7.3, 11.6.x <= 11.6.5, 10.11.x <= 10.11.20 fail to verify file deletion path which allows an admin with SAML system-console write permissions to delete arbitrary files outside the config directory from the server via the remove file endpoint.. Mattermost Advisory ID: MMSA-2026-00666

CVSS 3.1 · 5.5 Medium EPSS < 1% KEV · Not listed
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CVE-2026-10600

Denial of service via unbounded document content extraction in Mattermost Server

Mattermost versions 11.8.x <= 11.8.0, 11.7.x <= 11.7.3, 11.6.x <= 11.6.5, 10.11.x <= 10.11.20 fail to bound the time and resource consumption of server-side document content extraction which allows an authenticated user with file-upload permission to degrade file uploads for all users on the server via repeatedly uploading small documents that are cheap to upload but expensive to extract, saturating the shared extraction worker pool.. Mattermost Advisory ID: MMSA-2026-00694

CVSS 3.1 · 4.3 Medium EPSS < 1% KEV · Not listed
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Mattermost Mattermost Server
CVE-2026-10819

Mattermost Server Denial of Service via Animated GIF Emoji Upload

Mattermost versions 11.6.x <= 11.6.5, 10.11.x <= 10.11.20, 11.8.x <= 11.8.1, 11.7.x <= 11.7.4 fail to limit the number of frames and enforce the file size cap on animated GIF uploads, which allows an authenticated attacker to cause a denial of service via a crafted animated GIF uploaded as a custom emoji.. Mattermost Advisory ID: MMSA-2026-00695

CVSS 3.1 · 6.5 Medium EPSS < 1% KEV · Not listed
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Mattermost Mattermost Server
CVE-2026-9602

Mattermost Desktop App crashes when malformed arguments are provided to some exposed IPC methods

Mattermost Desktop App versions <=6.2 6.0.2 5.6.13.0 fail to validate payloads sent from the Mattermost Web App to the Desktop App which allows a malicious server owner to crash the Mattermost Desktop App via changing the payload of a method to a malformed one. Mattermost Advisory ID: MMSA-2026-00678

CVSS 3.1 · 5.7 Medium EPSS < 1% KEV · Not listed
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Mattermost Mattermost Desktop
CVE-2026-8075

Posting a malicious markdown image crashes the Mattermost Desktop App

Mattermost Desktop App versions <=6.2 5.5.13 6.0.2.0 fail to properly null check when checking for headers in the Mattermost Desktop App which allows any user to crash another channel members Desktop App via posting a malicious link with an embedded image that misses one of those headers. Mattermost Advisory ID: MMSA-2026-00668

CVSS 3.1 · 6.5 Medium EPSS < 1% KEV · Not listed
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Mattermost Mattermost Desktop
CVE-2026-9571

Deactivated user accounts can continue to obtain valid OAuth access tokens via refresh token grant in Mattermost

Mattermost versions 11.7.x <= 11.7.2, 11.6.x <= 11.6.4, 10.11.x <= 10.11.19 fail to invalidate OAuth refresh tokens upon user account deactivation, which allows a deactivated user or an attacker in possession of a valid refresh token to obtain new functional access tokens via the OAuth refresh token grant endpoint.. Mattermost Advisory ID: MMSA-2026-00680

CVSS 3.1 · 5.9 Medium EPSS < 1% KEV · Not listed
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Mattermost Mattermost Server
CVE-2026-10103

Authenticated remote cluster can modify or delete posts it does not own in Mattermost Connected Workspaces shared channels

Mattermost versions 11.7.x <= 11.7.2, 11.6.x <= 11.6.4, 10.11.x <= 10.11.19 fail to verify post ownership in the shared channel inbound sync handler, which allows an authenticated remote cluster to modify or delete posts authored by local users or other remotes via crafted sync messages referencing arbitrary post IDs in channels shared with that remote.. Mattermost Advisory ID: MMSA-2026-00689

CVSS 3.1 · 4.3 Medium EPSS < 1% KEV · Not listed
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CVE-2026-9708

Incoming webhook user attribution via unvalidated webhook owner

Mattermost versions 11.7.x <= 11.7.2, 11.6.x <= 11.6.4, 10.11.x <= 10.11.19 fail to validate that an assigned incoming webhook user has access to the target team or channel, which allows a requester with webhook management permissions to create posts or direct messages attributed to another user via crafted incoming webhook configuration and payloads.. Mattermost Advisory ID: MMSA-2026-00683

CVSS 3.1 · 4.9 Medium EPSS < 1% KEV · Not listed
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Mattermost Mattermost Server
CVE-2026-9824

Remote cluster metadata enumeration via /share-channel autocomplete

Mattermost versions 11.7.x <= 11.7.2, 11.6.x <= 11.6.4, 10.11.x <= 10.11.19 fail to check the manage_shared_channels permission in the /share-channel autocomplete handler, which allows an authenticated user without that permission to enumerate configured remote cluster connection metadata via slash command autocomplete.. Mattermost Advisory ID: MMSA-2026-00676

CVSS 3.1 · 4.3 Medium EPSS < 1% KEV · Not listed
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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
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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
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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
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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
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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
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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
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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
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Mattermost Server
CVE-2026-9699

Mattermost Agents plugin logs unsanitized OpenAI API keys on authentication errors

Mattermost Plugins versions <=11.6 10.18.11 11.3.6 11.6.5.0 fail to sanitize error responses from the OpenAI API before logging, which allows a user with access to server logs or support packets to obtain a valid or partially reconstructable OpenAI API key via inspection of mattermost.log entries generated during authentication failures. Mattermost Advisory ID: MMSA-2026-00609

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