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CVE-2026-86348

MS Calendar plugin: unrecovered handler panics from malformed post-action requests could crash the plugin process

Mattermost versions <=11.9 11.0.9 11.4.8 11.7.7 10.22.11.0 fail to recover from handler panics, which allows an authenticated user to crash the plugin via a post-action request with an unexpected field type.. Mattermost Advisory ID: MMSA-2026-00701

CVSS 3.1 · 4.3 Medium EPSS < 1% KEV · Not listed
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Mattermost Mattermost Server
CVE-2025-62690

Open redirect in error page when link opened in new tab

Mattermost versions 10.11.x <= 10.11.4 fail to validate redirect URLs on the /error page, which allows an attacker to redirect a victim to a malicious site via a crafted link opened in a new tab.

CVSS 3.1 · 3.1 Low EPSS < 1% KEV · Not listed
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Mattermost
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Mattermost Mattermost Server
CVE-2025-13352

Mattermost GitHub Plugin allows unauthorized GitHub reactions via reaction forwarding hijacking

Mattermost versions 10.11.x <= 10.11.6 and Mattermost GitHub plugin versions <=2.4.0 fail to validate plugin bot identity in reaction forwarding which allows attackers to hijack the GitHub reaction feature to make users add reactions to arbitrary GitHub objects via crafted notification posts.

CVSS 3.1 · 3 Low EPSS < 1% KEV · Not listed
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Mattermost
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Mattermost Mattermost Server
CVE-2026-82920

Mattermost ABAC parent policy bypass via policy update endpoint

Mattermost versions 11.9.x <= 11.9.0, 11.8.x <= 11.8.4, 11.7.x <= 11.7.7 fail to enforce authorization boundaries on the access control policy update endpoint which allows a channel or team administrator to detach a system-assigned ABAC parent policy via a crafted PUT /api/v4/access_control_policies request with an empty imports list.. Mattermost Advisory ID: MMSA-2026-00724

CVSS 3.1 · 5.5 Medium EPSS < 1% KEV · Not listed
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Mattermost
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Mattermost Mattermost Server
CVE-2026-86349

Mattermost Server Algorithmic DoS via Unbounded Markdown Block Nesting

Mattermost versions 11.9.x <= 11.9.0, 11.8.x <= 11.8.4, 11.7.x <= 11.7.8, 10.11.x <= 10.11.22 fail to limit the nesting depth in the server-side Markdown parser which allows an authenticated attacker to cause a denial of service (CPU resource exhaustion) via a crafted post containing deeply nested blockquotes or list items.. Mattermost Advisory ID: MMSA-2026-00707

CVSS 3.1 · 4.3 Medium EPSS < 1% KEV · Not listed
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Mattermost Mattermost Server
CVE-2026-12985

Mattermost DCR redirect URI allowlist bypass via improper URL component validation

Mattermost versions 11.9.x <= 11.9.0, 11.8.x <= 11.8.4, 11.7.x <= 11.7.7 Mattermost failed to validate Dynamic Client Registration redirect URIs by URL component (matching glob patterns against the raw URI string instead) which allows a remote unauthenticated attacker to register an OAuth client with an attacker-controlled callback host that bypasses the configured redirect URI allowlist via a crafted redirect URI that places an allowlisted host/path suffix inside the query string.. Mattermost Advisory ID: MMSA-2026-00700

CVSS 3.1 · 6.8 Medium EPSS < 1% KEV · Not listed
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Mattermost
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Mattermost Mattermost Server
CVE-2026-91181

Data Retention Teams Endpoint Leaks Private Team Invite ID

Mattermost versions 11.9.x <= 11.9.0, 11.8.x <= 11.8.4, 11.7.x <= 11.7.7, 10.11.x <= 10.11.22 Fail to sanitize Team objects returned by the data retention teams endpoint which allows an authenticated user holding only the read-only Data Retention Policy permission to obtain a private team's secret invite_id and email, and use it to join the team without authorization, via GET /api/v4/data_retention/policies/{policy_id}/teams.. Mattermost Advisory ID: MMSA-2026-00702

CVSS 3.1 · 6.5 Medium EPSS < 1% KEV · Not listed
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Mattermost
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Mattermost Mattermost Server
CVE-2026-95666

Unbounded post ID array in the bulk reactions endpoint allows denial of service

Mattermost versions 11.9.x <= 11.9.1, 11.8.x <= 11.8.5, 11.7.x <= 11.7.10, 11.10.x <= 11.10.1 fail to limit the length of the post ID array accepted by the bulk reactions endpoint which allows an authenticated user to cause excessive database load via a crafted request to {{POST /api/v4/posts/ids/reactions}}.. Mattermost Advisory ID: MMSA-2026-00771

CVSS 3.1 · 4.3 Medium EPSS < 1% KEV · Not listed
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Mattermost Mattermost Server
CVE-2026-96259

Mattermost server-side request forgery via OAuth endpoints configurable by a System Administrator

Mattermost versions 11.9.x <= 11.9.1, 11.8.x <= 11.8.5, 11.7.x <= 11.7.10, 11.10.x <= 11.10.1 fail to apply the internal-connection filter to OAuth endpoint requests, which allows a System Administrator to make the server issue requests to internal network addresses and read the responses via the configured OAuth token and userinfo endpoints.. Mattermost Advisory ID: MMSA-2026-00776

CVSS 3.1 · 5.5 Medium EPSS < 1% KEV · Not listed
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Mattermost
Products
Mattermost Mattermost Server
CVE-2026-96260

Mattermost server missing request body size limit on plugin routes allows denial of service by an authenticated user

Mattermost versions 11.9.x <= 11.9.1, 11.8.x <= 11.8.5, 11.7.x <= 11.7.10, 11.10.x <= 11.10.1 fail to enforce a request body size limit during CSRF validation of plugin requests which allows an authenticated user to exhaust server memory and cause a denial of service via a large request body sent to a plugin endpoint.. Mattermost Advisory ID: MMSA-2026-00775

CVSS 3.1 · 6.5 Medium EPSS < 1% KEV · Not listed
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Mattermost
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Mattermost Mattermost Server
CVE-2026-10542

Playbooks channel action update validation issue

Mattermost versions 11.9.x <= 11.9.0, 11.8.x <= 11.8.4, 11.7.x <= 11.7.7, 10.11.x <= 10.11.22 fail to validate channel action ownership which allows channel managers to update actions in other channels via the channel action update endpoint.. Mattermost Advisory ID: MMSA-2026-00692

CVSS 3.1 · 5 Medium EPSS < 1% KEV · Not listed
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Mattermost Mattermost Server
CVE-2026-10556

Unauthenticated webhook request with null notification entry could crash the Microsoft Calendar plugin.

Mattermost versions 11.9.x <= 11.9.0, 11.8.x <= 11.8.4, 11.7.x <= 11.7.7, 10.11.x <= 10.11.22 fail to validate null entries in Microsoft Graph webhook notification payloads, which allows an unauthenticated attacker to crash the Microsoft Calendar plugin process and deny calendar integration service to all users on the instance via a crafted {{POST}} request to the public webhook endpoint.. Mattermost Advisory ID: MMSA-2026-00693

CVSS 3.1 · 5.3 Medium EPSS < 1% KEV · Not listed
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Mattermost
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Mattermost Mattermost Server
CVE-2026-11993

Fix authenticated members disabling file content indexing server-wide via extraction pool exhaustion

Mattermost versions 11.9.x <= 11.9.0, 11.8.x <= 11.8.4, 11.7.x <= 11.7.7, 10.11.x <= 10.11.22 fail to properly enforce the limit of concurrent files being processed and handled failed files, which allows a user with permission to upload files to spawn more goroutines than intended and block the indexing of other files via uploading heavy files constantly to the server.. Mattermost Advisory ID: MMSA-2026-00696

CVSS 3.1 · 4.3 Medium EPSS < 1% KEV · Not listed
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Mattermost
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Mattermost Mattermost Server
CVE-2026-12882

Mattermost Markdown autolink parsing denial of service

Mattermost versions 11.9.x <= 11.9.0, 11.8.x <= 11.8.4, 11.7.x <= 11.7.7, 10.11.x <= 10.11.22 fail to parse Markdown autolinks with unmatched trailing closing parentheses in linear time, which allows an authenticated user with permission to create posts to cause excessive server CPU consumption and degrade availability for other users via specially crafted post or message attachment content. Mattermost Advisory ID: MMSA-2026-00703

CVSS 3.1 · 4.3 Medium EPSS < 1% KEV · Not listed
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Mattermost
Products
Mattermost Mattermost Server
CVE-2026-13417

Boards plugin denial of service via unvalidated block fields.properties

Mattermost versions 11.9.x <= 11.9.0, 11.8.x <= 11.8.4, 11.7.x <= 11.7.7, 10.11.x <= 10.11.22 fail to validate the type of `fields.properties` on block creation which allows an authenticated user with editor access to a board to crash the Boards plugin worker and trigger a denial of service via a child block whose `fields.properties` is a non-object value. Mattermost Advisory ID: MMSA-2026-00710

CVSS 3.1 · 4.3 Medium EPSS < 1% KEV · Not listed
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Mattermost Mattermost Server
CVE-2026-14259

Board archive import bypasses team board creation permissions

Mattermost versions 11.9.x <= 11.9.0, 11.8.x <= 11.8.4, 11.7.x <= 11.7.7, 10.11.x <= 10.11.22 fail to enforce board creation permissions when importing archive files which allows an authenticated non-guest team member to create Open or Private boards despite administrator restrictions via importing a crafted .boardarchive file. Mattermost Advisory ID: MMSA-2026-00712

CVSS 3.1 · 4.3 Medium EPSS < 1% KEV · Not listed
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Mattermost
Products
Mattermost Mattermost Server
CVE-2026-14344

Inconsistent authorization checks in Mattermost Boards endpoints

Mattermost versions 11.9.x <= 11.9.0, 11.8.x <= 11.8.4, 11.7.x <= 11.7.7, 10.11.x <= 10.11.22 fail to enforce the board-creation permission which allows an unauthorized authenticated user to create boards via the board duplicate, boards-and-blocks, and archive-import endpoints.. Mattermost Advisory ID: MMSA-2026-00715

CVSS 3.1 · 4.3 Medium EPSS < 1% KEV · Not listed
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Mattermost
Products
Mattermost Mattermost Server
CVE-2026-15814

Uploading a crafted image causes excessive memory allocation in the Mattermost Server

Mattermost versions 11.9.x <= 11.9.0, 11.8.x <= 11.8.4, 11.7.x <= 11.7.7, 10.11.x <= 10.11.22 fail to limit the amount of memory allocated when decoding uploaded image files which allows an authenticated user to cause excessive server memory consumption and potential denial of service via uploading a specially crafted image as a profile picture, channel file attachment, team icon, or custom brand image. Mattermost Advisory ID: MMSA-2026-00719

CVSS 3.1 · 6.5 Medium EPSS < 1% KEV · Not listed
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Mattermost
Products
Mattermost Mattermost Server
CVE-2026-5132

Unbounded zlib decompression in Calls SDP WebSocket messages

Mattermost versions 11.9.x <= 11.9.0, 11.8.x <= 11.8.4, 11.7.x <= 11.7.7, 10.11.x <= 10.11.22 fail to limit size of unpacked SDP messages compressed with zlib, which allows attacker to deny service or crash server via sending many SDP messages that unpack to large size.. Mattermost Advisory ID: MMSA-2026-00643

CVSS 3.1 · 6.5 Medium EPSS < 1% KEV · Not listed
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Mattermost
Products
Mattermost Mattermost Server
CVE-2026-8821

Playbooks run owner channel membership permission bypass

Mattermost versions 11.9.x <= 11.9.0, 11.8.x <= 11.8.4, 11.7.x <= 11.7.7, 10.11.x <= 10.11.22 fail to validate channel member-management permission during playbook run creation, allowing an authenticated channel member to add an arbitrary user to a restricted channel via the run owner field.. Mattermost Advisory ID: MMSA-2026-00677

CVSS 3.1 · 7.1 High EPSS < 1% KEV · Not listed
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Mattermost
Products
Mattermost Mattermost Server