CVE-2026-8821

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

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. An authenticated user with only the read-only Data Retention Policy permission can obtain a private team's secret invite_id and email and join the team without authorization via GET /api/v4/data_retention/policies/{policy_id}/teams.

Mattermost versions 11.9.x <= 11.9.0, 11.8.x <= 11.8.4, 11.7.x <= 11.7.7 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

Mattermost versions 11.9.x <= 11.9.0, 11.8.x <= 11.8.4, 11.7.x <= 11.7.8, and 10.11.x <= 10.11.22 fail to limit nesting depth in the server-side Markdown parser, allowing an authenticated attacker to cause denial of service (CPU exhaustion) via a crafted post with deeply nested blockquotes or list items.

Mattermost versions <=11.9, 11.0.9, 11.4.8, 11.7.7, and 10.22.11.0 fail to recover from handler panics, allowing an authenticated user to crash the plugin via a post-action request with an unexpected field type.

Mattermost versions 11.9.x <= 11.9.0, 11.8.x <= 11.8.4, and 11.7.x <= 11.7.7 fail to enforce authorization boundaries on the access control policy update endpoint, allowing 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 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 size of unpacked SDP messages compressed with zlib. An attacker can send many SDP messages that unpack to a large size, causing denial of service or server crash.

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. An authenticated user can cause excessive server memory consumption and potential denial of service by uploading a specially crafted image as a profile picture, channel file attachment, team icon, or custom brand image.