Improper Validation Of OAuth State Tokens

github.com/mattermost/mattermost-server is vulnerable to improper validation of OAuth state tokens. The vulnerability is due to insufficient validation during the OpenID Connect OAuth flow, which allows an attacker to manipulate authentication data and take over a user account under specific...

Improper Access Control

github.com/mattermost/mattermost-server is vulnerable to improper access control. The vulnerability is due to failure to validate user channel membership when attaching posts to Jira issues, which allows an authenticated attacker with Jira plugin access to read post content and attachments from...

Improper Authorization

github.com/mattermost/mattermost-server is vulnerable to improper authorization. The vulnerability is due to failure in validating the relationship between the post being updated and the MSTeams plugin OAuth flow, which allows an attacker to exploit this via a crafted OAuth redirect URL to edit...

Sensitive Information Exposure

github.com/mattermost/mattermost-server is vulnerable to sensitive information exposure. The vulnerability is due to improper sanitization of user data, which allows system administrators to access password hashes and MFA secrets via the POST /api/v4/users/userid/email/verify/member endpoint...

Improper Access Control

github.com/mattermost/mattermost-server is vulnerable to improper access control. The vulnerability is due to failure in enforcing the "Allow users to view archived channels" setting, which allows an attacker to access archived channel content and files via the "Open in Channel" functionality fro...

GHSA-MH4X-RMRX-3HP4 Mattermost has session spoofing due to lack of single-use consumption of guest magic link tokens enforcement

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 enforce atomic single-use consumption of guest magic link tokens, which allows an attacker with access to a valid magic link to establish multiple independent authenticated sessions via concurrent...

Mattermost doesn't validate whether users were correctly owned by the correct Connected Workspace

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...

EUVD-2026-22873

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...

Time-of-check Time-of-use (TOCTOU) Race Condition

Overview github.com/mattermost/mattermost/server/channels/app is a private-cloud Slack alternative Affected versions of this package are vulnerable to Time-of-check Time-of-use TOCTOU Race Condition in the authentication process. An attacker can gain unauthorized access to multiple authenticated...

GHSA-M7CF-4GH2-V4QG Mattermost doesn't validate CSRF tokens on an authentication endpoint

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...

Mattermost has session spoofing due to lack of single-use consumption of guest magic link tokens enforcement

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 enforce atomic single-use consumption of guest magic link tokens, which allows an attacker with access to a valid magic link to establish multiple independent authenticated sessions via concurrent...

GHSA-MXXH-FMJQ-J6X4 Mattermost doesn't validate whether users were correctly owned by the correct Connected Workspace

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...

EUVD-2026-22875

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...

CVE-2026-3590

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 enforce atomic single-use consumption of guest magic link tokens, which allows an attacker with access to a valid magic link to establish multiple independent authenticated sessions via concurrent...

CVE-2026-27769

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...

CVE-2026-28741

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...

CVE-2026-3590 Race Condition in Guest Magic Link Authentication Allows Token Reuse

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 enforce atomic single-use consumption of guest magic link tokens, which allows an attacker with access to a valid magic link to establish multiple independent authenticated sessions via concurrent...

CVE-2026-3590 Race Condition in Guest Magic Link Authentication Allows Token Reuse

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 enforce atomic single-use consumption of guest magic link tokens, which allows an attacker with access to a valid magic link to establish multiple independent authenticated sessions via concurrent...

CVE-2026-3590

Mattermost CVE-2026-3590 describes a race condition in the guest magic link authentication flow that fails to enforce atomic single-use for tokens. A valid magic link can be used to establish multiple independent authenticated sessions via concurrent requests on affected versions: Mattermost 10.1...

CVE-2026-3590 Race Condition in Guest Magic Link Authentication Allows Token Reuse

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 enforce atomic single-use consumption of guest magic link tokens, which allows an attacker with access to a valid magic link to establish multiple independent authenticated sessions via concurrent...