CVE-2026-18223 in Mattermost
Summary
by MITRE • 10/16/2026
Mattermost versions 11.10.x <= 11.10.0, 11.7.x <= 11.7.9, 11.9.x <= 11.9.1, 11.8.x <= 11.8.5 fail to validate the invited recipient's email address when redeeming team and guest invitation tokens, which allows an authenticated attacker to join a team or gain unauthorized access to private channels via a forwarded or leaked invitation link.. Mattermost Advisory ID: MMSA-2026-00747
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Analysis
by VulDB Data Team • 10/16/2026
The vulnerability identified in Mattermost versions 11.10.x through 11.8.5 represents a critical authentication bypass flaw rooted in the improper validation of invitation tokens during team and guest onboarding processes. Specifically, when an authenticated user attempts to redeem an invitation token for joining a new team or accessing private channels, the application fails to verify that the email address associated with the currently logged-in session matches the recipient specified within the invitation metadata. This logical error creates a significant gap in access control enforcement, allowing any authenticated account holder to bypass intended restrictions by simply possessing a valid invitation link, regardless of whether they are the intended beneficiary.
From a technical perspective, this flaw constitutes an authorization failure where the system relies on the possession of a token rather than verifying identity continuity between the session and the target resource. The attacker leverages forwarded or leaked invitation links to exploit this discrepancy. Since Mattermost treats the redemption process as valid based solely on the presence of an active token without cross-referencing it against the authenticated user's profile data, the security boundary is effectively neutralized for any internal actor who can obtain such a link. This behavior deviates from standard secure design principles which mandate strict binding between session identity and resource access permissions during sensitive operations like team joining or channel entry.
The operational impact of this vulnerability is severe, particularly in enterprise environments where data confidentiality and role-based access control are paramount. An authenticated attacker with minimal privileges can escalate their access by joining teams they were not originally invited to or entering private channels containing sensitive information. This leads directly to unauthorized disclosure of proprietary data, internal communications, and potentially confidential project details. Furthermore, because the vulnerability affects guest accounts as well, it compromises the isolation boundaries between external collaborators and internal staff, increasing the risk of lateral movement within the organization's digital infrastructure if an attacker gains a foothold through compromised or shared credentials.
This issue aligns with CWE-284, Improper Access Control, specifically highlighting failures in validating user identity against requested actions during resource access. In terms of offensive security frameworks, it maps to ATT&CK technique T1078, Valid Accounts, as the exploitation relies on existing authenticated sessions rather than credential theft or injection attacks. It also reflects aspects of CWE-639, Authorization Bypass Through User-Controlled Key, where the invitation token acts as a key that is not properly bound to the specific user context attempting its use.
To mitigate this risk, organizations running affected Mattermost versions must immediately upgrade to patched releases that enforce strict email address validation during the redemption process. Administrators should also review audit logs for unusual patterns of team joins or channel accesses originating from accounts different from those originally invited. Implementing additional monitoring alerts for rapid succession of invitation redemptions by single users can help detect potential exploitation attempts in real time. Until patches are applied, restricting the ability to forward invitations and enforcing strict email verification policies during account creation may provide limited defensive value against this specific logical flaw.