Mattermost CVE-2026-16045

Exploitation requires that the attacker controls an OAuth application that has been authorized by the target user, giving the app a valid delegated user token for that user's Mattermost account. The target Mattermost instance must be running version 11.7.0-11.7.6 or 10.11.0-10.11.21. The user must have granted the attacker-controlled OAuth app access to their account. No special network position is required - the attack is fully remote over standard HTTPS. Exploitation is limited by the need for a valid delegated OAuth token scoped to the victim user; an unauthenticated or fully external attacker cannot exploit this without first obtaining such a token. Environments that restrict OAuth app installations to a curated, administrator-approved list significantly reduce exposure.

The vendor-assigned CVSS score of 4.3 (Medium) reflects a genuinely moderate risk profile. The CVSS vector (AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:L) signals network accessibility with low attack complexity and no user interaction - favorable conditions for exploitation - but the scored impact is constrained to availability only at a low level. In practice, revoking a user's OAuth authorizations or PATs also modifies account state, suggesting integrity impact (I:L) may be slightly underscored by the vendor. No EPSS data was provided in the source intelligence, so exploitation probability cannot be quantified. The vulnerability is not listed in the CISA KEV catalog, and no POC code has been identified, making mass or opportunistic exploitation unlikely at this time. Real-world risk is highest in organizations that: (1) run Mattermost with many third-party integrations, (2) grant broad OAuth app permissions to users, and (3) have not restricted which apps can be installed. For such environments, a malicious or compromised OAuth app could silently sever a user's other integration tokens, causing operational disruption without triggering obvious security alerts.

An attacker who controls or has compromised a Mattermost OAuth application authorized by a target user submits API requests to the OAuth deauthorization or PAT management endpoints, using the delegated user token the app legitimately holds. The platform incorrectly accepts these calls as authorized, causing the target user's authorizations or tokens for other integrations - such as bots, CI/CD pipelines, or webhooks - to be revoked. The user's other connected services silently lose access, causing operational disruption with no obvious indication of the attack vector.