Mattermost CVE-2026-16453

Exploitation requires the target Mattermost server to have SAML single sign-on configured and enabled, and the attacker must be an already-authenticated SAML user (PR:L) of that instance. The specific prerequisite is that the attacker forges the team_id or invite_id fields within the SAML RelayState POST parameter, whose integrity the server fails to verify; no user interaction by a victim is needed (UI:N). Limiting factors: instances without SAML SSO are not affected, the attacker needs valid SAML credentials, and impact is bounded to gaining membership in teams (including invite-only teams) rather than full administrative compromise. Affected builds are 11.7.x <= 11.7.9, 11.8.x <= 11.8.5, 11.9.x <= 11.9.1, and 11.10.x <= 11.10.0.

This is a moderate-priority, authentication-gated authorization flaw rather than a mass-exploitation emergency. The CVSS 3.1 vector (AV:N/AC:L/PR:L/UI:N) indicates a network-reachable, low-complexity attack, but the PR:L requirement means the attacker must already hold valid SAML-authenticated credentials in the target Mattermost instance, which substantially narrows the threat population to insiders or users who already have some legitimate access. No EPSS score was provided in the input, so exploitation probability cannot be quantified. The CVE is not listed in CISA KEV and no public exploit code was referenced, so this is 'no public exploit identified at time of analysis' - do not treat it as actively exploited. SSVC-style reasoning: exploitation is not observed, the attack is not obviously automatable at scale (it targets specific team_id/invite_id values per instance), and technical impact is partial (unauthorized team membership rather than full system compromise). Note a tension between the vendor-assigned C:H and the actual described outcome (joining invite-only teams), which reads closer to scoped information disclosure than total confidentiality loss - the real-world impact depends on how sensitive the content of the joined teams is. Overall: patch on a normal prioritized cadence, prioritizing instances with SAML SSO enabled and sensitive invite-only teams.

An employee or contractor who already authenticates to Mattermost via SAML forges the team_id or invite_id field inside the unsigned SAML RelayState POST parameter during the SSO flow. Because the server never verifies RelayState integrity, the forged value causes the user to be added to an invite-only team they were never granted access to, exposing that team's restricted channels and content. No exploit code is publicly referenced; the attack requires a valid SAML account and crafting of a single parameter, consistent with the low attack complexity rating.