CVE-2026-16453

Mattermost SAML RelayState forgery allows unauthorized team membership

State: Published
Severity: 7.1 High
Published: Oct 16, 2026
Updated: Oct 16, 2026
Reserved: Jul 21, 2026

Description

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 verify the integrity of the SAML RelayState POST parameter which allows an authenticated SAML user to join any team, including invite-only teams, via forging the team_id or invite_id fields in the RelayState.. Mattermost Advisory ID: MMSA-2026-00727

Scores

CVSS	Score	Vector	Source
3.1	7.1 High	CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:L/A:N	Mattermost

Weaknesses

CWE-472 External Control of Assumed-Immutable Web Parameter

Affected Products

Product	Versions
MattermostMattermost	11.10.0 through 11.10.0Affected 11.7.0 through 11.7.9Affected 11.9.0 through 11.9.1Affected 11.8.0 through 11.8.5Affected 11.11.0Not affected 11.10.1Not affected 11.7.10Not affected 11.9.2Not affected 11.8.6Not affected

References

MMSA-2026-00727vendor-advisory

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