CVE-2026-16453 PUBLISHED

Mattermost SAML RelayState forgery allows unauthorized team membership

Assigner: Mattermost
Reserved: 21.07.2026 Published: 16.10.2026 Updated: 16.10.2026

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 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

Metrics

CVSS 3.1

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

Attack Vector	Network	Scope	Unchanged
Attack Complexity	Low	Confidentiality Impact	High
Privileges Required	Low	Integrity Impact	Low
User Interaction	None	Availability Impact	None

CVSS 3.1

Product Status

Vendor	Mattermost
Product	Mattermost
Versions	Default: unaffected affected from 11.10.0 to 11.10.0 (incl.) affected from 11.7.0 to 11.7.9 (incl.) affected from 11.9.0 to 11.9.1 (incl.) affected from 11.8.0 to 11.8.5 (incl.) Version 11.11.0 is unaffected Version 11.10.1 is unaffected Version 11.7.10 is unaffected Version 11.9.2 is unaffected Version 11.8.6 is unaffected

Solutions

Update Mattermost to versions 11.11.0, 11.10.1, 11.7.10, 11.9.2, 11.8.6 or higher.

Credits

Doyensec finder

References

MMSA-2026-00727

Problem Types

CWE-472: External Control of Assumed-Immutable Web Parameter CWE