Vulnerability · Published Oct 16, 2026 · Assigned by Mattermost
CVE-2026-16453Mattermost SAML RelayState forgery allows unauthorized team membership
High 7.1
In plain English
A high-severity security flaw in Mattermost from Mattermost. There are no confirmed reports of exploitation so far. The chance it is exploited in the next 30 days is estimated at 0.2% (higher than 9% of all known vulnerabilities).
Severity (CVSS 3.1)7.1High
Exploit odds (30 days)0.2%top 91.2% of all CVEs
Exploited in the wildNo reportsCISA SSVC: none
Attack toolingNoneno public tooling known
Official 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
Affected products (as reported)
| Vendor | Product | Versions |
|---|---|---|
| Mattermost | Mattermost | 11.10.0 – through 11.10.011.7.0 – through 11.7.911.9.0 – through 11.9.111.8.0 – through 11.8.5 |
Weakness: CWE-472 · Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:L/A:N
References
- MMSA-2026-00727 vendor-advisory