Vulnerability · Published Oct 16, 2026 · Assigned by Mattermost

CVE-2026-18223Mattermost invitation tokens not bound to invited email address allowing unauthorized team and private channel access

Medium 5.0

In plain English

A medium-severity incorrect authorization 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 5% of all known vulnerabilities).

Severity (CVSS 3.1)5.0Medium

Exploit odds (30 days)0.2%top 94.7% 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 validate the invited recipient's email address when redeeming team and guest invitation tokens, which allows an authenticated attacker to join a team or gain unauthorized access to private channels via a forwarded or leaked invitation link.. Mattermost Advisory ID: MMSA-2026-00747

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-863 · Vector: CVSS:3.1/AV:N/AC:H/PR:H/UI:N/S:U/C:H/I:L/A:N

References

MMSA-2026-00747 vendor-advisory