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

Medium 5.0 Published Updated

Authenticated attackers can join teams or access private channels by redeeming invitation tokens not bound to specific email addresses.

Exploited in the wild
Not known
Login needed
An administrator
Attackable
Over the internet
Victim action needed
Not needed
Fix
None listed

What is CVE-2026-18223?

CVE-2026-18223 is a medium-severity authentication bypass vulnerability in Mattermost Mattermost Server, published on 16 Oct 2026. An attacker needs administrator access to exploit it, with no action from the victim.

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

Weakness: CWE-863

Which versions are affected by CVE-2026-18223?

It affects 1 product.

Products include Mattermost Mattermost Server.

How do I fix CVE-2026-18223?

No fix has been published for CVE-2026-18223 yet. Limit exposure of the affected software until one is available.

Upgrade, and rotate credentials and sessions that may have been exposed. More about Authentication bypass.

Is CVE-2026-18223 being exploited?

There is no known exploitation of CVE-2026-18223 in the wild and no public exploit code.

All Mattermost Mattermost Server vulnerabilities

Sources

Also known as GHSA-j732-3464-9jvm.

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