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CVE-2026-18799 Mattermost Server fails to restrict Slack import account binding, allowing team admins to forge posts as existing users

Medium 4.3 Published Updated

Team admins can forge posts as existing users by exploiting unrestricted Slack import account binding, compromising data integrity and user identity.

Exploited in the wild
Not known
Login needed
Any signed-in user
Attackable
Over the internet
Victim action needed
Not needed
Fix
None listed

What is CVE-2026-18799?

CVE-2026-18799 is a medium-severity missing authorization vulnerability in Mattermost Mattermost Server, published on 16 Oct 2026. Any signed-in user can exploit it over the network, 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 restrict Slack import account binding to system administrator-initiated imports which allows an authenticated attacker with team admin privileges to force-add existing Mattermost users to their team and forge posts attributed to those users' identities via a crafted Slack export containing target users' email addresses.. Mattermost Advisory ID: MMSA-2026-00757

Weakness: CWE-862

Which versions are affected by CVE-2026-18799?

It affects 1 product.

Products include Mattermost Mattermost Server.

How do I fix CVE-2026-18799?

No fix has been published for CVE-2026-18799 yet. Limit exposure of the affected software until one is available.

Upgrade, and check permissions on every request on the server, not only in the interface. More about Missing authorization.

Is CVE-2026-18799 being exploited?

There is no known exploitation of CVE-2026-18799 in the wild and no public exploit code.

All Mattermost Mattermost Server vulnerabilities

Sources

Also known as GHSA-f4q7-3g3r-qrg8.

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