Vulnerability · Published Oct 16, 2026 · Assigned by Mattermost

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

Medium 4.3

In plain English

A medium-severity missing 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 10% of all known vulnerabilities).

Severity (CVSS 3.1)4.3Medium

Exploit odds (30 days)0.2%top 90.4% 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 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

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-862 · Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:L/A:N

References

MMSA-2026-00757 vendor-advisory