CVE-2026-18256: Private team metadata disclosure via the team search API

Severity: medium (CVSS 4.3 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.2% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2026-10-16 · updated 2026-10-29
Weakness: CWE-863

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 properly restrict private teams from public team search results, which allows an authenticated user without permission to list private teams to disclose the ID, name, and display name of private teams via a modified team search request. Mattermost Advisory ID: MMSA-2026-00748

Affected products

Mattermost: from 11.7.0 before 11.7.10; from 11.8.0 before 11.8.6; from 11.9.0 before 11.9.2; 11.10.0 (and their updates) — fixed in 11.7.10, 11.8.6, 11.9.2
Mattermost: from 11.10.0 through 11.10.0; from 11.7.0 through 11.7.9; from 11.9.0 through 11.9.1; from 11.8.0 through 11.8.5

How to fix

Upgrade to a fixed version: Mattermost 11.7.10, 11.8.6, 11.9.2. Check the vendor advisory for mitigations.

References

https://mattermost.com/security-updates

Get alerted when CVE-2026-18256 or the next vulnerability affects your stack