CVE-2026-6046: Plugin bot username conflict allows user account to be used as bot identity in Mattermost Server

Severity: medium (CVSS 5.3 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: 2026-06-12 · updated 2026-06-18
Weakness: CWE-200

Description

Mattermost versions 11.6.x <= 11.6.1, 11.5.x <= 11.5.4, 10.11.x <= 10.11.15, 10.11.x <= 10.11.16 fail to validate that a username returned during bot registration belongs to a bot account, which allows an unprivileged attacker to intercept private messages sent by plugins via direct message channels by pre-registering a user account with a predictable plugin bot username.. Mattermost Advisory ID: MMSA-2026-00649

Affected products

Mattermost: from 10.11.0 before 10.11.17; from 11.5.0 before 11.5.5; from 11.6.0 before 11.6.2 — fixed in 10.11.17, 11.5.5, 11.6.2

How to fix

Upgrade to a fixed version: Mattermost 10.11.17, 11.5.5, 11.6.2. Check the vendor advisory for mitigations.

References

https://mattermost.com/security-updates

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