Plugin bot username conflict allows user account to be used as bot identity in Mattermost Server

Summary

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

Timeline

1. Jun 12, 2026, 3:52 PM UTCCVE published
2. Jun 12, 2026, 5:18 PM UTCCISA SSVC assessment

   Exploitation: none · Automatable: no · Technical impact: partial

3. Jun 12, 2026, 5:18 PM UTCRecord last updated

Affected versions

• Mattermost Mattermost

  From 11.6.0 · through 11.6.1; From 11.5.0 · through 11.5.4; From 10.11.0 · through 10.11.15; From 10.11.0 · through 10.11.16

• github.com/mattermost/mattermost-serverGo

  From 10.11.0+incompatible · fixed in 10.11.17+incompatible; From 11.5.0+incompatible · fixed in 11.5.5+incompatible; From 11.6.0+incompatible · fixed in 11.6.1+incompatible

• github.com/mattermost/mattermost-server/v5Go

  All versions

• github.com/mattermost/mattermost-server/v6Go

  All versions

• github.com/mattermost/mattermost/server/v8Go

  From 8.0.0-20250731163400-5b955468ea1e · fixed in 8.0.0-20260428151657-c79c3831061a

• github.com/mattermost/mattermost-serverGo

  From 11.6.0 · fixed in 11.6.1

• github.com/mattermost/mattermost-serverGo

  From 11.5.0 · fixed in 11.5.5

• github.com/mattermost/mattermost-serverGo

  From 10.11.0 · fixed in 10.11.17

References 1