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MEDIUM 5.3

GHSA-3vmp-whvv-5v9v

Mattermost doesn't validate that a username returned during bot registration belongs to a bot account

Quick fix

GHSA-3vmp-whvv-5v9v — github.com/mattermost/mattermost-server: upgrade to the fixed version with the command below. 

go get github.com/mattermost/mattermost-server@v11.6.1

Details

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

Are you affected?

Enter the version of the package you're using.

Affected packages

Go / github.com/mattermost/mattermost-server
Introduced in: 11.6.0 Fixed in: 11.6.1
Fix go get github.com/mattermost/mattermost-server@v11.6.1
Go / github.com/mattermost/mattermost-server
Introduced in: 11.5.0 Fixed in: 11.5.5
Fix go get github.com/mattermost/mattermost-server@v11.5.5
Go / github.com/mattermost/mattermost-server
Introduced in: 10.11.0 Fixed in: 10.11.17
Fix go get github.com/mattermost/mattermost-server@v10.11.17
Go / github.com/mattermost/mattermost/server/v8
Introduced in: 8.0.0-20250731163400-5b955468ea1e Fixed in: 8.0.0-20260428151657-c79c3831061a
Fix go get github.com/mattermost/mattermost/server/v8@v8.0.0-20260428151657-c79c3831061a

References