Mattermost Shared Channel Upload Type Validation Bypass

Summary

Mattermost versions 10.8.x <= 10.8.3, 10.5.x <= 10.5.8, 9.11.x <= 9.11.17, 10.9.x <= 10.9.2, 10.10.x <= 10.10.0 fail to validate upload types in remote cluster upload sessions which allows a system admin to upload non-attachment file types via shared channels that could potentially be placed in arbitrary filesystem directories.

Timeline

1. Aug 21, 2025, 7:59 AM UTCCVE published
2. Aug 21, 2025, 1:57 PM UTCCISA SSVC assessment

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

3. Aug 21, 2025, 1:57 PM UTCRecord last updated

Affected versions

• Mattermost Mattermost

  From 10.8.0 · through 10.8.3; From 10.5.0 · through 10.5.8; From 9.11.0 · through 9.11.17; From 10.9.0 · through 10.9.2; From 10.10.0 · through 10.10.0

• github.com/mattermost/mattermost-serverGo

  From 10.8.0 · fixed in 10.8.4

• github.com/mattermost/mattermost-serverGo

  From 10.5.0 · fixed in 10.5.9

• github.com/mattermost/mattermost-serverGo

  From 9.11.0 · fixed in 9.11.18

• github.com/mattermost/mattermost-serverGo

  From 10.9.0 · fixed in 10.9.3

• github.com/mattermost/mattermost-serverGo

  From 10.10.0 · fixed in 10.10.1

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

  fixed in 8.0.0-20250708173752-d6b35c41f0ae5

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

  through 5.39.3

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

  through 5.7.2

• github.com/mattermost/mattermost-serverGo

  From 9.11.0+incompatible · fixed in 9.11.18+incompatible; From 10.5.0+incompatible · fixed in 10.5.9+incompatible; From 10.8.0+incompatible · fixed in 10.8.4+incompatible; From 10.9.0+incompatible · fixed in 10.9.3+incompatible; From 10.10.0+incompatible · fixed in 10.10.1+incompatible

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

  All versions

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

  All versions

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

  All versions

References 1