Denial of service via malformed User-Agent header in getBrowserVersion

Summary

Mattermost versions 11.3.x <= 11.3.0, 11.2.x <= 11.2.2, 10.11.x <= 10.11.10 fail to properly validate User-Agent header tokens which allows an authenticated attacker to cause a request panic via a specially crafted User-Agent header. Mattermost Advisory ID: MMSA-2026-00586

Timeline

1. Mar 16, 2026, 12:04 PM UTCCVE published
2. Mar 16, 2026, 1:41 PM UTCCISA SSVC assessment

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

3. Mar 16, 2026, 1:49 PM UTCRecord last updated

Affected versions

• Mattermost Mattermost

  From 11.3.0 · through 11.3.0; From 11.2.0 · through 11.2.2; From 10.11.0 · through 10.11.10

• github.com/mattermost/mattermost-serverGo

  From 10.11.0-rc1+incompatible · fixed in 10.11.11+incompatible; From 11.2.0-rc1+incompatible · fixed in 11.2.3+incompatible; From 11.3.0-rc1+incompatible · fixed in 11.3.1+incompatible

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

  All versions

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

  All versions

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

  fixed in 8.0.0-20260129181235-1346cf529aef

• github.com/mattermost/mattermost-serverGo

  fixed in 5.3.2-0.20260129181235-1346cf529aef

• github.com/mattermost/mattermost-serverGo

  From 10.11.0-rc1 · fixed in 10.11.11

• github.com/mattermost/mattermost-serverGo

  From 11.2.0-rc1 · fixed in 11.2.3

• github.com/mattermost/mattermost-serverGo

  From 11.3.0-rc1 · fixed in 11.3.1

References 1