medium4.3
CVE-2026-25783
Mattermost fails to properly validate User-Agent header tokens
Impact
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
Affected packages
| Ecosystem | Package | Affected | Fixed in |
|---|---|---|---|
| go | github.com/mattermost/mattermost/server/v8 | < 8.0.0-20260129181235-1346cf529aef | 8.0.0-20260129181235-1346cf529aef |
| go | github.com/mattermost/mattermost-server | < 5.3.2-0.20260129181235-1346cf529aef | 5.3.2-0.20260129181235-1346cf529aef |
Affected products
| Vendor | Product | Affected versions |
|---|---|---|
| Mattermost | Mattermost Server | from 10.11.0 before 10.11.11 from 11.2.0 before 11.2.3 from 11.3.0 before 11.3.1 |