Vulnerability · Published Mar 16, 2026 · Assigned by Mattermost
CVE-2026-25783Denial of service via malformed User-Agent header in getBrowserVersion
Medium 4.3
In plain English
A medium-severity security flaw in Mattermost from Mattermost. There are no confirmed reports of exploitation so far. The chance it is exploited in the next 30 days is estimated at 0.3% (higher than 23% of all known vulnerabilities). Fixed versions are available - upgrade the affected package.
Severity (CVSS 3.1)4.3Medium
Exploit odds (30 days)0.3%top 76.8% of all CVEs
Exploited in the wildNo reportsCISA SSVC: none
Attack toolingNoneno public tooling known
Official description
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 & fixed versions
| Ecosystem | Package | Fixed in | Advisory |
|---|---|---|---|
| Go | github.com/mattermost/mattermost-server | 11.2.3 | GHSA-2v3w-6g35-5f9v |
| Go | github.com/mattermost/mattermost-server | 5.3.2-0.20260129181235-1346cf529aef | GHSA-2v3w-6g35-5f9v |
| Go | github.com/mattermost/mattermost-server | 10.11.11 | GHSA-2v3w-6g35-5f9v |
| Go | github.com/mattermost/mattermost-server | 11.3.1 | GHSA-2v3w-6g35-5f9v |
| Go | github.com/mattermost/mattermost-server | 10.11.11+incompatible, 11.2.3+incompatible, 11.3.1+incompatible | GO-2026-4725 |
| Go | github.com/mattermost/mattermost-server/v5 | no fix listed | GO-2026-4725 |
| Go | github.com/mattermost/mattermost-server/v6 | no fix listed | GO-2026-4725 |
| Go | github.com/mattermost/mattermost/server/v8 | 8.0.0-20260129181235-1346cf529aef | GO-2026-4725 |
| Go | github.com/mattermost/mattermost/server/v8 | 8.0.0-20260129181235-1346cf529aef | GHSA-2v3w-6g35-5f9v |
Affected products (as reported)
| Vendor | Product | Versions |
|---|---|---|
| Mattermost | Mattermost | 11.3.0 – through 11.3.011.2.0 – through 11.2.210.11.0 – through 10.11.10 |
Weakness: CWE-1287 · Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:L
References
- MMSA-2026-00586 vendor-advisory