CVE-2026-5740Unauthenticated WebSocket binary frame causes denial of service in Mattermost Server
- 7.5HighCVSS base score
- 0.34%Exploitation in 30 daysEPSS · top 72% of all CVEs
- NoKnown exploitednot in CISA's KEV catalog
- NonePublic exploitsnone known yet
Description
Mattermost versions 11.6.x <= 11.6.0, 11.5.x <= 11.5.3, 11.4.x <= 11.4.4, 10.11.x <= 10.11.14 fail to properly validate msgpack-encoded WebSocket frames before memory allocation which allows an unauthenticated remote attacker to crash the server process and cause a full service outage for all users via a crafted binary WebSocket message sent to the public WebSocket endpoint.. Mattermost Advisory ID: MMSA-2026-00647
Timeline
ID reserved
By Mattermost
- 45 days later
· 45 days later
Published
Changes after publication (a new score, a public exploit, EPSS crossing 1, 10 or 50 %, leaving KEV) are kept since November 2026; earlier ones are not known.
Scores
| Scored by | CVSS | Score | Vector |
|---|---|---|---|
| MattermostCNA (assigner) | 3.1 | High7.5 | CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H |
| NVDNVD · Analyzed | 3.1 | High7.5 | CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H |
- EPSS (exploitation in 30 days)
- 0.34% · top 72% of all CVEs (12 Nov 2026)
- CISA SSVC
- Exploitation: none · Automatable: yes · Technical impact: partial
The list uses the assigner's score when there is one, then CISA's, then NVD's: High 7.5.
Affected products
- affected 11.6.0 – ≤ 11.6.0
- affected 11.5.0 – ≤ 11.5.3
- affected 11.4.0 – ≤ 11.4.4
- affected 10.11.0 – ≤ 10.11.14
- unaffected 11.7.0
- unaffected 11.6.1
- unaffected 11.5.4
- unaffected 11.4.5
- unaffected 10.11.15
Affected packages
From reviewed advisories of the GitHub Advisory Database.
- Gogithub.com/mattermost/mattermost-serverGHSA-w9m8-p4cc-4qj9 (opens in a new tab)
- Gogithub.com/mattermost/mattermost/server/v8GHSA-w9m8-p4cc-4qj9 (opens in a new tab)
What to do
Solution: Update Mattermost to versions 11.7.0, 11.6.1, 11.5.4, 11.4.5, 10.11.15 or higher.
References
References1
- MMSA-2026-00647 (opens in a new tab)vendor-advisory
Sources
The same CVE in the primary databases and aggregators, to check against.
- CVE.org record (opens in a new tab)
- CVE List JSON (GitHub) (opens in a new tab)
- NVD (opens in a new tab)
- FIRST EPSS (opens in a new tab)
- EUVD EUVD-2026-31426 (opens in a new tab)
- OSV.dev (opens in a new tab)
- GitHub Advisory GHSA-w9m8-p4cc-4qj9 (opens in a new tab)
- CIRCL Vulnerability-Lookup (opens in a new tab)
Badge for a README or a ticket
[](https://cveledger.app/cve/CVE-2026-5740)Record last updated .