# CVE-2026-5740

## Summary

- **CVE ID:** CVE-2026-5740
- **Severity:** HIGH
- **CVSS Score:** 7.5 (CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H)
- **CWE:** CWE-789
- **Published:** May 22, 2026
- **Last Modified:** May 22, 2026

## 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

## Affected Products

- Mattermost — Mattermost (11.6.0)
- Mattermost — Mattermost (11.5.0)
- Mattermost — Mattermost (11.4.0)
- Mattermost — Mattermost (10.11.0)
- Mattermost — Mattermost (11.7.0)
- Mattermost — Mattermost (11.6.1)
- Mattermost — Mattermost (11.5.4)
- Mattermost — Mattermost (11.4.5)
- Mattermost — Mattermost (10.11.15)

## References

- [CNA](https://mattermost.com/security-updates)

## Exploitation Prediction (EPSS)

- **EPSS Score:** 0.34%
- **EPSS Percentile:** 27.1

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_Exported from OnDuty AI Vulnerability Intelligence on 2026-09-10._