What is the CVSS score of EUVD-2026-31426?

EUVD-2026-31426 has a CVSS 3.1 base score of 7.5 (High). CVSS vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H. EPSS exploitation probability: 0.1%.

Which versions of Mattermost are affected by EUVD-2026-31426?

Mattermost server versions 10.11.0-10.11.14, 11.4.0-11.4.4, 11.5.0-11.5.3, and 11.6.0 are vulnerable to unauthenticated denial-of-service attacks that can crash the entire service, causing a complete…. A patch is available.

How to fix or mitigate EUVD-2026-31426?

Within 24 hours: inventory all Mattermost deployments and confirm current versions; implement temporary network-level restrictions on WebSocket access from untrusted networks. Within 7 days: test upgrade paths in staging environment and develop deployment plan for patched version once vendor releases it; enable incident response monitoring for service crashes. Within 30 days: deploy patched version to all production instances once available from vendor, or maintain permanent network compensating controls if vendor timeline extends beyond 30 days.

Mattermost EUVD-2026-31426

| CVE-2026-5740 HIGH

Memory Allocation with Excessive Size Value (CWE-789)

2026-05-22 Mattermost

GHSA-w9m8-p4cc-4qj9

Denial Of Service Mattermost

7.5

CVSS 3.1 · NVD

Severity by source

NVD PRIMARY

7.5 HIGH

AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

Primary rating from NVD · only source for this CVE.

CVSS VectorNVD

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

Attack Vector

Network

Attack Complexity

Low

Privileges Required

None

User Interaction

None

Scope

Unchanged

Confidentiality

None

Integrity

None

Availability

High

Lifecycle Timeline

Analysis Generated

May 22, 2026 - 11:30 vuln.today

DescriptionCVE.org

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

AnalysisAI

Denial of service in Mattermost server (versions 11.6.0, 11.5.0-11.5.3, 11.4.0-11.4.4, and 10.11.0-10.11.14) allows remote attackers to crash the server process by sending a crafted msgpack-encoded binary WebSocket frame to the public endpoint. The flaw stems from missing validation of frame sizes before memory allocation, enabling a full service outage for all users. No public exploit identified at time of analysis, and the CVSS 7.5 score reflects the unauthenticated network-reachable nature of the attack with high availability impact.

Technical ContextAI

Mattermost is an open-source team collaboration platform whose real-time messaging layer uses WebSockets to deliver presence, channel, and message events to connected clients. This issue resides in the WebSocket frame handler that decodes binary payloads using msgpack (MessagePack), a compact binary serialization format. The root cause maps to CWE-789 (Memory Allocation with Excessive Size Value): the server reads size or length fields from an attacker-controlled msgpack-encoded frame and allocates memory based on those values without first bounding or sanity-checking them, leading to resource exhaustion or process termination. Because the WebSocket endpoint is exposed to anonymous clients during the connection handshake, no prior session or token is required to reach the vulnerable code path.

RemediationAI

Patch available per vendor advisory MMSA-2026-00647 - administrators should upgrade to a fixed release on their current branch (a release higher than 11.6.0, 11.5.3, 11.4.4, or 10.11.14 respectively) as published at https://mattermost.com/security-updates; exact fix versions should be confirmed against that advisory before deployment. If immediate patching is not possible, restrict reachability of the WebSocket endpoint by placing Mattermost behind a reverse proxy or WAF that terminates and inspects WebSocket traffic, source-IP-allowlist the endpoint to corporate networks or VPN ranges, and rate-limit new WebSocket connections - these controls reduce exposure but will break legitimate mobile/desktop client access from outside the allowlist and may not stop a low-volume attack from an authorized network. Monitor the server process for unexpected restarts and configure the service supervisor (systemd, Kubernetes liveness probe) for fast restart so outages are bounded while the patch is rolled out.