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Mattermost doesn't properly validate msgpack-encoded WebSocket frames before memory allocation

High severity GitHub Reviewed Published May 26, 2026 to the GitHub Advisory Database • Updated Jun 29, 2026

Package

gomod github.com/mattermost/mattermost-server (Go)

Affected versions

= 11.6.0
>= 11.5.0, < 11.5.4
>= 11.4.0, < 11.4.5
>= 10.11.0, < 10.11.15

Patched versions

11.6.1
11.5.4
11.4.5
10.11.15
gomod github.com/mattermost/mattermost/server/v8 (Go)
< 8.0.0-20260410202636-17939826efa2
8.0.0-20260410202636-17939826efa2

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

References

Published by the National Vulnerability Database May 22, 2026
Published to the GitHub Advisory Database May 26, 2026
Last updated Jun 29, 2026
Reviewed Jun 29, 2026

Severity

High

CVSS overall score

This score calculates overall vulnerability severity from 0 to 10 and is based on the Common Vulnerability Scoring System (CVSS).
/ 10

CVSS v3 base metrics

Attack vector
Network
Attack complexity
Low
Privileges required
None
User interaction
None
Scope
Unchanged
Confidentiality
None
Integrity
None
Availability
High

CVSS v3 base metrics

Attack vector: More severe the more the remote (logically and physically) an attacker can be in order to exploit the vulnerability.
Attack complexity: More severe for the least complex attacks.
Privileges required: More severe if no privileges are required.
User interaction: More severe when no user interaction is required.
Scope: More severe when a scope change occurs, e.g. one vulnerable component impacts resources in components beyond its security scope.
Confidentiality: More severe when loss of data confidentiality is highest, measuring the level of data access available to an unauthorized user.
Integrity: More severe when loss of data integrity is the highest, measuring the consequence of data modification possible by an unauthorized user.
Availability: More severe when the loss of impacted component availability is highest.
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

EPSS score

Exploit Prediction Scoring System (EPSS)

This score estimates the probability of this vulnerability being exploited within the next 30 days. Data provided by FIRST.
(27th percentile)

Weaknesses

Memory Allocation with Excessive Size Value

The product allocates memory based on an untrusted, large size value, but it does not ensure that the size is within expected limits, allowing arbitrary amounts of memory to be allocated. Learn more on MITRE.

CVE ID

CVE-2026-5740

GHSA ID

GHSA-w9m8-p4cc-4qj9

Source code

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