CVE-2026-5740 - Unauthenticated WebSocket binary frame causes denial of service in Mattermost Server

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HIGH

Unauthenticated WebSocket binary frame causes denial of service in Mattermost Server

Published May 22, 2026

7.5

HIGH

CVSS 3.1

EPSS 0.57%

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

Vendor Mattermost Product Mattermost Default
unaffected
- Version
  10.11.0
  Status
  affected
  Constraints
  <=10.11.14
- Version
  11.4.0
  Status
  affected
  Constraints
  <=11.4.4
- Version
  11.5.0
  Status
  affected
  Constraints
  <=11.5.3
- Version
  11.6.0
  Status
  affected
  Constraints
  <=11.6.0
- Version
  10.11.15
  Status
  unaffected
  Constraints
  -
- Version
  11.4.5
  Status
  unaffected
  Constraints
  -
- Version
  11.5.4
  Status
  unaffected
  Constraints
  -
- Version
  11.6.1
  Status
  unaffected
  Constraints
  -
- Version
  11.7.0
  Status
  unaffected
  Constraints
  -

Default status is the baseline for the product, each version can override it (e.g. patched versions marked unaffected).

Vendor

Product

Default status

Versions

Mattermost

unaffected

Version	Status	Constraints
10.11.0	affected	<=10.11.14
11.4.0	affected	<=11.4.4
11.5.0	affected	<=11.5.3
11.6.0	affected	<=11.6.0
10.11.15	unaffected	-
11.4.5	unaffected	-
11.5.4	unaffected	-
11.6.1	unaffected	-
11.7.0	unaffected	-

github.com/mattermost/mattermost/server/v8

Go

Introduced 0 Fixed 8.0.0-20260410202636-17939826efa2
github.com/mattermost/mattermost-server/v5

Go

Introduced 0 Fixed not fixed
github.com/mattermost/mattermost-server/v6

Go

Introduced 0 Fixed not fixed
github.com/mattermost/mattermost-server

Go

Introduced 11.5.0+incompatible Fixed 11.5.4+incompatible
github.com/mattermost/mattermost-server

Go

Introduced 11.6.0+incompatible Fixed 11.6.1+incompatible
github.com/mattermost/mattermost-server

Go

Introduced 11.4.0+incompatible Fixed 11.4.5+incompatible
github.com/mattermost/mattermost-server

Go

Introduced 10.11.0+incompatible Fixed 10.11.15+incompatible

Ecosystem	Package	Introduced	Fixed
Go	github.com/mattermost/mattermost/server/v8	0	8.0.0-20260410202636-17939826efa2
Go	github.com/mattermost/mattermost-server/v5	0	not fixed
Go	github.com/mattermost/mattermost-server/v6	0	not fixed
Go	github.com/mattermost/mattermost-server	11.5.0+incompatible	11.5.4+incompatible
Go	github.com/mattermost/mattermost-server	11.6.0+incompatible	11.6.1+incompatible
Go	github.com/mattermost/mattermost-server	11.4.0+incompatible	11.4.5+incompatible
Go	github.com/mattermost/mattermost-server	10.11.0+incompatible	10.11.15+incompatible

Remediation

Vendor solution

Update Mattermost to versions 11.7.0, 11.6.1, 11.5.4, 11.4.5, 10.11.15 or higher.

Weaknesses (1)

CWE-789

References (5)

https://euvd.enisa.europa.eu/vulnerability/EUVD-2026-31426
Advisory
https://github.com/advisories/GHSA-w9m8-p4cc-4qj9
Advisory
https://github.com/mattermost/mattermost/commit/17939826efa20a97f087b3d390ec5136df350bae
https://mattermost.com/security-updates
vendor-advisoryVendor Advisory
https://nvd.nist.gov/vuln/detail/CVE-2026-5740

Link	Providers	Tags
https://euvd.enisa.europa.eu/vulnerability/EUVD-2026-31426		Advisory
https://github.com/advisories/GHSA-w9m8-p4cc-4qj9		Advisory
https://github.com/mattermost/mattermost/commit/17939826efa20a97f087b3d390ec5136df350bae
https://mattermost.com/security-updates		vendor-advisoryVendor Advisory
https://nvd.nist.gov/vuln/detail/CVE-2026-5740