CVE-2026-3473 - Improper file ownership validation in the Boards API allows unauthorised file access

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HIGH

Improper file ownership validation in the Boards API allows unauthorised file access

Published May 22, 2026

7.1

HIGH

CVSS 3.1

EPSS 0.25%

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 validate file ownership and access control, which allows an authenticated user to access and download files belonging to other users or teams via crafted Boards API requests using valid file IDs.. Mattermost Advisory ID: MMSA-2026-00620

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

Go

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

Go

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

Go

Introduced 10.11.0+incompatible Fixed 10.11.15+incompatible
github.com/mattermost/mattermost-server

Go

Introduced 11.5.0+incompatible Fixed 11.5.4+incompatible

Ecosystem	Package	Introduced	Fixed
Go	github.com/mattermost/mattermost-server	11.4.0+incompatible	11.4.5+incompatible
Go	github.com/mattermost/mattermost-server	11.6.0+incompatible	11.6.1+incompatible
Go	github.com/mattermost/mattermost-server	10.11.0+incompatible	10.11.15+incompatible
Go	github.com/mattermost/mattermost-server	11.5.0+incompatible	11.5.4+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-639

Authorization Bypass

References (4)

https://euvd.enisa.europa.eu/vulnerability/EUVD-2026-31429
Advisory
https://github.com/advisories/GHSA-7pf2-9c95-w332
Advisory
https://mattermost.com/security-updates
vendor-advisoryVendor Advisory
https://nvd.nist.gov/vuln/detail/CVE-2026-3473

Link	Providers	Tags
https://euvd.enisa.europa.eu/vulnerability/EUVD-2026-31429		Advisory
https://github.com/advisories/GHSA-7pf2-9c95-w332		Advisory
https://mattermost.com/security-updates		vendor-advisoryVendor Advisory
https://nvd.nist.gov/vuln/detail/CVE-2026-3473