High severity · published May 22, 2026
CVE-2026-3473
Improper file ownership validation in the Boards API allows unauthorised file access
7.1
CVSS
0.1%
EPSS
No
On KEV
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
CVSS vector
CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:L/A:N
Attack vector
Network
Complexity
Low
Privileges
Low
User interaction
None
Confidentiality
High
Integrity
Low
Availability
None
Scope
Unchanged
Exploitation
Is it being used?
Exploit prediction (EPSS)0.1% · higher than 4% of CVEs
Exploited in the wildNo evidence yet
CISA KEVNot listed
Ransomware campaign useNot known
Affected products
Which versions
MA
Mattermost · Mattermost
11.6.0 through 11.6.0; 11.5.0 through 11.5.3; 11.4.0 through 11.4.4; 10.11.0 through 10.11.14 · source: vendor
MS
Mattermost Server · Mattermost
10.11.0 before 10.11.15; 11.4.0 before 11.4.5; 11.5.0 before 11.5.4; 11.6.0 before 11.6.1 · source: NVD
References
NVD
https://nvd.nist.gov/vuln/detail/CVE-2026-3473
CVE.org
https://www.cve.org/CVERecord?id=CVE-2026-3473
Vendor Advisory
https://mattermost.com/security-updates
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