CVE-2026-4265: Guest user can upload files without permission across teams

Severity: medium (CVSS 4.3 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.2% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2026-03-16 · updated 2026-06-17
Weakness: CWE-863

Description

Mattermost versions 11.3.x <= 11.3.0, 11.2.x <= 11.2.2, 10.11.x <= 10.11.10 fail to validate team-specific upload_file permissions which allows a guest user to post files in channels where they lack upload_file permission via uploading files in a team where they have permission and reusing the file metadata in a POST request to a different team. Mattermost Advisory ID: MMSA-2025-00553

Affected products

Mattermost: from 10.11.0 before 10.11.11; from 11.2.0 before 11.2.3; from 11.3.0 before 11.3.1 — fixed in 10.11.11, 11.2.3, 11.3.1

How to fix

Upgrade to a fixed version: Mattermost 10.11.11, 11.2.3, 11.3.1. Check the vendor advisory for mitigations.

References

https://mattermost.com/security-updates

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