Mattermost <11.3.0/11.2.2/10.11.10: Guest upload_file bypass via metadata reuse
CVE-2026-4265 Published on March 16, 2026
Guest user can upload files without permission across teams
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
Vulnerability Analysis
CVE-2026-4265 can be exploited with network access, and requires small amount of user privileges. This vulnerability is considered to have a low attack complexity. The potential impact of an exploit of this vulnerability is considered to have no impact on confidentiality, with no impact on integrity, and no impact on availability.
Weakness Type
What is an AuthZ Vulnerability?
The software performs an authorization check when an actor attempts to access a resource or perform an action, but it does not correctly perform the check. This allows attackers to bypass intended access restrictions.
CVE-2026-4265 has been classified to as an AuthZ vulnerability or weakness.
Products Associated with CVE-2026-4265
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Affected Versions
Mattermost:- Version 11.3.0, <= 11.3.0 is affected.
- Version 11.2.0, <= 11.2.2 is affected.
- Version 10.11.0, <= 10.11.10 is affected.
- Version 11.4.0 is unaffected.
- Version 11.3.1 is unaffected.
- Version 11.2.3 is unaffected.
- Version 10.11.11 is unaffected.
Exploit Probability
EPSS (Exploit Prediction Scoring System) scores estimate the probability that a vulnerability will be exploited in the wild within the next 30 days. The percentile shows you how this score compares to all other vulnerabilities.