CVE-2026-4265 in Mattermost
Summary
by MITRE • 03/16/2026
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
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Analysis
by VulDB Data Team • 03/21/2026
This vulnerability represents a critical access control flaw in Mattermost's permission validation system that enables unauthorized file sharing across team boundaries. The issue stems from insufficient validation of team-specific upload permissions within the file sharing mechanism, allowing guest users to bypass intended security controls through a sophisticated metadata reuse attack pattern. The vulnerability affects multiple version streams including 11.3.x up to 11.3.0, 11.2.x up to 11.2.2, and 10.11.x up to 10.11.10, indicating a widespread impact across the platform's release cycles.
The technical exploitation occurs through a two-step process where malicious guest users first upload files to a team where they possess upload_file permissions, then leverage the resulting file metadata to post those same files in channels of different teams where they normally lack upload privileges. This approach effectively circumvents the intended permission boundaries by reusing valid file metadata while bypassing the actual file upload validation checks that should occur for each individual team and channel combination. The flaw demonstrates a clear violation of the principle of least privilege and represents a classic case of improper access control validation.
From an operational impact perspective, this vulnerability creates a significant security risk for organizations relying on Mattermost for collaborative communications, as it allows unauthorized file sharing between teams that should remain isolated. The attack vector enables guest users to potentially access and distribute sensitive information across team boundaries without proper authorization, undermining the intended security model of team-based access controls. This could lead to data leakage, unauthorized information sharing, and potential compliance violations in regulated environments where team isolation is mandatory.
The vulnerability aligns with CWE-285, which addresses improper authorization in access control systems, and represents a specific implementation flaw in the permission validation logic. From an ATT&CK framework perspective, this vulnerability maps to privilege escalation techniques through the use of valid file metadata to bypass access controls, potentially enabling information disclosure and lateral movement within the platform. Organizations should immediately implement the vendor-recommended patches and consider monitoring for unauthorized file sharing activities, particularly in scenarios involving guest users and cross-team file transfers. The mitigation strategy should include immediate version upgrades to patched releases and enhanced monitoring of file upload patterns to detect anomalous cross-team sharing behaviors.