CVE-2026-4265: Mattermost Server Auth Bypass Vulnerability

CVE-2026-4265 Overview

CVE-2026-4265 is an authorization bypass vulnerability affecting Mattermost Server that allows guest users to circumvent team-specific file upload permissions. The vulnerability exists due to improper validation of upload_file permissions when reusing file metadata across different teams. A malicious guest user can upload files in a team where they have permission, then reuse the file metadata in a POST request to post files in channels of a different team where they lack upload_file permission.

Critical Impact

Guest users can bypass file upload restrictions and post unauthorized files in restricted channels, potentially leading to information disclosure, policy violations, or abuse of privileged team spaces.

Affected Products

• Mattermost Server 11.3.x versions <= 11.3.0
• Mattermost Server 11.2.x versions <= 11.2.2
• Mattermost Server 10.11.x versions <= 10.11.10

Discovery Timeline

• 2026-03-16 - CVE-2026-4265 published to NVD
• 2026-03-18 - Last updated in NVD database

Technical Details for CVE-2026-4265

Vulnerability Analysis

This vulnerability is classified as CWE-863 (Incorrect Authorization) and stems from a failure to properly validate team-specific permissions during the file posting workflow. When a user uploads a file in Mattermost, the server generates file metadata that can be referenced in subsequent API requests. The authorization flaw occurs because the server validates upload_file permissions only at the initial upload stage but fails to re-validate these permissions when the file metadata is reused to post files in different team channels.

The attack requires network access and low-privilege authentication (a guest account), but does not require user interaction. The impact is limited to integrity violations, allowing unauthorized file posting in restricted channels without affecting confidentiality or availability of the system.

Root Cause

The root cause of CVE-2026-4265 is insufficient authorization validation in the Mattermost Server's file posting logic. The server architecture separates the file upload process from the channel posting process, but the permission checks for upload_file are not enforced at the posting stage. This creates a security gap where file metadata generated from a permitted upload can be transplanted into unauthorized contexts.

Specifically, the team-level upload_file permission is only checked when a file is initially uploaded, not when the file reference is subsequently used in a POST request to create a message with an attachment in a different team's channel.

Attack Vector

The attack follows this exploitation path:

1. The attacker authenticates as a guest user with upload_file permission in Team A
2. The attacker uploads a file to Team A, obtaining valid file metadata (file ID and references)
3. The attacker crafts a POST request to create a message in a channel within Team B (where they lack upload_file permission)
4. The attacker includes the file metadata from step 2 in the POST request body
5. The server fails to validate team-specific upload_file permissions and allows the file to be posted in Team B's channel

This attack leverages the network-accessible API and requires only guest-level authentication. The attacker must have valid credentials and membership in at least one team with upload permissions.

Detection Methods for CVE-2026-4265

Indicators of Compromise

• Unexpected file attachments appearing in channels from guest users who should not have upload permissions
• API requests containing file metadata references (file IDs) that originate from different team contexts
• Audit log entries showing file posts by guest users in teams with restricted upload permissions
• Correlation mismatches between file upload team context and file post team context

Detection Strategies

• Monitor Mattermost audit logs for guest user file posting activity, particularly in teams with restricted permissions
• Implement API request analysis to detect cross-team file metadata reuse patterns
• Configure alerting for file attachments posted by users without explicit upload_file permission in the target team
• Review guest user activity reports for anomalous posting behavior across multiple teams

Monitoring Recommendations

• Enable detailed audit logging for all file upload and posting operations in Mattermost
• Configure SIEM rules to correlate file upload events with subsequent file post events across different team contexts
• Implement periodic access control reviews to verify guest user permissions align with intended policies
• Monitor for unusual spikes in guest user file posting activity

How to Mitigate CVE-2026-4265

Immediate Actions Required

• Upgrade Mattermost Server to the latest patched version that addresses MMSA-2025-00553
• Review guest user permissions and consider temporarily restricting upload_file permissions until patching is complete
• Audit recent file posting activity by guest users to identify potential exploitation
• Implement network-level monitoring for suspicious API patterns if immediate patching is not possible

Patch Information

Mattermost has released security updates to address this vulnerability. Organizations should upgrade to patched versions as documented in the Mattermost Security Updates advisory. The fix ensures that team-specific upload_file permissions are validated at both the upload stage and the posting stage, preventing cross-team file metadata reuse attacks.

Affected versions require upgrade to:

• 11.3.x series: Upgrade beyond 11.3.0
• 11.2.x series: Upgrade beyond 11.2.2
• 10.11.x series: Upgrade beyond 10.11.10

Workarounds

• Temporarily revoke upload_file permissions for guest users across all teams until the patch can be applied
• Implement additional API gateway rules to block file posting requests that reference file metadata from different team contexts
• Configure channel moderation settings to require approval for file attachments from guest users
• Consider restricting guest users to a limited set of teams to reduce the attack surface

# Example: Review guest user permissions in Mattermost CLI
mmctl permissions role show Guest
# Audit recent file posts by guest users
mmctl post list --since "2026-03-01" --user-type guest --has-files