CVE-2026-18795: Mattermost Auth Bypass Vulnerability

CVE-2026-18795 Overview

CVE-2026-18795 is a missing authorization vulnerability [CWE-862] affecting multiple Mattermost server versions. The flaw stems from the server failing to enforce Direct Message (DM) and Group Message (GM) membership checks before processing post quarantine requests. An authenticated user can invoke the quarantine API directly to act on posts within DM or GM conversations they do not belong to. Mattermost tracks the issue as advisory MMSA-2026-00756.

Critical Impact
Any authenticated Mattermost user can quarantine posts in private DM and GM channels by issuing a direct API call, bypassing membership-based access controls.

Affected Products

Mattermost 11.10.x <= 11.10.0
Mattermost 11.9.x <= 11.9.1
Mattermost 11.8.x <= 11.8.5 and 11.7.x <= 11.7.9

Discovery Timeline

2026-10-16 - CVE-2026-18795 published to NVD
2026-10-16 - Last updated in NVD database

Technical Details for CVE-2026-18795

Vulnerability Analysis

The vulnerability is a server-side broken access control issue in Mattermost's post quarantine workflow. The quarantine endpoint relies on client-side context to determine channel membership rather than validating that the caller participates in the target DM or GM channel. An authenticated user who knows or guesses a post_id can send a direct API request and successfully quarantine that post. The result is loss of integrity for private conversations and limited availability impact for the affected post content.

Root Cause

The server does not perform a DM/GM membership check during post quarantine handling. The authorization gate assumed clients would scope requests to channels the user belongs to, so no backend verification tied the authenticated session to the channel owning the referenced post. This is a classic missing-authorization defect [CWE-862] where enforcement exists in the user interface but not at the API layer.

Attack Vector

Exploitation requires an authenticated account on the Mattermost workspace, with no user interaction from the victim. The attacker sends a direct HTTP request to the post quarantine API referencing a post_id belonging to a DM or GM they are not a member of. Because the server skips membership verification, the quarantine action succeeds. Attack complexity is elevated because the attacker must know or discover valid post identifiers for private conversations.

No verified public exploit code is available. Refer to the Mattermost Security Updates advisory for additional technical context.

Detection Methods for CVE-2026-18795

Indicators of Compromise

Audit log entries showing post quarantine actions performed by users who are not members of the target DM or GM channel.
Unexpected POST requests to the Mattermost post quarantine API endpoint originating from standard (non-admin) user sessions.
User reports of missing or quarantined messages inside DM or GM conversations.

Detection Strategies

Correlate Mattermost audit logs for quarantine events against channel membership tables to identify unauthorized actions.
Alert on anomalous API usage patterns where a single authenticated user issues quarantine calls across many distinct channels.
Baseline normal quarantine activity volumes per user and flag deviations for review.

Monitoring Recommendations

Forward Mattermost application and audit logs to a centralized SIEM for retention and correlation.
Enable verbose API logging on the Mattermost server to capture endpoint, user ID, and target post_id for every quarantine request.
Review privileged activity dashboards for post moderation actions on a recurring cadence.

How to Mitigate CVE-2026-18795

Immediate Actions Required

Upgrade Mattermost server to a fixed release above 11.10.0, 11.9.1, 11.8.5, or 11.7.9 per the vendor advisory.
Review audit logs for prior quarantine actions and restore any posts quarantined without legitimate authorization.
Rotate API tokens for users who exhibit anomalous quarantine behavior pending investigation.

Patch Information

Mattermost has released fixed versions addressing advisory MMSA-2026-00756. Administrators should consult the Mattermost Security Updates page for the exact patched build numbers for each supported release train and apply the upgrade following standard change management procedures.

Workarounds

Restrict Mattermost network access to trusted users while patching is pending to reduce the pool of potential attackers.
Increase audit log review frequency to rapidly identify unauthorized quarantine actions until the patch is deployed.
Communicate with end users to report missing DM or GM messages so administrators can investigate possible exploitation.

bash

# Verify the running Mattermost server version before and after patching
curl -sS https://<mattermost-host>/api/v4/config/client?format=old | \
  grep -E '"Version"|"BuildNumber"'