CVE-2026-2578: Mattermost Server Information Disclosure

CVE-2026-2578 Overview

CVE-2026-2578 is an Information Disclosure vulnerability affecting Mattermost Server versions 11.3.x <= 11.3.0. The vulnerability occurs when the application fails to preserve the redacted state of burn-on-read posts during deletion operations. This flaw allows channel members to access unrevealed burn-on-read message contents through the WebSocket post deletion event, potentially exposing sensitive information that was intended to be protected by the burn-on-read feature.

Critical Impact

Channel members can bypass the burn-on-read privacy feature and access confidential message contents that should remain redacted until explicitly revealed by the recipient.

Affected Products

• Mattermost Server versions 11.3.x <= 11.3.0
• Mattermost Enterprise Edition (if using affected server versions)
• Self-hosted and cloud deployments running vulnerable versions

Discovery Timeline

• 2026-03-16 - CVE-2026-2578 published to NVD
• 2026-03-18 - Last updated in NVD database

Technical Details for CVE-2026-2578

Vulnerability Analysis

This vulnerability falls under CWE-201 (Insertion of Sensitive Information Into Sent Data). The core issue lies in how Mattermost handles the deletion of burn-on-read posts when broadcasting WebSocket events to channel members.

Burn-on-read posts are a privacy feature designed to protect sensitive messages by keeping their contents hidden until the recipient explicitly chooses to view them. Once viewed, the message content is typically destroyed or marked as read. However, when a burn-on-read post is deleted, Mattermost Server versions 11.3.x <= 11.3.0 fail to maintain the redacted state of the message content within the WebSocket deletion event payload.

This means that even if a message was never revealed to a user, the full unredacted content may be included in the deletion event broadcast to all channel members subscribed to that channel's WebSocket feed.

Root Cause

The root cause stems from improper state management during post deletion operations. When constructing the WebSocket event for post deletion, the server does not properly check whether the post was a burn-on-read message or whether it had been revealed. Instead, it includes the full post content in the deletion event payload, bypassing the intended privacy controls that should keep unrevealed burn-on-read content hidden from unauthorized viewers.

Attack Vector

The attack vector is network-based and requires low privileges (authenticated channel membership). An attacker who is a member of a channel can exploit this vulnerability by:

1. Establishing a WebSocket connection to the Mattermost server as an authenticated user
2. Subscribing to channel events for channels they are a member of
3. Monitoring WebSocket messages for post deletion events
4. Extracting burn-on-read message contents from deletion event payloads before the messages were intended to be revealed

The vulnerability does not require any user interaction and can be exploited passively by simply monitoring WebSocket traffic. Any automated script or modified client could capture these events, making it trivial for malicious actors to collect sensitive information from burn-on-read messages that were never meant to be disclosed to them.

Detection Methods for CVE-2026-2578

Indicators of Compromise

• Unusual WebSocket connection patterns from specific users who maintain long-lived connections
• Clients logging or storing post deletion event payloads locally
• Third-party or modified Mattermost clients being used within the organization
• Unexpected data exfiltration patterns from users with access to sensitive channels

Detection Strategies

• Monitor WebSocket traffic for unusual patterns of deletion event consumption
• Audit client applications connecting to Mattermost for unauthorized modifications
• Review server logs for excessive WebSocket subscriptions to channels with sensitive content
• Implement network-level inspection for burn-on-read related API calls and WebSocket events

Monitoring Recommendations

• Enable detailed logging for WebSocket events and post deletion operations
• Deploy network monitoring to detect unusual data patterns in WebSocket communications
• Implement alerting for users connecting with unrecognized or modified clients
• Regularly audit channel memberships for users with access to sensitive burn-on-read content

How to Mitigate CVE-2026-2578

Immediate Actions Required

• Upgrade Mattermost Server to a patched version as soon as one is available
• Review and restrict channel membership for channels using burn-on-read functionality
• Audit recent burn-on-read message usage to assess potential exposure
• Consider temporarily disabling burn-on-read features until patched
• Notify users about the vulnerability and advise against sharing highly sensitive information via burn-on-read until a fix is applied

Patch Information

Mattermost has acknowledged this vulnerability under advisory MMSA-2026-00579. Organizations should monitor the Mattermost Security Updates page for patch availability and upgrade instructions. Apply the security patch immediately once released to remediate this vulnerability.

Workarounds

• Restrict channel membership to only trusted users for channels where burn-on-read is used
• Temporarily disable or discourage use of the burn-on-read feature until patched
• Implement network-level controls to monitor and audit WebSocket traffic
• Use alternative secure communication methods for highly sensitive information until the vulnerability is resolved
• Consider deploying a Web Application Firewall (WAF) to filter WebSocket traffic if feasible