CVE-2026-19621: Mattermost Information Disclosure Vulnerability

CVE-2026-19621 Overview

CVE-2026-19621 is an information disclosure vulnerability in Mattermost that exposes action integration secrets to authenticated channel members. The affected versions fail to strip sensitive secrets from post payloads before broadcasting them through the post_deleted websocket event and the threads REST API. An authenticated user with channel membership can harvest server-side webhook URLs and authentication context from interactive-message posts. The flaw is tracked under Mattermost Advisory ID MMSA-2026-00766 and falls under [CWE-200: Exposure of Sensitive Information to an Unauthorized Actor].

Critical Impact
Authenticated channel members can extract webhook URLs and authentication secrets from interactive-message posts, enabling impersonation of trusted integrations and lateral access to backend services.

Affected Products

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

Discovery Timeline

2026-10-16 - CVE-2026-19621 published to NVD
2026-10-16 - Last updated in NVD database

Technical Details for CVE-2026-19621

Vulnerability Analysis

Mattermost supports interactive messages where posts contain action integration definitions. These definitions include server-side secrets used to authenticate callback requests to external webhook endpoints. The affected server versions fail to sanitize these secrets from post payloads before transmitting them to clients over two distinct channels. The first path is the post_deleted websocket event, which broadcasts the full post object (including integration metadata) to all channel members when a post is deleted. The second path is the threads REST API, which returns thread responses containing the original post payload without stripping sensitive fields.

An authenticated user who is a member of a channel containing interactive-message posts can passively collect these secrets. The attacker does not need to control the posts or trigger specific actions beyond standard channel participation and API access.

Root Cause

The root cause is missing output sanitization in post serialization logic. Mattermost strips integration secrets from standard post read paths, but the equivalent sanitization is absent from the websocket broadcast handler for post deletion and from the threads API response builder. The integration secret field is intended for server-only use, and its leakage violates the trust boundary between the Mattermost server and connected clients.

Attack Vector

The attack requires network access to the Mattermost server and valid credentials for an account with membership in a channel that contains interactive-message posts. The attacker connects to the websocket endpoint and listens for post_deleted events, or queries the threads REST API for posts containing action integrations. Both paths return the webhook URL and authentication secret embedded in the post payload. With these values, the attacker can forge requests to the external integration endpoint, impersonating legitimate Mattermost traffic and bypassing the shared-secret validation that integrations typically rely on.

No verified public exploit is available at this time. Technical details are described in the Mattermost Security Updates advisory.

Detection Methods for CVE-2026-19621

Indicators of Compromise

Unusual volume of websocket subscriptions to channels containing interactive-message integrations from a single authenticated session.
High-frequency calls to the threads REST API targeting threads with action integration posts.
Outbound requests to integration webhook URLs originating from source IPs not associated with the Mattermost server.

Detection Strategies

Audit Mattermost server logs for authenticated users accessing an abnormal number of threads or posts containing actions or integration fields.
Correlate webhook endpoint access logs with the Mattermost server IP to identify requests originating from unexpected sources.
Review websocket session telemetry for clients subscribing to channels and immediately enumerating historical posts.

Monitoring Recommendations

Enable verbose API access logging on the Mattermost server and forward logs to a centralized SIEM for query and correlation.
Monitor integration webhook endpoints for authentication failures or anomalous payload structures that suggest replayed or forged requests.
Track user-agent and session anomalies against the websocket endpoint /api/v4/websocket.

How to Mitigate CVE-2026-19621

Immediate Actions Required

Upgrade Mattermost to a patched release above 11.10.0, 11.9.1, 11.8.5, or 11.7.9 as applicable to your deployment branch.
Rotate all action integration secrets and webhook authentication tokens for interactive messages posted on affected channels.
Audit channel membership for interactive-message channels and remove accounts that no longer require access.

Patch Information

Mattermost has released fixed versions addressing MMSA-2026-00766. Administrators should consult the Mattermost Security Updates page for the exact patched build numbers for each supported release branch and apply updates following standard Mattermost upgrade procedures.

Workarounds

Temporarily disable interactive-message integrations that embed sensitive secrets until the server is patched.
Restrict channel membership for channels containing integration posts to trusted administrators only.
Enforce network-level access controls on integration webhook endpoints so they accept requests only from the Mattermost server IP range.

bash

# Verify installed Mattermost version before and after patching
mattermost version

# Example firewall rule limiting webhook endpoint access to Mattermost server
iptables -A INPUT -p tcp --dport 8443 -s <mattermost_server_ip> -j ACCEPT
iptables -A INPUT -p tcp --dport 8443 -j DROP