CVE-2026-6333: Mattermost Server Auth Bypass Vulnerability

CVE-2026-6333 Overview

CVE-2026-6333 affects Mattermost Server versions 11.5.x up to 11.5.1 and 10.11.x up to 10.11.13. The flaw resides in how the server constructs response URLs for custom slash commands. Mattermost fails to validate the HTTP Host header before building the callback URL used to deliver slash command responses. An authenticated attacker can supply a spoofed Host header to redirect slash command responses to an attacker-controlled server. Mattermost tracks this issue as advisory MMSA-2026-00582. The vulnerability maps to [CWE-918] Server-Side Request Forgery (SSRF) and exposes outbound requests originating from the Mattermost server.

Critical Impact

Authenticated attackers can redirect slash command response traffic to attacker-controlled infrastructure by spoofing the Host header, enabling SSRF and data exposure scenarios.

Affected Products

• Mattermost Server 11.5.x versions up to and including 11.5.1
• Mattermost Server 10.11.x versions up to and including 10.11.13
• Mattermost custom slash command integrations relying on server-generated response URLs

Discovery Timeline

• 2026-05-18 - CVE-2026-6333 published to NVD
• 2026-05-19 - Last updated in NVD database

Technical Details for CVE-2026-6333

Vulnerability Analysis

Mattermost supports custom slash commands that allow integrations to receive command payloads and return responses asynchronously. When the server prepares a slash command invocation, it constructs response URLs that the integration can call back to. The server derives the host portion of these URLs from the inbound HTTP Host header without validating it against an allowlist or the configured SiteURL. An authenticated user who can issue requests to the slash command endpoint can therefore inject an arbitrary host value. The resulting response URL points at attacker infrastructure rather than the legitimate Mattermost deployment. This classifies as Server-Side Request Forgery because the server itself initiates outbound requests using the manipulated URL.

Root Cause

The root cause is missing input validation on the Host HTTP header during response URL construction. Mattermost trusts the client-supplied header value and concatenates it into the callback URL. The server should compare the header against the administratively configured SiteURL or an explicit allowlist before generating any URL used for outbound communication.

Attack Vector

Exploitation requires network access to the Mattermost server and a valid authenticated session. The attacker sends a crafted request to the slash command endpoint while setting the Host header to a domain they control. The server then issues outbound requests to that domain when delivering slash command responses, leaking command metadata, tokens, or other context-bearing payloads. No user interaction is required beyond the attacker's own authenticated session. No verified public proof-of-concept is currently available for CVE-2026-6333.

Detection Methods for CVE-2026-6333

Indicators of Compromise

• Outbound HTTP requests from the Mattermost server to domains that do not match the configured SiteURL.
• Slash command response traffic directed at unfamiliar hosts in web proxy or egress firewall logs.
• Inbound requests to slash command endpoints containing Host header values that differ from the canonical Mattermost FQDN.

Detection Strategies

• Inspect reverse proxy and load balancer logs for requests where the Host header does not match the expected Mattermost hostname.
• Correlate authenticated user sessions with outbound connections from the Mattermost server to non-allowlisted destinations.
• Review slash command integration audit logs for response URLs that resolve to unexpected external hosts.

Monitoring Recommendations

• Enable egress filtering on the Mattermost server and alert on any outbound HTTP/HTTPS traffic to destinations outside the approved integration list.
• Forward Mattermost access logs and proxy logs to a centralized analytics platform to baseline expected Host header values.
• Monitor for spikes in slash command invocations originating from a single account, which may indicate exploitation attempts.

How to Mitigate CVE-2026-6333

Immediate Actions Required

• Upgrade Mattermost Server to a fixed release beyond 11.5.1 for the 11.5 branch or beyond 10.11.13 for the 10.11 branch per the vendor advisory.
• Audit existing custom slash command integrations and rotate any tokens that may have been exposed through redirected callbacks.
• Restrict outbound network access from the Mattermost server to only the integration endpoints required for operation.

Patch Information

Mattermost has released fixed builds addressed in advisory MMSA-2026-00582. Refer to the Mattermost Security Updates page for the exact patched versions and release notes. Administrators should apply the upgrade according to the standard Mattermost deployment procedure for their installation type (Docker, Kubernetes Operator, or binary).

Workarounds

• Configure the reverse proxy in front of Mattermost to normalize or reject requests whose Host header does not match the canonical FQDN.
• Enforce strict egress firewall rules so the Mattermost server can only initiate outbound connections to known integration hosts.
• Temporarily disable custom slash command integrations until the upgrade is applied if patching cannot occur immediately.

# Example NGINX reverse proxy snippet enforcing the expected Host header
server {
    listen 443 ssl;
    server_name mattermost.example.com;

    if ($host != "mattermost.example.com") {
        return 421;
    }

    location / {
        proxy_set_header Host mattermost.example.com;
        proxy_pass http://mattermost_backend;
    }
}