CVE-2026-3116: Mattermost Plugins DOS Vulnerability

CVE-2026-3116 Overview

CVE-2026-3116 is a Resource Exhaustion vulnerability affecting Mattermost Plugins that allows authenticated attackers to cause service disruption through the webhook endpoint. The vulnerability stems from a failure to properly validate incoming request size, classified under CWE-400 (Uncontrolled Resource Consumption).

This vulnerability enables authenticated users to send oversized requests to the webhook endpoint, potentially overwhelming server resources and causing denial of service conditions for legitimate users of the Mattermost collaboration platform.

Critical Impact

Authenticated attackers can exploit the webhook endpoint to cause service disruption through resource exhaustion, impacting availability of the Mattermost collaboration platform.

Affected Products

• Mattermost Plugins versions <=11.4
• Mattermost Plugins versions 11.0.4, 11.1.3, 11.3.2
• Mattermost Plugins version 10.11.11.0

Discovery Timeline

• 2026-03-26 - CVE-2026-3116 published to NVD
• 2026-03-26 - Last updated in NVD database

Technical Details for CVE-2026-3116

Vulnerability Analysis

This vulnerability resides in the request handling mechanism of Mattermost Plugins, specifically within the webhook endpoint processing logic. The root issue is the absence of proper input size validation before processing incoming requests. When an authenticated user sends a request to the webhook endpoint, the application fails to enforce size limits, allowing arbitrarily large payloads to be processed.

The attack requires authenticated access to the Mattermost instance, meaning an attacker would need valid credentials or session access. However, once authenticated, the exploitation is straightforward with no user interaction required. The impact is limited to availability—there is no compromise of confidentiality or integrity, as the vulnerability only enables denial of service conditions.

Root Cause

The vulnerability is caused by missing input validation on request size in the webhook endpoint handler. The Mattermost Plugins component does not implement proper bounds checking or rate limiting on incoming webhook requests, allowing resource exhaustion through oversized payloads. This is a classic CWE-400 (Uncontrolled Resource Consumption) pattern where insufficient controls on external input lead to resource depletion.

Attack Vector

The attack vector is network-based, requiring authenticated access to the Mattermost server. An attacker with valid credentials can target the webhook endpoint by sending excessively large HTTP requests. The attack process involves:

1. Authenticating to the Mattermost instance with valid credentials
2. Identifying accessible webhook endpoints within the plugin infrastructure
3. Sending oversized HTTP requests to the webhook endpoint
4. Repeating the process to exhaust server resources and cause service disruption

The vulnerability affects webhook processing, which is commonly used for integrations and automated workflows. By flooding this endpoint with large requests, attackers can degrade performance or completely disrupt service availability for all users of the Mattermost instance.

Detection Methods for CVE-2026-3116

Indicators of Compromise

• Abnormally large HTTP POST requests directed at webhook endpoints (significantly exceeding typical payload sizes)
• Spike in memory or CPU utilization on Mattermost server resources correlating with webhook activity
• Increased error rates or timeouts in webhook processing logs
• Degraded application performance or service unavailability following webhook traffic spikes

Detection Strategies

• Monitor HTTP request sizes to webhook endpoints and alert on requests exceeding expected thresholds
• Implement application-level logging for webhook request processing, including payload sizes and processing times
• Configure infrastructure monitoring to detect resource exhaustion patterns (memory, CPU, connection pool depletion)
• Analyze authentication logs to identify authenticated users generating excessive or unusually large webhook requests

Monitoring Recommendations

• Establish baseline metrics for normal webhook traffic volume and request sizes
• Configure alerting for sudden increases in resource consumption associated with webhook processing
• Review Mattermost server logs for repeated large request patterns from individual authenticated users
• Implement network-level monitoring to track request sizes before they reach the application layer

How to Mitigate CVE-2026-3116

Immediate Actions Required

• Update Mattermost Plugins to the latest patched version as soon as security updates are available
• Review and restrict webhook endpoint access to only necessary authenticated users
• Implement web application firewall (WAF) rules to limit request body size on webhook endpoints
• Monitor for exploitation attempts while awaiting patch deployment

Patch Information

Mattermost has issued security advisory MMSA-2026-00589 addressing this vulnerability. Organizations should consult the Mattermost Security Updates page for the latest patch information and upgrade instructions. Ensure all affected Mattermost Plugins versions (<=11.4, 11.0.4, 11.1.3, 11.3.2, and 10.11.11.0) are updated to patched releases.

Workarounds

• Configure reverse proxy or load balancer to enforce maximum request body size limits on webhook endpoints
• Implement rate limiting for authenticated users accessing webhook functionality
• Temporarily restrict webhook endpoint access to essential integrations only until patches can be applied
• Enable enhanced monitoring and alerting for webhook endpoint activity to detect exploitation attempts

# Example nginx configuration to limit request size on webhook endpoints
# Add to server or location block for Mattermost
location /plugins/webhook {
    client_max_body_size 1m;
    limit_req zone=webhook_limit burst=10 nodelay;
    proxy_pass http://mattermost_backend;
}