CVE-2026-26233: Mattermost Server DoS Vulnerability

CVE-2026-26233 Overview

CVE-2026-26233 is a Denial of Service vulnerability in Mattermost Server caused by a failure to implement rate limiting on login requests. This vulnerability allows unauthenticated remote attackers to crash and restart the server by exploiting HTTP/2 single packet attacks with 100 or more parallel login requests. The lack of proper rate limiting on authentication endpoints makes the server susceptible to resource exhaustion attacks.

Critical Impact

Unauthenticated attackers can cause server crashes and restarts, disrupting team communication and collaboration services for all Mattermost users.

Affected Products

• Mattermost Server versions 11.4.x <= 11.4.0
• Mattermost Server versions 11.3.x <= 11.3.1
• Mattermost Server versions 11.2.x <= 11.2.3
• Mattermost Server versions 10.11.x <= 10.11.11

Discovery Timeline

• 2026-03-25 - CVE-2026-26233 published to NVD
• 2026-03-26 - Last updated in NVD database

Technical Details for CVE-2026-26233

Vulnerability Analysis

This vulnerability stems from insufficient request rate limiting on the Mattermost Server login endpoint. When the server receives a large volume of concurrent login requests, particularly through HTTP/2 multiplexing capabilities, it fails to properly throttle or limit these requests. The HTTP/2 single packet attack technique allows an attacker to bundle 100 or more login requests into a single network packet, bypassing traditional network-level rate limiting that might exist at the infrastructure layer. This overwhelms the server's authentication processing resources, leading to service disruption. The weakness is classified as CWE-400 (Uncontrolled Resource Consumption), indicating the server does not adequately manage the computational resources consumed when processing authentication requests.

Root Cause

The root cause is the absence of proper rate limiting mechanisms on the login API endpoint. Mattermost Server fails to implement controls that would limit the number of authentication requests processed within a given time window from a single source or in aggregate. This allows attackers to flood the authentication system with requests faster than the server can handle, exhausting available resources such as CPU, memory, and database connections dedicated to authentication processing.

Attack Vector

The attack is network-based and can be executed by any unauthenticated remote attacker with network access to the Mattermost Server. The attacker leverages HTTP/2's multiplexing feature to send numerous login requests simultaneously within a single TCP connection. By crafting a single network packet containing 100 or more parallel login requests, the attacker can bypass network-level protections and deliver a concentrated burst of requests directly to the application layer. The server attempts to process all requests concurrently, resulting in resource exhaustion that triggers a crash and subsequent restart.

The vulnerability is exploitable without any authentication or special privileges, requiring only network connectivity to the target server's login endpoint.

Detection Methods for CVE-2026-26233

Indicators of Compromise

• Sudden spikes in failed authentication attempts from single or multiple source IP addresses
• Abnormally high HTTP/2 connection multiplexing with numerous concurrent login requests
• Server crash logs showing resource exhaustion during authentication processing
• Unexpected Mattermost Server restarts correlating with high login request volumes

Detection Strategies

• Implement network monitoring to detect HTTP/2 connections with unusually high numbers of concurrent streams targeting the login endpoint
• Configure Web Application Firewall (WAF) rules to alert on burst patterns of authentication requests
• Monitor server resource metrics (CPU, memory, database connections) for anomalous spikes during login processing
• Review Mattermost Server logs for patterns indicating repeated authentication request floods

Monitoring Recommendations

• Set up alerting thresholds for login request rates that exceed normal operational baselines
• Monitor for server availability disruptions and correlate with authentication endpoint traffic patterns
• Implement distributed logging to capture authentication request patterns across load-balanced deployments
• Track HTTP/2 connection statistics for signs of multiplexing abuse

How to Mitigate CVE-2026-26233

Immediate Actions Required

• Upgrade Mattermost Server to the latest patched version that addresses the rate limiting vulnerability
• Implement external rate limiting at the reverse proxy or load balancer level for the login endpoint
• Configure WAF rules to limit the number of authentication requests per source IP within a time window
• Consider temporarily enabling HTTP/2 request limiting or disabling HTTP/2 if the server cannot be immediately patched

Patch Information

Mattermost has released security updates addressing this vulnerability. Organizations should upgrade to patched versions as documented in the Mattermost Security Updates page. The advisory for this vulnerability is tracked as MMSA-2025-00566.

Workarounds

• Deploy rate limiting at the reverse proxy layer (e.g., nginx, HAProxy) to throttle requests to the /api/v4/users/login endpoint
• Configure connection limits for HTTP/2 to restrict the number of concurrent streams per connection
• Implement geographic or IP-based access restrictions if Mattermost is only accessed from known networks
• Use a CDN or DDoS protection service with application-layer rate limiting capabilities

# Example nginx rate limiting configuration for Mattermost login endpoint
limit_req_zone $binary_remote_addr zone=login_limit:10m rate=10r/s;

location /api/v4/users/login {
    limit_req zone=login_limit burst=20 nodelay;
    proxy_pass http://mattermost_backend;
}