CVE-2026-5308: Mattermost Server DoS Vulnerability

CVE-2026-5308 Overview

CVE-2026-5308 is a denial of service vulnerability in Mattermost Server. The flaw exists because plugin HTTP endpoints fail to enforce request body size limits. An attacker can send crafted oversized HTTP requests to exhaust server resources and disrupt service availability.

The vulnerability affects Mattermost Server versions 11.6.x up to 11.6.0, 11.5.x up to 11.5.3, 11.4.x up to 11.4.4, and 10.11.x up to 10.11.14. Mattermost tracks this issue as advisory MMSA-2026-00646. The weakness maps to [CWE-400] Uncontrolled Resource Consumption.

Critical Impact

Unauthenticated remote attackers can trigger denial of service against Mattermost servers by sending oversized HTTP requests to plugin endpoints, impacting collaboration availability for all users.

Affected Products

• Mattermost Server 11.6.x through 11.6.0
• Mattermost Server 11.5.x through 11.5.3, and 11.4.x through 11.4.4
• Mattermost Server 10.11.x through 10.11.14

Discovery Timeline

• 2026-05-22 - CVE-2026-5308 published to NVD
• 2026-05-22 - Last updated in NVD database

Technical Details for CVE-2026-5308

Vulnerability Analysis

Mattermost extends server functionality through plugins that register HTTP endpoints. These endpoints accept inbound HTTP requests routed through the Mattermost API surface. The affected versions do not validate or cap the size of incoming request bodies before processing.

An attacker can submit HTTP requests containing oversized payloads to any plugin endpoint. The server allocates memory and processing resources proportional to the request body. Repeated or sufficiently large requests exhaust available resources, causing service degradation or full denial of service.

The vulnerability requires no authentication and no user interaction. The attack originates over the network against any reachable Mattermost instance running an affected version with plugins enabled. Impact is limited to availability, with no confidentiality or integrity consequences.

Root Cause

The root cause is missing enforcement of request body size limits on plugin HTTP endpoints. The HTTP request handler accepts arbitrarily large bodies without applying a maximum size policy. This omission allows uncontrolled resource consumption [CWE-400].

Attack Vector

An unauthenticated remote attacker crafts an oversized HTTP request targeting a plugin endpoint exposed by the Mattermost server. The attacker sends one or more such requests to exhaust memory, CPU, or network buffers. The result is denial of service against the Mattermost instance, blocking legitimate collaboration traffic.

No verified exploit code is publicly available. The EPSS score indicates a low probability of exploitation in the near term, but the network-accessible, unauthenticated nature of the flaw warrants prompt patching.

Detection Methods for CVE-2026-5308

Indicators of Compromise

• HTTP requests with abnormally large Content-Length headers targeting /plugins/ paths on the Mattermost server.
• Repeated POST or PUT requests to plugin endpoints from a single source IP within short intervals.
• Sudden spikes in Mattermost process memory consumption or CPU utilization without corresponding user activity.
• Mattermost server logs showing request handler errors, timeouts, or out-of-memory conditions.

Detection Strategies

• Inspect reverse proxy or load balancer logs for requests to /plugins/* routes exceeding expected payload sizes.
• Correlate Mattermost server resource metrics with inbound HTTP traffic volume to identify resource exhaustion patterns.
• Deploy web application firewall rules that flag oversized request bodies sent to plugin endpoints.

Monitoring Recommendations

• Alert on memory utilization above defined thresholds on Mattermost application servers.
• Track HTTP 5xx response rates and request duration metrics for plugin endpoint paths.
• Forward Mattermost server logs and reverse proxy access logs to a centralized SIEM for correlation and retention.

How to Mitigate CVE-2026-5308

Immediate Actions Required

• Upgrade Mattermost Server to a fixed release above the affected version ranges as published by Mattermost.
• Restrict network exposure of the Mattermost server to trusted networks until the patch is applied.
• Enforce request body size limits at an upstream reverse proxy or load balancer protecting Mattermost.

Patch Information

Mattermost has released patched versions addressing CVE-2026-5308 under advisory MMSA-2026-00646. Refer to the Mattermost Security Updates page for fixed release numbers and upgrade guidance. Administrators should plan upgrades from affected branches 10.11.x, 11.4.x, 11.5.x, and 11.6.x to the corresponding remediated builds.

Workarounds

• Configure an upstream reverse proxy such as NGINX or HAProxy to enforce a maximum client body size on requests routed to /plugins/ endpoints.
• Disable non-essential plugins to reduce the attack surface until patches are deployed.
• Apply rate limiting on plugin endpoint paths to slow abuse from any single source.

# Example NGINX configuration enforcing a body size limit on plugin routes
location /plugins/ {
    client_max_body_size 1m;
    limit_req zone=plugin_zone burst=10 nodelay;
    proxy_pass http://mattermost_backend;
}