CVE-2026-2325: Mattermost Server DOS Vulnerability

CVE-2026-2325 Overview

CVE-2026-2325 is a resource exhaustion vulnerability in Mattermost Server that affects the start meeting API endpoint. The flaw stems from missing request body size limits on /api/v1/meetings, allowing an authenticated attacker to submit oversized HTTP POST requests. The server consumes excessive memory and processing resources when handling these crafted requests, leading to denial of service conditions. Mattermost tracks this issue under advisory MMSA-2026-00608 and classifies it under CWE-770: Allocation of Resources Without Limits or Throttling. The vulnerability impacts availability without compromising data confidentiality or integrity.

Critical Impact

Authenticated users can exhaust server resources and disrupt Mattermost availability for all users by sending oversized requests to the meetings API.

Affected Products

• Mattermost Server versions 11.5.x up to and including 11.5.1
• Mattermost Server versions 10.11.x up to and including 10.11.13
• Mattermost Server versions 11.4.x up to and including 11.4.3

Discovery Timeline

• 2026-05-18 - CVE-2026-2325 published to NVD
• 2026-05-18 - Last updated in NVD database

Technical Details for CVE-2026-2325

Vulnerability Analysis

The vulnerability resides in the meeting initiation handler exposed at /api/v1/meetings. The endpoint accepts HTTP POST requests without enforcing an upper bound on the request body size. An authenticated attacker can submit a crafted oversized payload that forces the server to allocate excessive memory and CPU cycles while parsing the request. Repeated requests, or even a single sufficiently large request, can degrade service or crash the process. The EPSS score of 0.042% reflects low observed exploitation activity, but exploitation requires only standard authenticated access, which lowers the practical barrier inside collaboration platforms.

Root Cause

The root cause is the absence of input size validation and throttling on the start meeting API. The handler reads and processes the entire request body before applying any business logic checks. Without a configured maximum body size or streaming guard, the server treats arbitrarily large payloads as valid input. This pattern maps directly to CWE-770, where resources are allocated without proportional limits relative to the requester.

Attack Vector

Exploitation requires network access to the Mattermost API and valid user credentials. An attacker authenticates as any standard user and issues an HTTP POST request to /api/v1/meetings with a deliberately oversized body. The server attempts to parse and buffer the payload, consuming memory and CPU. Repeated parallel requests amplify the impact and can render the Mattermost instance unresponsive for legitimate users. No user interaction or elevated privileges are needed beyond a standard authenticated session.

No verified public proof-of-concept code is available. Refer to the Mattermost Security Updates advisory for vendor-supplied technical details.

Detection Methods for CVE-2026-2325

Indicators of Compromise

• Unusually large HTTP POST requests directed at the /api/v1/meetings endpoint, particularly bodies exceeding typical meeting payload sizes of a few kilobytes.
• Sudden memory or CPU spikes on Mattermost application servers correlated with authenticated API traffic.
• Repeated 5xx responses or worker process restarts originating from the meeting service handler.

Detection Strategies

• Inspect reverse proxy and web server access logs for POST requests to /api/v1/meetings with Content-Length headers above expected thresholds.
• Correlate authenticated session identifiers against request volume to identify single accounts generating disproportionate API load.
• Deploy web application firewall rules that flag oversized request bodies to Mattermost API paths.

Monitoring Recommendations

• Track Mattermost process memory consumption and garbage collection pauses through APM or host telemetry.
• Alert on sustained increases in API error rates, request latency, or queue depth on the meeting endpoint.
• Forward Mattermost and reverse proxy logs to a centralized log analytics platform for anomaly detection on payload size and request frequency.

How to Mitigate CVE-2026-2325

Immediate Actions Required

• Upgrade Mattermost Server to a fixed release above 11.5.1, 10.11.13, or 11.4.3 as specified in the vendor advisory.
• Restrict access to the Mattermost API to trusted network ranges where feasible, reducing the attack surface for authenticated abuse.
• Audit recent authentication and API activity for signs of abusive request patterns against /api/v1/meetings.

Patch Information

Mattermost has released patched versions addressing CVE-2026-2325 under advisory MMSA-2026-00608. Administrators should consult the Mattermost Security Updates page for the corresponding fixed versions and apply the upgrade following standard Mattermost deployment procedures.

Workarounds

• Configure the upstream reverse proxy, such as NGINX or HAProxy, to enforce a strict maximum request body size for the /api/v1/meetings path.
• Apply per-user rate limiting at the proxy or API gateway layer to throttle excessive requests to meeting endpoints.
• Disable or restrict the meetings feature for untrusted user roles until the patch is applied.

# NGINX example: cap request body size for the meetings endpoint
location /api/v1/meetings {
    client_max_body_size 64k;
    limit_req zone=mm_api burst=5 nodelay;
    proxy_pass http://mattermost_upstream;
}