CVE-2025-14822: Mattermost Server DOS Vulnerability

CVE-2025-14822 Overview

CVE-2025-14822 is a CPU Exhaustion vulnerability affecting Mattermost Server versions 10.11.x through 10.11.8. The vulnerability stems from improper input validation when processing hashtags in posts. An authenticated attacker can exploit this flaw by sending a single HTTP request containing a post with thousands of space-separated tokens, causing the server to exhaust CPU resources and resulting in a denial of service condition.

Critical Impact

Authenticated attackers can trigger CPU exhaustion with a single malicious HTTP request, potentially rendering the Mattermost collaboration platform unavailable for all users.

Affected Products

• Mattermost Server versions 10.11.x <= 10.11.8

Discovery Timeline

• 2026-01-16 - CVE-2025-14822 published to NVD
• 2026-01-20 - Last updated in NVD database

Technical Details for CVE-2025-14822

Vulnerability Analysis

This vulnerability is classified under CWE-407 (Inefficient Algorithmic Complexity) and CWE-770 (Allocation of Resources Without Limits or Throttling). The core issue lies in the hashtag processing functionality within Mattermost Server, which fails to validate the size of input before processing. When a user creates a post containing an extremely large number of space-separated tokens, the server attempts to process each potential hashtag without implementing proper resource limits or input size validation.

The vulnerability is network-accessible and requires low attack complexity, though authentication is required. The impact is primarily on availability, as the CPU exhaustion can significantly degrade or completely halt server operations.

Root Cause

The root cause of CVE-2025-14822 is the absence of input size validation in the hashtag extraction and processing logic. The Mattermost Server parses incoming post content to identify and index hashtags for searchability. However, the implementation does not enforce limits on:

1. The total number of tokens that can be processed in a single post
2. The computational resources allocated to hashtag parsing operations
3. The maximum post content size that triggers hashtag processing

This allows an attacker to craft a payload with thousands of space-separated tokens, each requiring computational cycles to evaluate as a potential hashtag.

Attack Vector

The attack is executed over the network by an authenticated user submitting a malicious post through the Mattermost API or web interface. The attacker constructs a post body containing an excessive number of space-separated tokens (potentially thousands), which are then processed by the vulnerable hashtag parsing routine. Since the server does not limit input size before processing, a single request can consume significant CPU resources.

The attack mechanism involves submitting a post with thousands of space-separated tokens to the Mattermost API endpoint responsible for creating or editing posts. The server's hashtag processing routine iterates through each token without resource constraints, leading to CPU exhaustion. For detailed technical information, see the Mattermost Security Updates page.

Detection Methods for CVE-2025-14822

Indicators of Compromise

• Unusual CPU utilization spikes on Mattermost Server instances, particularly during post creation operations
• HTTP POST requests to message/post endpoints containing abnormally large payloads with high token counts
• Server response time degradation or timeout errors correlating with specific user activity
• Log entries indicating prolonged processing times for post submissions

Detection Strategies

• Monitor API endpoints for POST requests to /api/v4/posts or similar endpoints with unusually large request body sizes
• Implement alerting on CPU utilization thresholds for Mattermost Server processes
• Deploy Web Application Firewall (WAF) rules to detect and block requests with excessive space-separated tokens in post content
• Review application logs for abnormal hashtag processing durations

Monitoring Recommendations

• Configure resource monitoring dashboards to track CPU usage patterns on Mattermost Server instances
• Set up automated alerts for sustained CPU spikes exceeding normal operational baselines
• Enable detailed request logging on reverse proxies or load balancers fronting Mattermost deployments
• Implement rate limiting on post creation endpoints to limit potential abuse

How to Mitigate CVE-2025-14822

Immediate Actions Required

• Update Mattermost Server to a patched version beyond 10.11.8 immediately
• Review access controls and audit user accounts with posting privileges
• Implement rate limiting on message posting endpoints at the network layer
• Configure resource limits (CPU quotas, cgroups) for Mattermost Server processes as a defense-in-depth measure

Patch Information

Mattermost has addressed this vulnerability in versions following 10.11.8. Organizations should upgrade to the latest available version of Mattermost Server. Patch details and updated packages can be obtained from the official Mattermost Security Updates page.

Workarounds

• Implement WAF rules to limit the maximum size of POST request bodies to message endpoints
• Configure reverse proxy or load balancer request size limits to reject abnormally large post submissions
• Apply network-level rate limiting to throttle excessive requests from individual authenticated users
• Consider temporarily restricting posting privileges for non-essential users until patches are applied

# Example nginx rate limiting configuration for Mattermost
# Add to server block or location block for Mattermost proxy

# Define rate limiting zone
limit_req_zone $binary_remote_addr zone=mattermost_posts:10m rate=10r/s;

# Apply to post creation endpoint
location /api/v4/posts {
    limit_req zone=mattermost_posts burst=20 nodelay;
    client_max_body_size 1m;
    proxy_pass http://mattermost_backend;
}