CVE-2026-24458: Mattermost Server DOS Vulnerability

CVE-2026-24458 Overview

CVE-2026-24458 is a resource exhaustion vulnerability affecting Mattermost Server that allows unauthenticated attackers to overload the server's CPU and memory resources. The vulnerability stems from improper handling of excessively long passwords during login attempts, enabling attackers to submit multi-megabyte password strings that consume significant computational resources during processing.

Critical Impact
Unauthenticated attackers can cause denial of service conditions on Mattermost Server instances by submitting login requests with extremely large password payloads, potentially disrupting team communications and collaboration.

Affected Products

Mattermost Server version 11.3.x up to and including 11.3.0
Mattermost Server version 11.2.x up to and including 11.2.2
Mattermost Server version 10.11.x up to and including 10.11.10

Discovery Timeline

March 16, 2026 - CVE-2026-24458 published to NVD
March 18, 2026 - Last updated in NVD database

Technical Details for CVE-2026-24458

Vulnerability Analysis

This vulnerability is classified under CWE-770 (Allocation of Resources Without Limits or Throttling). The core issue lies in Mattermost Server's authentication mechanism, which fails to implement proper input length validation or resource constraints when processing password inputs during login attempts.

When a user submits a login request, the server processes the password field without first checking its length against reasonable boundaries. This allows an attacker to craft HTTP POST requests containing password fields with multi-megabyte payloads. The server then attempts to process these oversized strings through its authentication pipeline, including potential password hashing operations, which are computationally expensive by design.

The network-accessible nature of this vulnerability means any unauthenticated remote attacker can exploit it without requiring prior access to the system. The attack requires no user interaction and can be automated to sustain pressure on targeted servers.

Root Cause

The root cause is the absence of input validation for password length before resource-intensive processing begins. Password hashing algorithms like bcrypt are intentionally designed to be computationally expensive to prevent brute-force attacks. However, when combined with unbounded input acceptance, this computational cost becomes a liability rather than a security feature. The server allocates memory for the oversized password string and expends CPU cycles attempting to process it, without any early rejection based on reasonable password length limits.

Attack Vector

The attack is executed over the network by sending HTTP POST requests to the Mattermost login endpoint. An attacker constructs login requests with valid-looking usernames but with password fields containing multi-megabyte strings. Each request forces the server to allocate memory for the large password and potentially begin cryptographic operations on it.

By sending multiple concurrent requests with oversized passwords, an attacker can rapidly exhaust server resources. The attack does not require any authentication, valid credentials, or special privileges. The server's availability is impacted as CPU and memory become saturated processing the malicious requests, potentially causing service degradation or complete denial of service for legitimate users.

Detection Methods for CVE-2026-24458

Indicators of Compromise

Abnormally large HTTP POST requests to the /api/v4/users/login endpoint
Login attempts with password field sizes exceeding typical bounds (e.g., greater than 100 KB)
Sudden spikes in memory usage on Mattermost Server processes
Elevated CPU utilization correlated with authentication request processing

Detection Strategies

Implement web application firewall (WAF) rules to inspect and block login requests with oversized password fields
Configure network monitoring to alert on HTTP POST requests exceeding reasonable size thresholds to authentication endpoints
Monitor Mattermost Server process resource consumption for anomalous patterns indicative of resource exhaustion attacks
Review authentication logs for high volumes of failed login attempts from single IP addresses or ranges

Monitoring Recommendations

Enable detailed request logging on reverse proxies or load balancers fronting Mattermost Server
Set up alerting thresholds for CPU and memory utilization on Mattermost Server hosts
Implement rate limiting on authentication endpoints to reduce the impact of sustained attack attempts
Monitor for connection timeouts and service degradation that may indicate ongoing denial of service conditions

How to Mitigate CVE-2026-24458

Immediate Actions Required

Upgrade Mattermost Server to a patched version that addresses MMSA-2026-00587
Implement request size limits at the reverse proxy or load balancer level before traffic reaches Mattermost Server
Configure rate limiting on the login endpoint to restrict the number of authentication attempts per IP address
Monitor server resource utilization and set up automated alerting for abnormal consumption patterns

Patch Information

Mattermost has released security updates to address this vulnerability. Organizations should upgrade to patched versions as documented in the Mattermost Security Updates advisory (Advisory ID: MMSA-2026-00587). Review the advisory for specific version numbers that contain the fix for each affected release branch.

Workarounds

Configure web server or reverse proxy to reject HTTP POST requests to authentication endpoints exceeding a reasonable size (e.g., 1 KB for password fields)
Implement IP-based rate limiting on the login endpoint to reduce the effectiveness of sustained attacks
Deploy a web application firewall with rules to detect and block requests with abnormally large form field values
Consider implementing CAPTCHA or similar challenge mechanisms on the login page to slow automated attacks

bash

# Example nginx configuration to limit request body size for login endpoint
location /api/v4/users/login {
    client_max_body_size 16k;
    limit_req zone=login_limit burst=5 nodelay;
    proxy_pass http://mattermost_backend;
}