CVE-2026-24458: 
vulnerability analysis and mitigation

CVE-2026-24458 is a denial-of-service vulnerability in Mattermost Server caused by improper handling of excessively long passwords during login attempts. Affected versions include 10.11.x ≤ 10.11.10, 11.2.x ≤ 11.2.2, and 11.3.x ≤ 11.3.0. The vulnerability was published on March 16, 2026, and is tracked under Mattermost Advisory ID MMSA-2026-00587. It carries a CVSS v3.1 base score of 7.5 (High) (Mattermost Security, Red Hat CVE).

The root cause is classified as CWE-770 (Allocation of Resources Without Limits or Throttling). Mattermost Server fails to enforce any size limit or throttling on the password field submitted during login, allowing an unauthenticated attacker to send HTTP login requests containing multi-megabyte password strings. Processing these oversized inputs triggers intensive CPU and memory consumption — likely due to password hashing operations (e.g., bcrypt) applied to arbitrarily large inputs — without any prior length validation. No authentication, special privileges, or user interaction is required to trigger the condition (Mattermost Security, Red Hat CVE).

Successful exploitation results in a denial-of-service condition, exhausting the server's CPU and memory resources and rendering the Mattermost instance unavailable to legitimate users. The attack has no impact on confidentiality or data integrity — only availability is affected. Because no authentication is required, any network-accessible Mattermost deployment is at risk, and sustained attacks could cause prolonged service outages for organizations relying on Mattermost for team communications (Mattermost Security).

1. Reconnaissance: Identify internet-facing or network-accessible Mattermost Server instances running versions 10.11.0–10.11.10, 11.2.0–11.2.2, or 11.3.0 using tools like Shodan, Censys, or direct version enumeration via the Mattermost API endpoint (e.g., /api/v4/system/ping).
2. Craft oversized login request: Prepare an HTTP POST request to the Mattermost login endpoint (e.g., /api/v4/users/login) with a JSON body containing a valid or arbitrary username and a multi-megabyte password string (e.g., several MB of random characters).
3. Send repeated requests: Transmit the crafted request repeatedly — either in rapid succession or concurrently — to the target server. Each request forces the server to attempt processing (and likely hashing) the oversized password, consuming significant CPU and memory.
4. Achieve denial of service: Sustained submission of such requests exhausts server resources, causing degraded performance or complete unavailability of the Mattermost service for legitimate users (Mattermost Security).

• Network: High volume of HTTP POST requests to /api/v4/users/login from one or more source IPs; requests with unusually large Content-Length headers (megabytes in size) targeting the login endpoint.
• Logs: Mattermost application logs showing repeated failed login attempts with abnormally large request bodies; web server or reverse proxy logs recording oversized POST requests to the authentication endpoint.
• System: Sudden spikes in CPU utilization and memory consumption on the Mattermost server process without a corresponding increase in legitimate user activity; potential OOM (out-of-memory) killer events in system logs (/var/log/syslog or dmesg) related to the Mattermost process.

Mattermost has released patched versions addressing this vulnerability: upgrade to 10.11.11 or later (for 10.11.x deployments), 11.2.3 or later (for 11.2.x deployments), or 11.3.1 or later (for 11.3.x deployments). As an interim workaround if immediate patching is not possible, implement network-level rate limiting on login endpoint requests (e.g., via a reverse proxy or WAF) and enforce maximum request body size limits. Monitor server CPU and memory utilization for anomalous spikes indicative of exploitation attempts (Mattermost Security, Red Hat CVE).

Red Hat has tracked and published this CVE in their security advisory database, indicating relevance to distributions packaging Mattermost. openSUSE also issued a security announcement referencing this vulnerability. No notable independent researcher commentary or significant social media discussion has been identified at this time (Red Hat CVE, openSUSE Announce).