CVE-2026-2456: 
vulnerability analysis and mitigation

CVE-2026-2456 is a denial-of-service vulnerability in Mattermost Server caused by the failure to limit the size of responses from integration action endpoints. An authenticated attacker can exploit this to cause server memory exhaustion by configuring a malicious integration server that returns an arbitrarily large response when a user clicks an interactive message button. Affected versions include Mattermost Server 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-2026-00571. It carries a CVSS v3.1 base score of 5.7 (Medium) (Mattermost Security, ENISA EUVD).

The root cause is classified as CWE-789 (Memory Allocation with Excessive Size Value): Mattermost does not enforce any upper bound on the size of HTTP responses received from third-party integration action endpoints. When a user clicks an interactive message button, the Mattermost server makes a request to the configured integration action URL and reads the response without restriction. A malicious integration server can return an arbitrarily large payload, causing the server to allocate excessive memory while processing it. Exploitation requires the attacker to control or compromise an integration endpoint registered within the Mattermost instance and to induce a legitimate user to click the associated interactive button (Mattermost Security, ENISA EUVD).

Successful exploitation results in server memory exhaustion, rendering the Mattermost server unavailable to legitimate users (high availability impact). There is no confidentiality or integrity impact — the vulnerability is limited to a denial-of-service condition. Because Mattermost is a team collaboration platform, an outage could disrupt internal communications and workflows across all users of the affected instance (Mattermost Security, ENISA EUVD).

1. Gain authenticated access: Obtain a valid Mattermost account with permissions to create or modify integrations (e.g., slash commands, outgoing webhooks, or interactive message actions).
2. Set up a malicious integration server: Deploy an HTTP server that, when called by Mattermost's integration action handler, returns an HTTP response with an arbitrarily large body (e.g., hundreds of megabytes or gigabytes of data).
3. Register the malicious integration: Configure the malicious server's URL as an integration action endpoint within the Mattermost instance (e.g., as an interactive button action URL in a custom integration or bot).
4. Post an interactive message: Send a message containing an interactive button linked to the malicious integration endpoint to a channel where legitimate users are active.
5. Trigger the exploit: Wait for or socially engineer a legitimate user to click the interactive button, causing the Mattermost server to make a request to the malicious endpoint and attempt to read the oversized response, exhausting server memory and causing a denial of service (Mattermost Security, ENISA EUVD).

• Logs: Mattermost server logs showing repeated or sustained HTTP requests to an unusual or external integration action URL; out-of-memory (OOM) errors or crash reports in server logs coinciding with user interaction with interactive message buttons.
• Network: Outbound connections from the Mattermost server to an unfamiliar or newly registered integration endpoint URL; large or sustained data transfers from an integration server back to the Mattermost server.
• Process/System: Sudden spike in memory consumption by the Mattermost server process (mattermost) leading to OOM kills or service restarts; system-level OOM killer events logged in /var/log/syslog or dmesg referencing the Mattermost process.
• Configuration: Presence of newly added or recently modified integration action endpoints pointing to external or unrecognized URLs in the Mattermost admin console.

Mattermost has released patched versions addressing this vulnerability: upgrade to 10.11.11 or later (for 10.11.x users), 11.2.3 or later (for 11.2.x users), or 11.3.1 or later (for 11.3.x users). As a workaround, administrators should audit all configured integration endpoints to ensure they point to trusted, internally controlled servers, and consider restricting integration creation/modification permissions to trusted users only. Patches are available via the official Mattermost security updates page (Mattermost Security).

The vulnerability received routine coverage from automated CVE tracking services and security feeds shortly after publication on March 16, 2026, including mentions on CVEFeed, VulDB, and Bluesky CVE notification accounts. An openSUSE security announcement was also issued in relation to this CVE. No notable independent researcher commentary or significant media coverage has been identified beyond standard vulnerability disclosure channels (openSUSE Security).