CVE-2026-6340: Mattermost Server DoS Vulnerability

CVE-2026-6340 Overview

CVE-2026-6340 affects Mattermost Server versions 11.5.x <= 11.5.1, 10.11.x <= 10.11.13, and 11.4.x <= 11.4.3. The server fails to validate 7zip archive structure before processing uploaded files. An authenticated attacker can upload a specially crafted 7zip file containing excessive folder declarations to exhaust server memory. This memory exhaustion triggers a denial of service condition against the Mattermost instance. The flaw is tracked under Mattermost Advisory ID MMSA-2026-00573 and classified as [CWE-789] Memory Allocation with Excessive Size Value.

Critical Impact

Authenticated attackers can crash Mattermost servers by uploading malformed 7zip archives, disrupting collaboration services for all users on the affected instance.

Affected Products

• Mattermost Server 11.5.x through 11.5.1
• Mattermost Server 11.4.x through 11.4.3
• Mattermost Server 10.11.x through 10.11.13

Discovery Timeline

• 2026-05-18 - CVE-2026-6340 published to NVD
• 2026-05-19 - Last updated in NVD database

Technical Details for CVE-2026-6340

Vulnerability Analysis

The vulnerability resides in the Mattermost Server file upload handling pipeline. When users upload 7zip archives, the server begins parsing the archive structure without first validating that the declared metadata is reasonable. An attacker can craft a 7zip file that declares an excessive number of folder entries in its header. The server then attempts to allocate memory proportional to the declared folder count. This unbounded allocation drives the process toward memory exhaustion and termination by the operating system or runtime.

The vulnerability requires authentication and exploits a missing input validation control on attacker-supplied archive metadata. Because the attack relies on a single small upload, it can be executed repeatedly across sessions to maintain a denial of service condition.

Root Cause

The root cause is improper validation of the 7zip archive header structure before processing, mapped to [CWE-789] Memory Allocation with Excessive Size Value. The parser trusts the folder declaration count provided in the archive metadata and uses that value to drive memory allocation without applying upper bounds. Trusting attacker-supplied size fields without sanity checks is a well-documented anti-pattern in archive and serialization libraries.

Attack Vector

The attack is network-based and requires a valid authenticated session on the Mattermost server. An attacker uploads a crafted 7zip file through any feature that accepts archive attachments. No user interaction beyond the attacker's own upload action is needed. Successful exploitation impacts availability only — confidentiality and integrity of stored data are not affected by this flaw.

The vulnerability mechanism is described in prose because no verified proof-of-concept code is publicly available. See the Mattermost Security Updates advisory for vendor-supplied technical detail.

Detection Methods for CVE-2026-6340

Indicators of Compromise

• Sudden Mattermost server process termination correlated with OutOfMemory errors in application or system logs.
• File upload events containing .7z archives with abnormally small file sizes but unusually long parsing durations.
• Repeated archive upload requests from a single authenticated user prior to a server crash.

Detection Strategies

• Monitor Mattermost server process memory utilization for rapid spikes that coincide with file upload API calls.
• Inspect upload telemetry for .7z MIME types and correlate against server resource pressure metrics.
• Alert on oom-killer events on hosts running Mattermost server processes.

Monitoring Recommendations

• Forward Mattermost application logs, kernel OOM logs, and upload audit records to a centralized SIEM for correlation.
• Track per-user upload frequency and archive size distributions to baseline normal behavior.
• Configure alerting on consecutive server restarts within short time windows.

How to Mitigate CVE-2026-6340

Immediate Actions Required

• Upgrade Mattermost Server to a fixed release as specified in the Mattermost Security Updates advisory for MMSA-2026-00573.
• Restrict archive upload permissions to trusted user groups until patching is complete.
• Review recent upload logs for suspicious .7z uploads from authenticated accounts.

Patch Information

Mattermost has released fixed versions addressing CVE-2026-6340. Administrators running 11.5.x <= 11.5.1, 11.4.x <= 11.4.3, or 10.11.x <= 10.11.13 must upgrade to the patched releases identified in advisory MMSA-2026-00573. Refer to the Mattermost Security Updates page for exact patched version numbers and upgrade guidance.

Workarounds

• Disable file uploads or block .7z archive types at an upstream reverse proxy until the patch is applied.
• Enforce file size limits on uploads and apply per-process memory cgroup limits to contain impact.
• Increase logging verbosity for the file ingestion subsystem to detect exploitation attempts.

# Example NGINX reverse proxy rule to block 7zip uploads
location /api/v4/files {
    if ($http_content_type ~* "application/x-7z-compressed") {
        return 415;
    }
    client_max_body_size 10m;
    proxy_pass http://mattermost_backend;
}