CVE-2026-6340 — Denial of Service
CVE-2026-6340 — Mattermost versions 11.5.x <= 11.5.1, 10.11.x <= 10.11.13, 11.4.x <= 11.4.3 fail to validate 7zip archive structure before processing which allows an authenticated attacker to cause server memory exhaustion and denial of service via uploading a specially crafted 7zip file with excess
What This Means For You
- If your environment is affected by CWE-789, review your exposure and prioritize patching based on your environment. Monitor vendor advisories for CVE-2026-6340 updates and patches.
Related ATT&CK Techniques
🛡️ Detection Rules
3 rules · 6 SIEM formats3 detection rules auto-generated for this incident, mapped to MITRE ATT&CK. Sigma YAML is free — export to any SIEM format via the Intel Bot.
MMSA-2026-00573: Mattermost 7zip Archive DoS - Free Tier
title: MMSA-2026-00573: Mattermost 7zip Archive DoS - Free Tier
id: scw-2026-05-18-ai-1
status: experimental
level: medium
description: |
Detects an authenticated user uploading a crafted 7zip file to Mattermost, targeting the vulnerability CVE-2026-6340. The exploit involves excessive folder declarations within the 7zip archive structure, leading to server memory exhaustion and denial of service. This rule specifically looks for the file upload API endpoint with a .7z extension and indicators of archive manipulation in the URI query.
author: SCW Feed Engine (AI-generated)
date: 2026-05-18
references:
- https://shimiscyberworld.com/posts/nvd-CVE-2026-6340/
tags:
- attack.impact
- attack.t1499
logsource:
category: webserver
detection:
selection:
cs-uri|contains:
- '/api/v4/files/upload'
cs-method|exact: 'POST'
cs-uri-query|contains:
- '.7z'
selection_base:
sc-status|exact: '200'
selection_indicators:
uri|contains:
- 'archive/folder/declarations'
condition: selection AND selection_base AND selection_indicators
falsepositives:
- Legitimate administrative activity
Source: Shimi's Cyber World · License & reuse
Indicators of Compromise
| ID | Type | Indicator |
|---|---|---|
| CVE-2026-6340 | vulnerability | CVE-2026-6340 |
| CWE-789 | weakness | CWE-789 |
Written by SCW Vulnerability Desk
Source & Attribution
| Source Platform | NVD |
| Channel | National Vulnerability Database |
| Published | May 18, 2026 at 11:16 UTC |
This content was curated and summarized by Shimi's Cyber World for informational purposes. It is not copied or republished in full. All intellectual property rights remain with the original author and source.
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