CVE-2021-33175
📋 TL;DR
CVE-2021-33175 is a denial of service vulnerability in EMQ X Broker where specially crafted untrusted inputs cause excessive memory consumption, leading to application termination. This affects all EMQ X Broker deployments prior to version 4.2.8 that process untrusted MQTT messages. Organizations using vulnerable versions as message brokers are at risk.
💻 Affected Systems
- EMQ X Broker
📦 What is this software?
⚠️ Risk & Real-World Impact
Worst Case
Complete service outage of the EMQ X Broker, disrupting all MQTT message processing and potentially affecting dependent IoT/industrial systems.
Likely Case
Intermittent broker crashes causing message loss and service disruption until the broker restarts.
If Mitigated
Minimal impact with proper input validation and memory monitoring in place.
🎯 Exploit Status
Exploitation requires sending specially crafted MQTT messages to trigger memory exhaustion.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: 4.2.8 and later
Vendor Advisory: https://www.emqx.com/en/blog/emqx-4-2-8-release-notes
Restart Required: Yes
Instructions:
1. Download EMQ X Broker version 4.2.8 or later from official sources. 2. Stop the current EMQ X service. 3. Backup configuration and data. 4. Install the updated version. 5. Restart the EMQ X service.
🔧 Temporary Workarounds
Input Validation Filtering
allImplement network-level filtering to block malformed MQTT packets before they reach the broker.
Memory Limit Configuration
linuxConfigure operating system memory limits to prevent complete system exhaustion.
ulimit -v [LIMIT_IN_KB] (Linux)
🧯 If You Can't Patch
- Deploy network-level protections (firewalls/WAFs) to filter malicious MQTT traffic.
- Isolate vulnerable brokers in segmented networks with strict access controls.
🔍 How to Verify
Check if Vulnerable:
Check EMQ X Broker version; if below 4.2.8, the system is vulnerable.Check Version:
emqx_ctl status | grep 'EMQ X Broker'Verify Fix Applied:
Confirm version is 4.2.8 or higher and monitor for abnormal memory usage patterns.📡 Detection & Monitoring
Log Indicators:
- Rapid memory consumption spikes in system logs
- EMQ X process termination logs
- Out of memory errors in application logs
Network Indicators:
- Unusual volume of malformed MQTT packets
- Connection spikes from single sources
SIEM Query:
source="emqx.log" AND ("out of memory" OR "killed" OR memory>90%)