CVE-2025-56353
📋 TL;DR
A memory leak vulnerability in tinyMQTT allows attackers to cause denial of service by sending malformed UTF-8 strings in topic filters. Each malformed subscription request causes heap memory allocation without proper cleanup, leading to unbounded memory consumption. This affects any system running the vulnerable tinyMQTT broker.
💻 Affected Systems
- tinyMQTT
📦 What is this software?
Tinymqtt by Justdoit0910
⚠️ Risk & Real-World Impact
Worst Case
Complete broker crash and service disruption due to memory exhaustion, potentially affecting all connected MQTT clients and dependent systems.
Likely Case
Degraded broker performance and eventual service interruption under sustained attack, impacting message delivery reliability.
If Mitigated
Minimal impact with proper network controls and monitoring, though memory pressure may still occur during attacks.
🎯 Exploit Status
Exploitation requires only the ability to send MQTT subscription packets with malformed UTF-8 payloads. No authentication or special privileges needed.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: Any version after commit 6226ade15bd4f97be2d196352e64dd10937c1962
Vendor Advisory: https://github.com/JustDoIt0910/tinyMQTT/issues/19
Restart Required: Yes
Instructions:
1. Update to latest tinyMQTT version from GitHub repository. 2. Rebuild the broker from source. 3. Restart the tinyMQTT service to apply the fix.
🔧 Temporary Workarounds
Rate limit subscription requests
linuxImplement network-level rate limiting on MQTT subscription packets to reduce attack impact
iptables -A INPUT -p tcp --dport 1883 -m limit --limit 10/min --limit-burst 20 -j ACCEPT
iptables -A INPUT -p tcp --dport 1883 -j DROP
🧯 If You Can't Patch
- Implement strict network segmentation to isolate MQTT broker from untrusted networks
- Deploy memory monitoring with automated alerts for abnormal heap growth patterns
🔍 How to Verify
Check if Vulnerable:
Check if tinyMQTT version is from commit 6226ade15bd4f97be2d196352e64dd10937c1962 or earlier by examining build metadata or source codeCheck Version:
Check git commit hash or build timestamp in tinyMQTT output/logsVerify Fix Applied:
Test with malformed UTF-8 subscription packets and monitor memory usage for sustained growth📡 Detection & Monitoring
Log Indicators:
- Unusual volume of subscription requests
- Memory allocation errors in broker logs
- Repeated malformed UTF-8 parsing errors
Network Indicators:
- High frequency of SUBSCRIBE packets from single source
- Large payloads in subscription packets
- Invalid UTF-8 sequences in MQTT traffic
SIEM Query:
source="mqtt_broker.log" AND ("malformed" OR "memory" OR "subscription") | stats count by src_ip