CVE-2021-25676
📋 TL;DR
This vulnerability in Siemens industrial networking devices allows attackers to cause a denial-of-service by repeatedly attempting SSH authentication. When triggered, the device automatically reboots, disrupting network operations. Affected users include organizations using Siemens RUGGEDCOM RM1224, SCALANCE M-800, SCALANCE S615, or SCALANCE SC-600 devices.
💻 Affected Systems
- RUGGEDCOM RM1224
- SCALANCE M-800
- SCALANCE S615
- SCALANCE SC-600
📦 What is this software?
⚠️ Risk & Real-World Impact
Worst Case
Persistent attacker causes repeated device reboots, leading to extended network downtime and potential safety impacts in industrial environments.
Likely Case
Accidental or targeted SSH brute-force attempts trigger device reboots, causing temporary network disruption.
If Mitigated
With proper network segmentation and access controls, impact is limited to isolated network segments.
🎯 Exploit Status
Exploitation requires only SSH access and ability to send multiple failed authentication attempts. No special tools needed beyond standard SSH clients.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: V2.1.3 for SCALANCE SC-600; contact Siemens for other products
Vendor Advisory: https://cert-portal.siemens.com/productcert/pdf/ssa-296266.pdf
Restart Required: Yes
Instructions:
1. Download firmware update from Siemens support portal. 2. Backup device configuration. 3. Apply firmware update following Siemens documentation. 4. Verify update and restore configuration if needed.
🔧 Temporary Workarounds
Restrict SSH Access
allLimit SSH access to trusted IP addresses only using firewall rules.
# Configure firewall to allow SSH only from specific IPs
# Example: iptables -A INPUT -p tcp --dport 22 -s trusted_ip -j ACCEPT
# iptables -A INPUT -p tcp --dport 22 -j DROP
Disable SSH if Not Needed
linuxTurn off SSH service if remote administration is not required.
# Disable SSH service
# systemctl stop sshd
# systemctl disable sshd
🧯 If You Can't Patch
- Implement network segmentation to isolate affected devices from untrusted networks.
- Deploy intrusion detection systems to monitor for SSH brute-force attempts and alert on suspicious activity.
🔍 How to Verify
Check if Vulnerable:
Check device firmware version via web interface or CLI. For SCALANCE SC-600, check if version is >=2.1 and <2.1.3. For other devices, check if running V6.3.Check Version:
# For Siemens devices, typically via web interface or CLI command like 'show version'Verify Fix Applied:
After patching, verify firmware version shows patched version and test that multiple failed SSH attempts no longer trigger reboot.📡 Detection & Monitoring
Log Indicators:
- Multiple failed SSH authentication attempts from same source
- Device reboot events following SSH failures
- SSH service restart logs
Network Indicators:
- High volume of SSH connection attempts to device port 22
- SSH protocol anomalies
SIEM Query:
source="device_logs" (event_type="ssh_failed" count>5 within 60s) OR event_type="system_reboot"