CVE-2021-31884
📋 TL;DR
This vulnerability affects Siemens building automation controllers and related products. It allows attackers to exploit a DHCP client flaw where the hostname option isn't properly null-terminated, potentially leading to out-of-bounds memory operations. This can result in denial-of-service, information disclosure, or remote code execution on affected systems.
💻 Affected Systems
- APOGEE MBC (PPC)
- APOGEE MEC (PPC)
- APOGEE PXC Compact
- APOGEE PXC Modular
- Capital VSTAR
- Desigo PXC00-E.D
- Desigo PXC00-U
- Desigo PXC001-E.D
- Desigo PXC100-E.D
- Desigo PXC12-E.D
- Desigo PXC128-U
- Desigo PXC200-E.D
- Desigo PXC22-E.D
- Desigo PXC22.1-E.D
- Desigo PXC36.1-E.D
- Desigo PXC50-E.D
- Desigo PXC64-U
- Desigo PXM20-E
- Nucleus NET
- Nucleus ReadyStart V3
- Nucleus Source Code
- TALON TC Compact
- TALON TC Modular
📦 What is this software?
Apogee Modular Building Controller Firmware by Siemens
View all CVEs affecting Apogee Modular Building Controller Firmware →
Apogee Modular Equiment Controller Firmware by Siemens
View all CVEs affecting Apogee Modular Equiment Controller Firmware →
Nucleus Net by Siemens
⚠️ Risk & Real-World Impact
Worst Case
Remote code execution leading to complete system compromise, allowing attackers to manipulate building automation systems, disrupt operations, or pivot to other network segments.
Likely Case
Denial-of-service conditions causing building automation controllers to crash or become unresponsive, disrupting HVAC, lighting, or other building management functions.
If Mitigated
Limited impact if systems are isolated from untrusted networks and DHCP is controlled, though risk remains from internal threats.
🎯 Exploit Status
Exploitation requires network access to send malicious DHCP responses. No authentication needed as DHCP is typically unauthenticated.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: V3.5.4 for APOGEE PXC Compact/Modular (BACnet), V2.8.19 for APOGEE PXC Compact/Modular (P2 Ethernet), V6.30.016 for Desigo products, V2017.02.4 for Nucleus ReadyStart V3
Vendor Advisory: https://cert-portal.siemens.com/productcert/pdf/ssa-044112.pdf
Restart Required: Yes
Instructions:
1. Identify affected products and versions. 2. Download appropriate firmware updates from Siemens Industrial Security. 3. Apply updates following Siemens' building automation update procedures. 4. Restart affected controllers. 5. Verify successful update and functionality.
🔧 Temporary Workarounds
Network Segmentation
allIsolate building automation systems from untrusted networks and implement strict network controls.
DHCP Server Hardening
allConfigure trusted DHCP servers to only provide valid, properly formatted hostname options.
🧯 If You Can't Patch
- Segment building automation networks completely from corporate/IT networks
- Implement strict firewall rules blocking unnecessary traffic to/from affected controllers
- Monitor for unusual DHCP traffic or controller crashes
🔍 How to Verify
Check if Vulnerable:
Check controller firmware versions against affected version lists. Review Siemens security advisories SSA-044112, SSA-114589, SSA-620288 for specific product mappings.Check Version:
Product-specific commands vary. Typically accessed through Siemens building automation management software or controller web interfaces.Verify Fix Applied:
Verify firmware version is at or above patched versions: V3.5.4, V2.8.19, V6.30.016, or V2017.02.4 depending on product.📡 Detection & Monitoring
Log Indicators:
- Unexpected controller restarts or crashes
- DHCP request/response anomalies in network logs
- Memory error messages in controller logs
Network Indicators:
- Unusual DHCP traffic to building automation controllers
- Malformed DHCP packets with manipulated hostname options
SIEM Query:
Search for: (event_category:dhcp AND (hostname_length > normal) OR (malformed_dhcp)) OR (device_type:building_automation AND (crash OR restart))🔗 References
- https://cert-portal.siemens.com/productcert/pdf/ssa-044112.pdf
- https://cert-portal.siemens.com/productcert/pdf/ssa-114589.pdf
- https://cert-portal.siemens.com/productcert/pdf/ssa-620288.pdf
- https://cert-portal.siemens.com/productcert/pdf/ssa-044112.pdf
- https://cert-portal.siemens.com/productcert/pdf/ssa-114589.pdf
- https://cert-portal.siemens.com/productcert/pdf/ssa-620288.pdf
