CVE-2021-37185: Siemens SIMATIC Drive Controller DoS Flaw

CVE-2021-37185 Overview

CVE-2021-37185 is a denial-of-service vulnerability affecting multiple Siemens SIMATIC programmable logic controller (PLC) product families. An unauthenticated attacker can exploit this vulnerability by sending specially crafted packets over TCP port 102, which is used for industrial communication protocols, to cause the affected PLC to become unresponsive. Recovery from this attack requires a manual restart of the affected device, resulting in potential downtime for critical industrial control systems.

This vulnerability is particularly concerning in operational technology (OT) environments where PLCs control manufacturing processes, utilities, and other critical infrastructure. The ability for an unauthenticated, remote attacker to disrupt PLC operations without any user interaction makes this a significant threat to industrial environments.

Critical Impact

Unauthenticated remote attackers can cause denial-of-service conditions in industrial control systems, requiring manual device restart and potentially disrupting critical manufacturing or infrastructure operations.

Affected Products

• Siemens SIMATIC Drive Controller family (versions >= V2.9.2 and < V2.9.4)
• Siemens SIMATIC ET 200SP Open Controller CPU 1515SP PC2 (versions >= V21.9 and < V21.9.4)
• Siemens SIMATIC S7-1200 CPU family (versions >= V4.5.0 and < V4.5.2)
• Siemens SIMATIC S7-1500 CPU family (versions >= V2.9.2 and < V2.9.4)
• Siemens SIMATIC S7-1500 Software Controller (versions >= V21.9 and < V21.9.4)
• Siemens SIMATIC S7-PLCSIM Advanced (versions >= V4.0 and < V4.0 SP1)
• Siemens TIM 1531 IRC (versions < V2.3.6)
• Siemens SIPLUS variants of affected products

Discovery Timeline

• 2022-02-09 - CVE-2021-37185 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2021-37185

Vulnerability Analysis

This vulnerability stems from improper handling of network packets on TCP port 102, which is the standard port for S7comm and ISO-TSAP industrial communication protocols used by Siemens PLCs. The flaw relates to CWE-672 (Operation on a Resource after Expiration or Release), indicating that the PLC firmware improperly processes resources during network communication handling.

When the vulnerable PLCs receive specially prepared malicious packets on port 102, the device fails to properly handle the input, leading to a condition where the CPU becomes unresponsive. This is a network-based attack that requires no authentication and no user interaction, making it highly exploitable from any system with network access to the PLC.

The impact is limited to availability—there is no known impact on confidentiality or integrity of the system. However, in industrial environments, loss of availability can have severe consequences, including halted production lines, safety system failures, or disruption of critical infrastructure services.

Root Cause

The root cause is tied to CWE-672: Operation on a Resource after Expiration or Release. The PLC firmware contains a flaw in how it manages resources during the processing of network communications on TCP port 102. When receiving malformed or specially crafted packets, the firmware attempts to operate on resources that are no longer valid, causing the system to enter an unstable state that manifests as a denial-of-service condition.

Attack Vector

The attack is conducted over the network, targeting TCP port 102, which is commonly exposed for industrial communication in SIMATIC environments. The attack characteristics include:

• Network-based exploitation: Attackers can reach the vulnerable service from any network-connected system
• No authentication required: The vulnerability can be exploited by unauthenticated attackers
• No user interaction needed: The attack is fully automated and requires no action from operators
• Manual recovery required: After successful exploitation, the affected PLC must be manually restarted to restore normal operations

Attackers with network access to industrial control system networks containing vulnerable Siemens PLCs can craft malicious TCP packets targeting port 102. The attack does not require any prior knowledge of the PLC configuration or credentials.

Detection Methods for CVE-2021-37185

Indicators of Compromise

• Unexpected PLC restarts or unresponsive PLCs requiring manual intervention
• Anomalous traffic patterns on TCP port 102 from unusual source addresses
• Increased connection attempts or malformed packets on the S7comm/ISO-TSAP communication channel
• PLC diagnostic logs indicating communication errors or watchdog timeouts

Detection Strategies

• Implement network intrusion detection systems (IDS) with rules for S7comm protocol anomalies on TCP port 102
• Deploy industrial control system (ICS) specific security monitoring tools that understand SIMATIC communication protocols
• Monitor for unusual connection patterns or packet characteristics targeting PLC communication ports
• Configure SIEM systems to alert on PLC availability changes or unexpected restart events

Monitoring Recommendations

• Establish baseline network traffic patterns for TCP port 102 and alert on deviations
• Implement continuous availability monitoring for all SIMATIC PLCs in the environment
• Log and analyze all network connections to PLCs from non-authorized engineering workstations
• Deploy OT network segmentation with monitoring at zone boundaries

How to Mitigate CVE-2021-37185

Immediate Actions Required

• Update all affected SIMATIC products to the patched firmware versions specified by Siemens
• Implement network segmentation to restrict access to TCP port 102 to only authorized engineering stations
• Apply firewall rules to block unauthorized network access to PLC communication ports
• Review and verify physical security controls for industrial control system networks

Patch Information

Siemens has released firmware updates to address this vulnerability. Organizations should update to the following minimum versions:

• SIMATIC Drive Controller family: Update to version V2.9.4 or later
• SIMATIC ET 200SP Open Controller CPU 1515SP PC2: Update to version V21.9.4 or later
• SIMATIC S7-1200 CPU family: Update to version V4.5.2 or later
• SIMATIC S7-1500 CPU family: Update to version V2.9.4 or later
• SIMATIC S7-1500 Software Controller: Update to version V21.9.4 or later
• SIMATIC S7-PLCSIM Advanced: Update to version V4.0 SP1 or later
• TIM 1531 IRC: Update to version V2.3.6 or later

Refer to the Siemens Security Advisory SSA-838121 for complete details on affected versions and remediation guidance.

Workarounds

• Configure cell protection features in TIA Portal to restrict network access to PLCs
• Implement network access control lists (ACLs) to limit connections to TCP port 102 from trusted IP addresses only
• Deploy industrial firewalls or demilitarized zones (DMZ) to separate PLC networks from corporate and external networks
• Consider using VPN connections for remote access to engineering workstations that require PLC communication

# Example firewall rule to restrict access to S7comm port (TCP 102)
# Allow only authorized engineering workstation IP addresses
iptables -A INPUT -p tcp --dport 102 -s 10.10.10.50 -j ACCEPT
iptables -A INPUT -p tcp --dport 102 -s 10.10.10.51 -j ACCEPT
iptables -A INPUT -p tcp --dport 102 -j DROP