CVE-2021-31881 – This vulnerability in Siemens Capital Embedded ... (How to Fix)

Q: How do I fix CVE-2021-31881?

1. Download patch from Siemens Industrial Security portal. 2. Apply update to affected systems. 3. Restart systems to activate fix. 4. Verify version shows V2303 or later.

Q: What is the severity of CVE-2021-31881?

CVE-2021-31881 has a CVSS score of 7.1 (HIGH). This vulnerability in Siemens Capital Embedded AR Classic products allows attackers to cause denial-of-service conditions by sending specially crafted DHCP OFFER messages with malformed Vendor options. The DHCP client fails to validate option lengths, leading to crashes or service disruption. Affected systems include all versions of AR Classic 431-422 and AR Classic R20-11 versions before V2303.

📋 TL;DR

This vulnerability in Siemens Capital Embedded AR Classic products allows attackers to cause denial-of-service conditions by sending specially crafted DHCP OFFER messages with malformed Vendor options. The DHCP client fails to validate option lengths, leading to crashes or service disruption. Affected systems include all versions of AR Classic 431-422 and AR Classic R20-11 versions before V2303.

💻 Affected Systems

Products:

Siemens Capital Embedded AR Classic 431-422
Siemens Capital Embedded AR Classic R20-11

Versions: All versions of 431-422; R20-11 versions < V2303

Operating Systems: Embedded systems using affected AR Classic software

Default Config Vulnerable: ⚠️ Yes

Notes: Vulnerability exists when DHCP client is enabled and processing OFFER messages. Industrial control systems using these products are primarily affected.

📦 What is this software?

⚠️ Risk & Real-World Impact

🔴

Worst Case

Complete system crash requiring manual reboot, disrupting critical industrial operations and potentially causing safety issues in embedded control systems.

🟠

Likely Case

DHCP client crashes, losing network connectivity until service restart, disrupting communication in industrial environments.

🟢

If Mitigated

Minimal impact with proper network segmentation and DHCP server controls preventing malicious DHCP messages from reaching vulnerable devices.

🌐 Internet-Facing: MEDIUM - Requires DHCP traffic from untrusted sources, but industrial systems often have limited internet exposure.

🏢 Internal Only: HIGH - Internal attackers or compromised devices on the same network segment can easily exploit this vulnerability.

🎯 Exploit Status

Public PoC: ✅ No

Weaponized: UNKNOWN

Unauthenticated Exploit: ⚠️ Yes

Complexity: LOW

Exploitation requires network access to send DHCP packets. No authentication needed. Complexity is low as DHCP is a standard protocol.

🛠️ Fix & Mitigation

✅ Official Fix

Patch Version: V2303 for AR Classic R20-11

Vendor Advisory: https://cert-portal.siemens.com/productcert/html/ssa-044112.html

Restart Required: Yes

Instructions:

1. Download patch from Siemens Industrial Security portal. 2. Apply update to affected systems. 3. Restart systems to activate fix. 4. Verify version shows V2303 or later.

🔧 Temporary Workarounds

Network Segmentation

all

Isolate affected systems on separate VLANs with strict DHCP server controls

DHCP Server Hardening

all

Configure trusted DHCP servers only and block rogue DHCP servers

🧯 If You Can't Patch

Implement strict network segmentation to isolate vulnerable systems from untrusted networks
Deploy network monitoring to detect and block malicious DHCP traffic patterns

🔍 How to Verify

Check if Vulnerable:

Check system version against affected versions list. If running AR Classic R20-11 < V2303 or any 431-422 version, system is vulnerable.

Check Version:

System-specific command varies by implementation. Typically accessed through device management interface or console.

Verify Fix Applied:

Verify system version shows V2303 or later for R20-11. For 431-422, check with Siemens for specific patch verification.

📡 Detection & Monitoring

Log Indicators:

DHCP client crashes
Network interface resets
Unexpected service restarts

Network Indicators:

Malformed DHCP OFFER packets with abnormal Vendor option lengths
DHCP traffic from unauthorized sources

SIEM Query:

source="dhcp" AND (message="client crash" OR message="malformed option")

📊 Metadata

CVE ID: CVE-2021-31881

CVSS v3 Score: 7.1 (HIGH)

CVSS Vector: CVSS:3.1/AV:A/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:H

CWE: CWE-125

Published: November 9, 2021

Last Updated: November 21, 2024

🔗 References

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