CVE-2021-25663 – This vulnerability in Siemens Capital Embedded ... (How to Fix)

Q: How do I fix CVE-2021-25663?

1. Identify affected Siemens products and versions. 2. Download and apply vendor patches from Siemens support portal. 3. Restart affected devices. 4. Verify patch installation and system functionality.

Q: What is the severity of CVE-2021-25663?

CVE-2021-25663 has a CVSS score of 7.5 (HIGH). This vulnerability in Siemens Capital Embedded AR Classic, Nucleus NET, and related products allows attackers to cause denial of service by sending specially crafted IPv6 packets with malicious extension header lengths. The affected function enters an infinite loop, potentially crashing or freezing the device. This impacts industrial control systems, embedded devices, and networking equipment using these Siemens software components.

📋 TL;DR

This vulnerability in Siemens Capital Embedded AR Classic, Nucleus NET, and related products allows attackers to cause denial of service by sending specially crafted IPv6 packets with malicious extension header lengths. The affected function enters an infinite loop, potentially crashing or freezing the device. This impacts industrial control systems, embedded devices, and networking equipment using these Siemens software components.

💻 Affected Systems

Products:

Capital Embedded AR Classic 431-422
Capital Embedded AR Classic R20-11
Nucleus NET
Nucleus ReadyStart V3
Nucleus ReadyStart V4
Nucleus Source Code

Versions: All versions for most products; R20-11 < V2303; ReadyStart V3 < V2017.02.4; ReadyStart V4 < V4.1.0

Operating Systems: Embedded systems using affected Siemens software

Default Config Vulnerable: ⚠️ Yes

Notes: Affects IPv6 stack implementation; IPv4 traffic is not affected. Devices must have IPv6 enabled or processing IPv6 packets.

📦 What is this software?

⚠️ Risk & Real-World Impact

🔴

Worst Case

Complete system freeze or crash requiring physical reset, potentially disrupting critical industrial processes or network infrastructure.

🟠

Likely Case

Denial of service affecting device availability, potentially causing operational disruption in industrial environments.

🟢

If Mitigated

Limited impact if network segmentation and filtering prevent malicious IPv6 packets from reaching vulnerable devices.

🌐 Internet-Facing: MEDIUM - Devices directly exposed to internet could be targeted, but exploitation requires IPv6 connectivity and specific packet crafting.

🏢 Internal Only: MEDIUM - Internal attackers or compromised internal systems could exploit this to disrupt operations.

🎯 Exploit Status

Public PoC: ✅ No

Weaponized: UNKNOWN

Unauthenticated Exploit: ⚠️ Yes

Complexity: MEDIUM

Exploitation requires crafting specific IPv6 packets with malicious extension header lengths. No authentication needed as it's a network-based attack.

🛠️ Fix & Mitigation

✅ Official Fix

Patch Version: Capital Embedded AR Classic R20-11 V2303; Nucleus ReadyStart V3 V2017.02.4; Nucleus ReadyStart V4 V4.1.0

Vendor Advisory: https://cert-portal.siemens.com/productcert/html/ssa-248289.html

Restart Required: Yes

Instructions:

1. Identify affected Siemens products and versions. 2. Download and apply vendor patches from Siemens support portal. 3. Restart affected devices. 4. Verify patch installation and system functionality.

🔧 Temporary Workarounds

IPv6 Traffic Filtering

linux

Block or filter malicious IPv6 packets at network perimeter

# Example firewall rule to block suspicious IPv6 extension headers
ip6tables -A INPUT -p ipv6 --ipv6-ihl 0 -j DROP

Disable IPv6

linux

Disable IPv6 functionality on affected devices if not required

# Disable IPv6 on Linux
sysctl -w net.ipv6.conf.all.disable_ipv6=1
sysctl -w net.ipv6.conf.default.disable_ipv6=1

🧯 If You Can't Patch

Implement strict network segmentation to isolate vulnerable devices
Deploy intrusion detection/prevention systems to monitor for malicious IPv6 traffic

🔍 How to Verify

Check if Vulnerable:

Check device software version against affected versions list. Review system logs for IPv6 processing errors or crashes.

Check Version:

Vendor-specific commands vary; consult Siemens documentation for version checking on specific products.

Verify Fix Applied:

Verify installed version matches patched versions. Test IPv6 functionality remains operational without crashes.

📡 Detection & Monitoring

Log Indicators:

System crashes or freezes
High CPU usage spikes
IPv6 packet processing errors
Kernel panics related to network stack

Network Indicators:

Unusual IPv6 traffic patterns
Malformed IPv6 packets with extension headers
Traffic spikes to industrial control systems

SIEM Query:

source="network_firewall" dest_ip=* proto="ipv6" header_length>40 | stats count by dest_ip

📊 Metadata

CVE ID: CVE-2021-25663

CVSS v3 Score: 7.5 (HIGH)

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

CWE: CWE-835

Published: April 22, 2021

Last Updated: March 11, 2025

🔗 References

https://cert-portal.siemens.com/productcert/html/ssa-248289.html
https://cert-portal.siemens.com/productcert/pdf/ssa-248289.pdf Vendor Advisory
https://us-cert.cisa.gov/ics/advisories/icsa-21-103-05 Third Party Advisory,US Government Resource
https://cert-portal.siemens.com/productcert/html/ssa-248289.html
https://cert-portal.siemens.com/productcert/pdf/ssa-248289.pdf Vendor Advisory
https://us-cert.cisa.gov/ics/advisories/icsa-21-103-05 Third Party Advisory,US Government Resource

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🔗 Related Vulnerabilities

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