CVE-2021-27391 – This CVE describes a critical buffer overflow v... (How to Fix)

Q: How do I fix CVE-2021-27391?

1. Download firmware updates from Siemens Industry Online Support; 2. Backup device configuration; 3. Apply firmware update following Siemens documentation; 4. Restart device; 5. Verify firmware version and functionality.

Q: What is the severity of CVE-2021-27391?

CVE-2021-27391 has a CVSS score of 9.8 (CRITICAL). This CVE describes a critical buffer overflow vulnerability in Siemens APOGEE and TALON building automation controllers. Unauthenticated remote attackers can exploit improper bounds checking in the HTTP Host parameter parsing to execute arbitrary code with root privileges. Affected devices include various APOGEE MBC, MEC, PXC Compact/Modular and TALON TC Compact/Modular controllers with specific firmware versions.

📋 TL;DR

This CVE describes a critical buffer overflow vulnerability in Siemens APOGEE and TALON building automation controllers. Unauthenticated remote attackers can exploit improper bounds checking in the HTTP Host parameter parsing to execute arbitrary code with root privileges. Affected devices include various APOGEE MBC, MEC, PXC Compact/Modular and TALON TC Compact/Modular controllers with specific firmware versions.

💻 Affected Systems

Products:

APOGEE MBC (PPC) (P2 Ethernet)
APOGEE MEC (PPC) (P2 Ethernet)
APOGEE PXC Compact (BACnet)
APOGEE PXC Compact (P2 Ethernet)
APOGEE PXC Modular (BACnet)
APOGEE PXC Modular (P2 Ethernet)
TALON TC Compact (BACnet)
TALON TC Modular (BACnet)

Versions: APOGEE MBC/MEC (P2 Ethernet): >= V2.6.3; APOGEE PXC Compact/Modular (BACnet): < V3.5.3; APOGEE PXC Compact/Modular (P2 Ethernet): >= V2.8; TALON TC Compact/Modular (BACnet): < V3.5.3

Operating Systems: Embedded controller firmware

Default Config Vulnerable: ⚠️ Yes

Notes: All affected devices with web interface enabled are vulnerable by default. The vulnerability requires HTTP access to the device's web server.

📦 What is this software?

⚠️ Risk & Real-World Impact

🔴

Worst Case

Complete system compromise with root-level code execution, allowing attackers to take full control of building automation systems, manipulate HVAC controls, disable safety systems, or pivot to other network segments.

🟠

Likely Case

Remote code execution leading to disruption of building operations, data exfiltration, or ransomware deployment on critical infrastructure systems.

🟢

If Mitigated

Limited impact if devices are behind firewalls with strict network segmentation and proper access controls, though the vulnerability remains exploitable by authorized internal users.

🌐 Internet-Facing: HIGH

🏢 Internal Only: HIGH

🎯 Exploit Status

Public PoC: ✅ No

Weaponized: UNKNOWN

Unauthenticated Exploit: ⚠️ Yes

Complexity: LOW

The vulnerability is unauthenticated and requires only HTTP access, making exploitation straightforward for attackers with network access to vulnerable devices.

🛠️ Fix & Mitigation

✅ Official Fix

Patch Version: APOGEE PXC Compact/Modular (BACnet): V3.5.3 or later; TALON TC Compact/Modular (BACnet): V3.5.3 or later; APOGEE MBC/MEC (P2 Ethernet): Update to latest available version; APOGEE PXC Compact/Modular (P2 Ethernet): Update to latest available version

Vendor Advisory: https://cert-portal.siemens.com/productcert/pdf/ssa-944498.pdf

Restart Required: Yes

Instructions:

1. Download firmware updates from Siemens Industry Online Support; 2. Backup device configuration; 3. Apply firmware update following Siemens documentation; 4. Restart device; 5. Verify firmware version and functionality.

🔧 Temporary Workarounds

Network Segmentation and Access Control

all

Restrict network access to affected devices using firewalls and VLAN segmentation

Disable Web Interface

all

Disable HTTP/HTTPS web server if not required for operations

🧯 If You Can't Patch

Implement strict network segmentation to isolate affected devices from untrusted networks
Deploy intrusion detection systems to monitor for exploitation attempts and anomalous HTTP traffic

🔍 How to Verify

Check if Vulnerable:

Check device firmware version via web interface or management console and compare against affected versions listed in vendor advisory

Check Version:

Check via device web interface or Siemens management tools (specific commands vary by device model)

Verify Fix Applied:

Verify firmware version is updated to patched versions: APOGEE PXC/TALON TC (BACnet) >= V3.5.3; APOGEE MBC/MEC/PXC (P2 Ethernet) latest available version

📡 Detection & Monitoring

Log Indicators:

Unusual HTTP requests with malformed Host headers
Multiple failed buffer overflow attempts in web server logs
Unexpected process creation or system reboots

Network Indicators:

HTTP requests with unusually long Host headers to building automation controllers
Traffic to/from building automation systems from unauthorized IP addresses

SIEM Query:

source="web_server_logs" AND (Host:*[>1024] OR "buffer overflow" OR "segmentation fault")

📊 Metadata

CVE ID: CVE-2021-27391

CVSS v3 Score: 9.8 (CRITICAL)

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

CWE: CWE-120

Published: September 14, 2021

Last Updated: April 23, 2025

🔗 References

https://cert-portal.siemens.com/productcert/pdf/ssa-944498.pdf Patch,Vendor Advisory
https://cert-portal.siemens.com/productcert/pdf/ssa-944498.pdf Patch,Vendor Advisory

📤 Share & Export

📄 Export Markdown 📋 Export JSON

🔗 Related Vulnerabilities

If you're affected by CVE-2021-27391, you might also want to check these: