CVE-2021-27391 is a critical buffer overflow vulnerability affecting multiple Siemens APOGEE and TALON building automation system devices, specifically APOGEE MBC (PPC) (P2 Ethernet), APOGEE MEC (PPC) (P2 Ethernet), APOGEE PXC Compact and Modular (BACnet and P2 Ethernet), and TALON TC Compact and Modular (BACnet). The vulnerability arises from improper bounds checking in the web server component of these devices when parsing the HTTP Host header parameter. This classic buffer overflow (CWE-120) allows an unauthenticated remote attacker to send a specially crafted HTTP request with an overly long Host header, causing the device to overwrite memory beyond the allocated buffer. Exploitation can lead to arbitrary code execution with root privileges on the affected device. The vulnerability impacts all versions from V2.6.3 onward for APOGEE MBC and MEC, versions prior to V3.5.3 for APOGEE PXC Compact and Modular (BACnet), and versions from V2.8 onward for APOGEE PXC Compact and Modular (P2 Ethernet), as well as TALON TC Compact and Modular (BACnet) versions prior to V3.5.3. The CVSS v3.1 base score is 9.8 (critical), reflecting the vulnerability’s ease of exploitation (network vector, no privileges or user interaction required) and its severe impact on confidentiality, integrity, and availability. Although no public exploits have been reported in the wild to date, the vulnerability’s nature and severity make it a high-risk threat, especially given the root-level code execution capability. Siemens building automation systems are widely used in critical infrastructure and commercial buildings to control HVAC, lighting, and other essential services, making this vulnerability particularly dangerous if exploited in operational environments.

For European organizations, the exploitation of CVE-2021-27391 could have severe consequences. Siemens APOGEE and TALON systems are commonly deployed in commercial buildings, industrial facilities, and critical infrastructure such as hospitals, data centers, and government buildings across Europe. Successful exploitation could allow attackers to gain root-level control over these building management systems, potentially disrupting HVAC, lighting, and safety systems. This could lead to operational downtime, physical safety risks, and compromise of sensitive facility management data. Furthermore, attackers could use compromised devices as footholds for lateral movement within enterprise networks, escalating the impact beyond the building automation environment. The critical nature of this vulnerability combined with the widespread deployment of affected Siemens products in Europe increases the risk of targeted attacks, especially against high-value or strategic assets. The lack of authentication and user interaction requirements lowers the barrier for attackers, increasing the likelihood of exploitation if devices are exposed to untrusted networks or insufficiently segmented environments.

To mitigate CVE-2021-27391, European organizations should take the following specific actions: 1) Immediately identify and inventory all Siemens APOGEE and TALON devices in their environment, focusing on the affected versions listed. 2) Apply Siemens’ official patches or firmware updates as soon as they become available; if patches are not yet released, contact Siemens support for interim mitigation guidance. 3) Restrict network access to affected devices by implementing strict network segmentation and firewall rules, allowing management interfaces only from trusted internal networks or VPNs. 4) Disable or restrict HTTP access to the web server interface where possible, or replace it with secure management protocols that include input validation. 5) Monitor network traffic for anomalous HTTP requests with suspiciously long Host headers or other malformed inputs targeting building automation devices. 6) Implement intrusion detection/prevention systems (IDS/IPS) with signatures tuned to detect exploitation attempts of this buffer overflow. 7) Conduct regular security audits and vulnerability scans focused on industrial control and building management systems to detect unpatched or exposed devices. 8) Train facility management and IT teams on the risks associated with building automation system vulnerabilities and the importance of timely patching and network controls. These measures go beyond generic advice by emphasizing network-level controls, monitoring for specific attack vectors, and proactive asset management tailored to Siemens APOGEE and TALON systems.