CVE-2023-40547 is a remote code execution vulnerability identified in Shim, a bootloader component used by Red Hat Enterprise Linux 7 to facilitate secure boot processes. Shim is responsible for loading and verifying the integrity of boot components. The vulnerability stems from Shim's improper handling of HTTP responses during the early boot phase, where it trusts attacker-controlled values without sufficient validation. An attacker capable of intercepting or manipulating HTTP traffic—via a Man-in-the-Middle (MitM) attack or by compromising the boot server—can craft a malicious HTTP response that triggers an out-of-bounds write in Shim's memory. This out-of-bounds write allows the attacker to overwrite critical memory regions, leading to arbitrary code execution with system-level privileges before the operating system fully boots. This early-stage compromise can result in complete system takeover, bypassing traditional OS-level security controls. The vulnerability is rated with a CVSS 3.1 score of 8.3, indicating high severity due to its impact on confidentiality, integrity, and availability, combined with the complexity of exploitation requiring network-level access and control over the boot environment. No public exploits have been reported yet, but the potential for targeted attacks against critical infrastructure or enterprise systems is significant. The vulnerability affects Red Hat Enterprise Linux 7 installations using Shim for secure boot, which is common in enterprise and server environments.

For European organizations, this vulnerability poses a significant risk primarily to environments running Red Hat Enterprise Linux 7 with Shim-based secure boot enabled. The ability to execute code during the early boot phase means attackers can implant persistent, stealthy malware that survives OS reinstalls and evades detection by traditional security tools. This can lead to full system compromise, data breaches, disruption of critical services, and potential lateral movement within networks. Sectors such as finance, government, telecommunications, and critical infrastructure, which often rely on RHEL 7 for server and network appliance deployments, are particularly vulnerable. The requirement for a MitM or boot server compromise limits the attack surface but also implies that organizations with remote or distributed boot environments, or those using network booting (PXE) with HTTP-based delivery, are at higher risk. The impact extends to confidentiality (data exposure), integrity (system and data manipulation), and availability (system downtime or destruction). Given the widespread use of RHEL in European enterprise and public sectors, the threat could affect a broad range of organizations if exploited.

1. Secure the boot environment by restricting physical and network access to boot servers and infrastructure to prevent unauthorized interception or modification of boot traffic. 2. Employ network-level protections such as strong encryption (e.g., HTTPS with validated certificates) and network segmentation to protect HTTP boot traffic from MitM attacks. 3. Monitor and audit boot server configurations and logs for signs of compromise or unauthorized changes. 4. Apply any available patches or updates from Red Hat addressing this vulnerability as soon as they are released. 5. Consider upgrading to newer versions of Red Hat Enterprise Linux that may include improved secure boot mechanisms or mitigations. 6. Implement hardware-based secure boot features and trusted platform modules (TPM) to enhance boot integrity verification. 7. Use intrusion detection systems capable of monitoring early boot processes and network boot traffic anomalies. 8. Educate system administrators on the risks of network booting and the importance of securing boot infrastructure. 9. Regularly review and update incident response plans to include scenarios involving boot-level compromises.