CVE-2023-40547 in shim
Summary
by MITRE • 01/25/2024
A remote code execution vulnerability was found in Shim. The Shim boot support trusts attacker-controlled values when parsing an HTTP response. This flaw allows an attacker to craft a specific malicious HTTP request, leading to a completely controlled out-of-bounds write primitive and complete system compromise. This flaw is only exploitable during the early boot phase, an attacker needs to perform a Man-in-the-Middle or compromise the boot server to be able to exploit this vulnerability successfully.
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Analysis
by VulDB Data Team • 02/05/2026
The vulnerability identified as CVE-2023-40547 represents a critical remote code execution flaw within the Shim boot support module that operates as a foundational component in UEFI-based systems. Shim serves as a bootloader that facilitates the secure boot process by verifying signatures of subsequent bootloaders before execution. This particular vulnerability arises from insufficient validation of HTTP response data that Shim processes during the boot initialization phase, creating a dangerous trust relationship with attacker-controlled inputs that can be manipulated through network-based attacks.
The technical implementation of this vulnerability stems from improper input validation within Shim's HTTP parsing functionality, where the bootloader accepts and processes untrusted network data without adequate sanitization or bounds checking. This flaw manifests as an out-of-bounds write primitive that allows attackers to overwrite memory locations beyond the intended buffer boundaries, effectively providing them with arbitrary code execution capabilities. The vulnerability specifically targets the early boot phase when Shim is actively processing HTTP responses, typically during secure boot initialization or firmware updates, making the attack surface highly constrained yet extremely impactful.
The operational impact of this vulnerability extends beyond simple remote code execution to encompass complete system compromise during the critical boot process. Attackers can exploit this flaw to gain root-level access to systems before traditional security controls take effect, effectively bypassing hardware-based security measures and operating system protections. The exploitation requires network-based attack vectors such as man-in-the-middle attacks or compromise of boot servers, which aligns with the attack pattern described in the attack tactics and techniques framework where adversaries target the initial system boot process to establish persistent access. This vulnerability directly maps to CWE-121, which describes heap-based buffer overflow conditions, and aligns with attack techniques documented in the MITRE ATT&CK framework under initial access and execution phases.
The exploitation of CVE-2023-40547 requires attackers to establish a position within the network to intercept or manipulate HTTP traffic destined for the Shim bootloader, typically through network-based attacks such as DNS spoofing, ARP cache poisoning, or direct network interception. The attack is constrained by the timing requirements of the boot process, meaning that successful exploitation must occur during the specific window when Shim is actively processing HTTP responses rather than during normal system operation. Organizations affected by this vulnerability should implement immediate mitigations including network segmentation, secure boot enforcement, and firmware updates to address the underlying parsing logic that permits untrusted HTTP data to influence system behavior. The vulnerability highlights the critical importance of validating all network inputs within boot processes and demonstrates how attackers can leverage trust relationships within early boot stages to achieve complete system compromise.