CVE-2024-26240: Windows 10 1507 Secure Boot Bypass Flaw

CVE-2024-26240 Overview

CVE-2024-26240 is a Secure Boot security feature bypass vulnerability affecting a broad range of Microsoft Windows client and server operating systems. The flaw is categorized under [CWE-20] Improper Input Validation and carries a CVSS 3.1 base score of 8.0. Successful exploitation allows an attacker on an adjacent network to bypass the Unified Extensible Firmware Interface (UEFI) Secure Boot protections that verify boot component integrity. Exploitation requires user interaction, but a successful attack impacts confidentiality, integrity, and availability of the target system. Microsoft addressed the issue through its April 2024 security update cycle.

Critical Impact

An attacker who bypasses Secure Boot can load unsigned or malicious boot components, enabling persistent pre-OS code execution such as bootkits that survive OS reinstallation.

Affected Products

• Microsoft Windows 10 (versions 1507, 1607, 1809, 21H2, 22H2)
• Microsoft Windows 11 (versions 21H2, 22H2, 23H2)
• Microsoft Windows Server 2008, 2012, 2016, 2019, 2022, and 2022 23H2

Discovery Timeline

• 2024-04-09 - CVE-2024-26240 published to the National Vulnerability Database (NVD)
• 2024-04-09 - Microsoft published its security update guidance for CVE-2024-26240
• 2026-06-17 - Last updated in the NVD database

Technical Details for CVE-2024-26240

Vulnerability Analysis

CVE-2024-26240 is a Secure Boot bypass rooted in improper input validation ([CWE-20]) within Windows boot-path components. Secure Boot is a UEFI feature that validates the cryptographic signatures of firmware modules, bootloaders, and early OS components before executing them. When validation logic fails to correctly parse or verify attacker-influenced inputs, an adversary can cause the boot chain to accept content that should have been rejected.

The vulnerability requires adjacent network access and user interaction, indicating an attack path where a user is induced to interact with a crafted resource reachable on the local network segment. Once the bypass succeeds, the integrity guarantees of Secure Boot no longer hold for that host, and the attacker can influence code executed before the operating system kernel loads.

Root Cause

The underlying weakness is improper input validation in a Secure Boot-related component. Windows boot components fail to fully validate untrusted input, allowing the verification flow to be subverted. Microsoft has not published deeper internal details beyond the advisory, and no proof-of-concept has been publicly released.

Attack Vector

The attack vector is Adjacent Network (AV:A) with low attack complexity and no privileges required, but user interaction is required. A plausible exploitation chain involves an attacker on the same broadcast domain, VLAN, or link-local segment delivering a crafted boot resource or update payload that the target consumes during a boot or update workflow. Successful bypass leads to loading of unauthorized boot components, potentially enabling bootkit installation, tampering with BitLocker or measured-boot state, and persistent compromise beneath the operating system.

No verified public exploit code is available for CVE-2024-26240. Refer to the Microsoft Security Update for CVE-2024-26240 for authoritative technical guidance.

Detection Methods for CVE-2024-26240

Indicators of Compromise

• Unexpected changes to UEFI variables, the EFI system partition, or entries in the BCD (Boot Configuration Data) store.
• Measured Boot logs (TPM PCR values) that diverge from a known-good baseline after a reboot.
• Unsigned or unexpected drivers, bootloaders, or .efi binaries present on the EFI system partition.
• Windows event log entries indicating Secure Boot policy changes or boot integrity failures.

Detection Strategies

• Compare current Secure Boot state and DBX revocation list against expected values using Confirm-SecureBootUEFI and Get-SecureBootPolicy.
• Monitor for anomalous writes to the EFI system partition and boot configuration data outside of authorized Windows Update activity.
• Leverage TPM attestation and Measured Boot log analysis to detect divergence in the pre-OS chain of trust.
• Correlate boot-time telemetry with adjacent-network activity such as rogue PXE, DHCP, or update servers.

Monitoring Recommendations

• Ingest Windows boot, TPM, and Secure Boot events into a centralized SIEM for continuous baselining.
• Alert on new or modified files under the EFI system partition on managed endpoints.
• Track deployment status of the April 2024 Windows security update across the estate to identify unpatched hosts.
• Watch for unauthorized DHCP, PXE, or SMB services on internal network segments that could host malicious boot payloads.

How to Mitigate CVE-2024-26240

Immediate Actions Required

• Apply the April 2024 Microsoft security update that addresses CVE-2024-26240 on all affected Windows client and server versions.
• Prioritize patching hosts with TPM-backed BitLocker, domain controllers, and systems on segments where adjacent-network exposure is high.
• Verify that Secure Boot remains enabled in UEFI firmware after patching and audit for firmware downgrade attempts.
• Confirm DBX (Secure Boot revocation database) updates are installed alongside the OS patch.

Patch Information

Microsoft has released fixes for CVE-2024-26240 across all supported Windows 10, Windows 11, and Windows Server versions listed in the advisory. Refer to the Microsoft Security Update Guide for CVE-2024-26240 for the specific KB articles applicable to each affected build.

Workarounds

• Restrict access to trusted network segments and disable unused PXE or network boot options on endpoints where feasible.
• Enforce network segmentation and 802.1X authentication to reduce adjacent-network exposure required by the attack vector.
• Require user interaction only through trusted, signed workflows and educate users to avoid unverified boot or update prompts.
• Enable TPM-based Measured Boot and remote attestation to detect boot integrity deviations if a bypass is attempted.

# Verify Secure Boot state and applied updates on Windows
Confirm-SecureBootUEFI
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