EUVD-2026-43801
GHSA-vm4m-9p6f-gcg7
Severity by source
AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
Local attacker with low privileges bypasses Secure Boot without user interaction; boot-chain compromise yields high confidentiality, integrity, and availability impact on the same system (scope unchanged).
Primary rating from Vendor (microsoft).
CVSS VectorVendor: microsoft
Lifecycle Timeline
2DescriptionCVE.org
Improperly implemented security check for standard in Windows Secure Boot allows an authorized attacker to bypass a security feature locally.
AnalysisAI
Security feature bypass in Windows Secure Boot (CWE-358) allows a locally authenticated attacker on affected Windows 10, Windows 11, and Windows Server (2016 through 2025) systems to defeat the boot-integrity trust chain due to an improperly implemented standard security check. Because Secure Boot is the gate that blocks unsigned/tampered bootloaders and rootkits, a successful bypass can enable pre-OS persistence and undermine downstream protections such as BitLocker and Measured Boot. There is no public exploit identified at time of analysis and the CVE is not listed in CISA KEV; Microsoft has published a patch via its update guide.
Technical ContextAI
Secure Boot is a UEFI firmware feature that verifies the cryptographic signature of every component in the boot chain (firmware drivers, bootloader, OS loader) against keys held in the platform's UEFI variables (db/dbx/KEK/PK) before execution, preventing bootkits and unsigned code from loading. CWE-358 (Improperly Implemented Security Check for Standard) indicates the flaw is not a missing check but a check that deviates from the specified standard behavior - the validation logic exists but can be satisfied or circumvented in a way the specification does not intend, allowing an untrusted component to pass verification. The CPE data scopes this to Microsoft's own boot-path components across Windows 10 (1607, 1809, 21H2, 22H2), Windows 11 (24H2, 25H2, 26H1), and Windows Server 2016/2019/2022/2025 including Server Core installations, indicating a shared OS-level Secure Boot enforcement component rather than a third-party bootloader.
RemediationAI
Patch available per vendor advisory - apply the Microsoft security update for CVE-2026-49783 to every affected Windows 10, Windows 11, and Windows Server (including Server Core) build; the exact fixed KB/build per version is listed at https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-49783. Note that Secure Boot fixes frequently require an accompanying dbx (revocation list) update or a two-stage enablement to avoid bricking dual-boot or older-media scenarios, so validate the advisory's deployment guidance before mass rollout. Until patched, reduce exposure by restricting local and low-privilege code execution on sensitive hosts (limit local admin/interactive logon, enforce application control such as WDAC/AppLocker), keep BitLocker enabled with TPM+PIN so that boot-chain tampering triggers recovery-key prompts, and monitor Measured Boot / TPM attestation logs for unexpected boot-component changes - accepting that these are detective/limiting controls, not a substitute for the fix.
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External POC / Exploit Code
Leaving vuln.today