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CVE-2026-49783 is a vulnerability in Microsoft Windows Secure Boot that stems from an improperly implemented security check for a standardized verification routine. Secure Boot is a UEFI-based security feature designed to ensure that only cryptographically signed, trusted bootloaders, drivers, and operating system kernels are loaded during the system startup process. It relies on a chain of trust anchored in the platform firmware, where each component in the boot sequence must be signed by a trusted certificate authority (CA) and verified before execution. The vulnerability arises because a specific security-relevant check—likely related to the validation of a particular UEFI variable, boot policy, or signature verification step—was not correctly implemented according to the design specification of the underlying standard.
An authorized attacker with local access and low privileges can exploit this flaw to bypass the Secure Boot verification mechanism. The attack vector is local (AV:L), the attack complexity is low (AC:L), and no user interaction is required (UI:N). The attacker does not need elevated privileges—only a standard authenticated user account suffices (PR:L). The scope is unchanged (S:U), but the impact on confidentiality, integrity, and availability is high (C:H/I:H/A:H), earning a CVSS v3.1 base score of 7.8 (HIGH).
The root cause is mapped to CWE-358: Improperly Implemented Security Check for Standard. This indicates that the software either omits or incorrectly implements one or more security-relevant checks that are mandated by a standardized algorithm, protocol, or technique. In the context of Secure Boot, such a check could involve the verification of digital signatures, the enforcement of boot policy restrictions, or the validation of UEFI runtime variables. By bypassing this check, an attacker can load unsigned or malicious boot components, effectively breaking the chain of trust that Secure Boot is designed to enforce.
The vulnerability affects a wide range of Windows versions, including Windows 10 (1607, 1809, 21H2, 22H2), Windows 11 (24H2, 25H2, 26H1), and Windows Server (2016, 2019, 2022, 2025). Specific build thresholds have been identified as vulnerable, and Microsoft released security updates on July 14, 2026, to address the issue. The patches increment the build numbers to versions that are no longer susceptible to this bypass. Administrators are urged to apply these updates immediately, especially on systems where boot-chain integrity is critical, such as BitLocker-protected endpoints, privileged workstations, virtualization hosts, and servers exposed to untrusted local users. As of the patch release, Microsoft had not observed public disclosure or active exploitation in the wild, but given the local attack requirements and the potential for chaining with other vulnerabilities, the risk remains significant for environments where an attacker can already obtain a foothold.
DailyCVE Form:
Platform: Windows Secure Boot
Version: 10/11/Server (multiple)
Vulnerability: Security Feature Bypass
Severity: HIGH (7.8)
date: July 14, 2026
Prediction: July 14, 2026
What Undercode Say: Analytics & Detection
Administrators can use the following PowerShell and Bash commands to assess Secure Boot status, enumerate UEFI variables, and check for vulnerable build versions.
Check Secure Boot status (PowerShell)
Confirm-SecureBootUEFI
Get current OS build number
List all UEFI variables (requires admin)
Get-SecureBootPolicy
Verify if the system is vulnerable based on build number
$build = (Get-ItemProperty "HKLM:SOFTWARE\Microsoft\Windows NT\CurrentVersion").CurrentBuild
if ($build -lt 14393 -or ($build -eq 14393 -and (Get-ItemProperty "HKLM:SOFTWARE\Microsoft\Windows NT\CurrentVersion").UBR -lt 9339)) {
Write-Host "Vulnerable to CVE-2026-49783"
}
On Linux (for dual-boot or UEFI shell environments) Check Secure Boot status via efivar sudo efivar -l | grep -i secureboot Read the SecureBoot variable (returns 1 if enabled) sudo hexdump -C /sys/firmware/efi/efivars/SecureBoot- List all boot entries sudo efibootmgr -v Check for vulnerable shim versions (if applicable) strings /boot/efi/EFI//shim.efi | grep -i "version"
Exploit: Practical Attack Scenario
While no public exploit is available as of the patch date, the vulnerability can be weaponized in a multi-stage attack:
1. Initial Access – The attacker gains a low-privileged local account on the target machine (e.g., via phishing, credential reuse, or physical access).
2. Privilege Escalation & Secure Boot Bypass – The attacker executes a crafted UEFI application or manipulates the boot configuration to trigger the improperly implemented security check. This allows the loading of an unsigned bootkit or malicious bootloader.
3. Persistence – The malicious boot component installs itself into the UEFI firmware or the boot partition, ensuring it runs before the operating system loads.
4. Payload Delivery – The bootkit can disable security software, tamper with the kernel, and maintain persistence across reboots and OS reinstalls.
A simplified proof-of-concept (hypothetical) might involve:
// Pseudo-code for triggering the bypass
// Note: This is illustrative, not a working exploit
EFI_STATUS BypassSecureBoot() {
// Manipulate a UEFI variable that is improperly validated
EFI_GUID gEfiGlobalVariableGuid = EFI_GLOBAL_VARIABLE;
UINT32 Attributes = EFI_VARIABLE_BOOTSERVICE_ACCESS | EFI_VARIABLE_RUNTIME_ACCESS;
// Write a malformed payload to a trusted variable
SetVariable(L"SecureBootBypass", &gEfiGlobalVariableGuid, Attributes, sizeof(payload), &payload);
// The flawed check fails to validate the integrity of the variable
// Allowing the boot process to proceed with untrusted code
return EFI_SUCCESS;
}
Protection: Mitigation and Hardening
- Apply Microsoft Updates Immediately – Install the July 2026 Patch Tuesday updates (released July 14, 2026) which include the fix for CVE-2026-49783. Ensure systems are updated to the following build thresholds or later:
- Windows 10 1607: ≥ 14393.9339
- Windows 10 1809: ≥ 17763.9020
- Windows 10 21H2/22H2: ≥ 19044.7548 / 19045.7548
- Windows 11 24H2: ≥ 26100.8875
- Windows 11 25H2: ≥ 26200.8875
- Windows 11 26H1: ≥ 28000.2269
- Windows Server 2016: ≥ 14393.9339
- Windows Server 2019: ≥ 17763.9020
- Windows Server 2022: ≥ 20348.5386
- Windows Server 2025: ≥ 26100.33158
- Enable Secure Boot in Firmware – Ensure Secure Boot is enabled in the UEFI firmware settings. Use `Confirm-SecureBootUEFI` to verify.
- Use TPM + BitLocker – Combine Secure Boot with TPM and BitLocker to provide additional protection against bootkits and unauthorized boot components.
- Monitor UEFI Variables – Implement auditing for changes to UEFI variables, especially those related to Secure Boot policies.
- Restrict Local Access – Limit physical and remote local access to systems, especially those handling sensitive data or operating in high-security environments.
Impact: Consequences of Successful Exploitation
- Confidentiality (High) – An attacker can read sensitive data by loading a malicious bootkit that intercepts OS-level encryption keys or memory contents.
- Integrity (High) – The attacker can modify system files, kernel structures, and security policies, effectively compromising the entire trust model of the platform.
- Availability (High) – The bootkit can render the system unbootable, cause denial of service, or perform destructive actions during the boot process.
- Persistence – Bootkits installed via this bypass can survive OS reinstalls, firmware updates (if not properly applied), and traditional antivirus scans, making them extremely difficult to detect and remove.
- Chained Attacks – This vulnerability is a powerful enabler for advanced persistent threats (APTs) and ransomware operators, allowing them to establish a persistent, low-level presence that can evade most endpoint detection and response (EDR) solutions.
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Sources:
Reported By: nvd.nist.gov
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