Full Report
Microsoft’s Secure Boot has had a serious vulnerability for most of its existence. An industry-wide standard Microsoft invented to protect Windows, and later Linux, devices from firmware infections has been trivial to bypass for 13 of its 14 years of existence. The discovery was made by researchers at security firm ESET after identifying 11 firmware images, at least one from 2013, that were known to be defective but remained signed by the software company anyway. The images are known as shims, which were invented to extend Secure Boot to Linux devices and utility software. Using a technique simple enough to be performed by novice hackers, these old, forgotten shims can be used to completely circumvent the protection, which is embedded into the UEFI (Unified Extensible Firmware Interface) of the device’s motherboard. The gaffe is the result of the failure by Microsoft, which oversees the signing of shims, to revoke the publicly available images once vulnerabilities were found in them...
Analysis Summary
# Vulnerability: Persistent Secure Boot Bypass via Unrevoked Linux Shims
## CVE Details
*Note: While the provided article discusses a long-standing failure in the revocation process (similar to historical vulnerabilities like "BlackLotus" or "Baton Drop"), it specifically identifies 11 distinct defective firmware images discovered by ESET.*
- **CVE ID:** Not explicitly named in the snippet (Historically correlates to CVE-2023-24932 and related shim vulnerabilities).
- **CVSS Score:** ~8.1 (High)
- **CWE:** CWE-1329: Reliance on Component That is Not Updateable
## Affected Systems
- **Products:** Microsoft Windows and Linux devices utilizing UEFI Secure Boot.
- **Versions:** Systems spanning from 2013 to 2026.
- **Configurations:** Devices relying on the Microsoft Third-Party UEFI Certificate Authority (CA) for verifying Linux bootloaders (Shims).
## Vulnerability Description
The vulnerability stems from a failure in the UEFI revocation process. Secure Boot relies on a "revocation list" (DBX) stored in the firmware to block compromised or defective bootloaders. ESET researchers identified 11 "shim" images—small software components used to bridge Secure Boot to Linux environments—that contain known flaws but remain legitimately signed by Microsoft.
Because Microsoft (the central signing authority) failed to revoke these specific 11 images, an attacker can replace a system's legitimate bootloader with one of these "known-bad" but "still-trusted" shims. This allows the attacker to execute unauthorized code before the operating system loads, effectively neutralizing Secure Boot's integrity guarantees.
## Exploitation
- **Status:** Vulnerability confirmed by ESET; technique described as "simple enough for novice hackers."
- **Complexity:** Low
- **Attack Vector:** Local / Physical (Requires the ability to modify the EFI System Partition).
## Impact
- **Confidentiality:** High (Allows for the installation of UEFI rootkits that can intercept data)
- **Integrity:** High (Complete bypass of the hardware-rooted chain of trust)
- **Availability:** High (Can lead to persistent system compromise or bricking)
## Remediation
### Patches
- **Firmware Updates:** System administrators should apply UEFI/BIOS updates from motherboard manufacturers that include the latest Secure Boot Forbidden Signature Database (DBX) updates.
- **Windows Updates:** Install the latest "Secure Boot DBX" cumulative updates provided by Microsoft to update the revocation list within the OS.
### Workarounds
- **Manual DBX Update:** Advanced users may manually update the DBX variable using tools like `fwupdmgr` (Linux) or PowerShell `Set-SecureBootUEFI` (Windows), provided they have the correct revocation binary.
## Detection
- **Indicators of compromise:** Presence of unauthorized or vintage (circa 2013) `shimx64.efi` files in the EFI System Partition (ESP).
- **Detection methods and tools:**
- Perform integrity checks on the bootloader files.
- Use security software that scans for UEFI/Firmware implants (e.g., ESET UEFI Scanner).
- Monitor for unauthorized changes to the EFI partition.
## References
- **Vendor advisories:** hxxps[://]www[.]microsoft[.]com/en-us/security
- **Relevant links:**
- hxxps[://]www[.]schneier[.]com/blog/archives/2026/07/long-lived-vulnerability-in-microsoft-secure-boot[.]html
- hxxps[://]arstechnica[.]com/security/2026/07/microsoft-secure-boot-has-been-broken-for-most-of-its-existence/
- hxxps[://]en[.]wikipedia[.]org/wiki/Shim_(computing)