ESET reported that 11 old, Microsoft-signed UEFI shim bootloaders can bypass Secure Boot and help an attacker run bootkit malware during startup. Microsoft revoked the reported binaries in a June 9, 2026 dbx update, according to ESET. Install current Windows and Secure Boot updates, including the applicable dbx revocation. The finding describes a potential exposure, not evidence that every PC is vulnerable or infected.
What the Secure Boot flaw does
In a disclosure dated July 14, 2026, ESET researcher Martin Smolár reported 11 vulnerable UEFI shim bootloaders, versions 0.9 and earlier, that had been signed by Microsoft. A shim is an early-boot program used in some systems to continue the boot process. ESET says the vulnerable binaries could be used to bypass UEFI Secure Boot and execute untrusted code during startup, potentially enabling a bootkit.
The reported case is identified as CVE-2026-8863 and CVE-2026-10797. ESET says it reported its findings, including a proof of concept, to CERT/CC on February 16, 2026, and that Microsoft revoked the reported binaries in the June 9, 2026 Patch Tuesday dbx update. ESET Research’s Martin Smolár recommends: “To protect your systems against this threat, install the latest Microsoft dbx updates.”
The 11 figure counts identified shim binaries; it is not an estimate of affected computers. The cited disclosures do not establish how many devices were attacked or infected, or that the flaw is being actively exploited.
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Does this affect my PC?
ESET describes the relevant condition as a UEFI-based system that trusts Microsoft’s Microsoft Corporation UEFI CA 2011 third-party certificate. The risk is not limited to a computer that already has one of the affected Linux distributions or utilities installed: an attacker could bring a vulnerable shim to another system that trusts that certificate. That is a potential exposure condition, not proof of compromise.
| What to check | Why it matters | What to do |
|---|---|---|
| Whether the system trusts Microsoft Corporation UEFI CA 2011 | ESET says this trust setting is relevant to whether the reported shims could be accepted, regardless of the installed operating system. | Check the device’s Secure Boot configuration using the applicable vendor or organization guidance; the available guidance does not establish one universal check for every PC. |
| Whether the applicable dbx revocation update is installed | ESET says Microsoft revoked the reported binaries in the June 9, 2026 dbx update. | Install current Windows and Secure Boot updates, and follow Microsoft or OEM guidance for confirming update status. |
| Whether the device needs OEM firmware support | Some Secure Boot changes can depend on device firmware and vendor support. | Check the computer maker’s instructions if updates are missing, fail, or require firmware action. |
| Whether updates are personally or organization-managed | Managed devices may follow a controlled deployment schedule rather than an individual user’s update settings. | Ask the IT administrator to confirm the dbx update and any required firmware steps. |
ESET says Windows 11 Secured-core PCs should have Microsoft’s third-party UEFI signing option disabled by default. Treat that as a qualified expectation, not a guarantee for every configuration; verify the actual device setting and applicable vendor instructions.
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How do I update Secure Boot?
- Install available Windows updates. On Windows 11, open Settings > Windows Update and select Check for updates. Install the available updates and restart if prompted. Ensure the applicable Secure Boot dbx revocation update is included; ESET identifies the latest Microsoft dbx updates as the mitigation for the reported shims.
- Check device-specific instructions. Consult the computer maker’s support guidance for firmware requirements or update-status checks. Microsoft says some devices may need an OEM firmware update for Secure Boot certificate updates, and firmware behavior can vary by device.
- For a work or school computer, use the organization’s process. Contact IT rather than applying firmware changes outside the managed deployment process.
- Keep Secure Boot enabled. Do not turn it off as a workaround unless an informed administrator or the OEM gives a device-specific reason.
A PC that starts normally is not necessarily current on Secure Boot protections. Microsoft says a device without updated Secure Boot certificates may still start and install ordinary Windows updates while lacking future early-boot protections; that status alone does not show that the vulnerable shim is present or that the device is compromised.
How the shim revocation differs from the 2011 certificate expiration
These are two concurrent but separate Secure Boot maintenance issues. ESET’s reported vulnerability concerns old shim binaries signed by Microsoft and their potential to bypass Secure Boot; the June 9, 2026 dbx update revokes the reported binaries. Separately, Microsoft says Secure Boot certificates originally issued in 2011 begin expiring in June 2026, and it is delivering a new set of 2023 certificates to maintain future Secure Boot protections.
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Microsoft says most personal Windows devices receive the 2023 certificate updates through Microsoft-managed updates, while some devices may need an OEM firmware update. Keep both items in view: the dbx revocation addresses the reported vulnerable shims, while the certificate transition maintains the trust chain as older certificates expire.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Should I disable Secure Boot?
No—not as a general response to this flaw or to the certificate transition. Microsoft warns that disabling Secure Boot significantly reduces device protection, removes safeguards against boot-level malware, and can create security and compliance risks. Keep it enabled unless a qualified administrator or the device maker provides a specific reason and instructions for that machine.
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