CVSS 7.1: Cisco UCS UEFI Shell Memory Commands Enable Secure Boot Bypass

Cisco UCS servers and appliances contain a flaw that allows attackers to bypass UEFI Secure Boot protections and run unauthorized software on the devices.

The risk

The vulnerability, identified as CVE-2026-20293, stems from the availability of memory write commands within the UEFI Shell when Secure Boot is enabled. According to Cisco PSIRT, an attacker can select the UEFI Shell boot option at startup and use these commands to modify UEFI memory variables. This manipulation allows the attacker to overwrite Secure Boot-related memory values, effectively bypassing validation checks. The Cisco advisory rates the security impact as high, with a CVSS base score of 7.1.

Who is exposed

The flaw affects Cisco UCS Servers and UCS-based appliances running vulnerable BIOS versions with UEFI Secure Boot enabled. Affected server products include the 5000 Series Enterprise Network Compute Systems. Cisco appliances based on preconfigured UCS C-Series Servers are also at risk if they support Secure Boot and expose KVM or virtual KVM console access. This includes Application Policy Infrastructure Controller Servers and Secure Firewall Management Center Appliances. For these specific appliances, physical KVM access is required to exploit the vulnerability.

Cost and compliance

Cisco states that there are no workarounds for this vulnerability. The company has released software updates that address the issue by removing UEFI Shell commands that allow memory modifications when Secure Boot is enabled. The advisory, first published in September 2026, lists several Cisco Bug IDs associated with the fix. Organizations must apply these updates to mitigate the risk, as no temporary mitigations are available.

What should you do?

Managers should verify if their UCS servers and appliances are running vulnerable BIOS versions. Teams should prioritize applying the latest software updates released by Cisco to remove the dangerous memory commands. Until updates are applied, restrict physical access to devices and monitor for unauthorized boot attempts.

The background in plain language

UEFI Secure Boot is a security standard that ensures a device only boots using software trusted by the original equipment manufacturer. It checks digital signatures before loading the operating system. The UEFI Shell is a command-line interface used for debugging and controlling boot options. When this shell allows direct memory writes while Secure Boot is active, attackers can alter the system's trust state before the operating system starts, bypassing the security check entirely.

Background: Secure Boot

Secure Boot validates every boot component using digital signatures against trusted keys stored in the firmware. It ensures only authorized software runs during startup, blocking rootkits that target the pre-operating system environment, though it does not protect the system once the OS is fully loaded.

Read the full guide: Secure Boot Mechanism: Step-by-Step Verification and Limits

What to do and how to stay safe: Cisco

• Check your inventory for Cisco UCS servers and appliances running vulnerable BIOS versions with Secure Boot enabled.
• Apply the software updates released by Cisco that remove memory modification commands from the UEFI Shell.
• Restrict physical access to servers and disable unnecessary KVM or virtual console access where possible.
• Monitor boot logs for unauthorized access to the UEFI Shell or unexpected boot option changes.

Step-by-step guide: Zero-Day Vulnerabilities Meaning: How Unpatched Flaws Bypass Your Defenses

General security guidance from the Vector Tribune newsroom. It is not confirmed advice from the organisations named in this story.

Frequently asked questions

What is the CVSS score for CVE-2026-20293?

The CVSS base score is 7.1, which Cisco rates as high severity.

Can this vulnerability be exploited remotely?

No, exploitation requires either valid user credentials for a user or admin account or physical access to the device.

Are there any workarounds for this vulnerability?

No, Cisco states there are no workarounds; organizations must apply the released software updates.