Cisco UCS servers face UEFI Secure Boot bypass enabling unauthorized code execution via memory writes

Cisco has released software updates to address a high-severity vulnerability in its Unified Computing System servers and related appliances. The flaw, tracked as CVE-2026-20293, allows attackers to bypass UEFI Secure Boot validation checks. This security feature bypass occurs because the UEFI Shell implementation permits memory write commands even when Secure Boot is active on the device.

The risk

An attacker can exploit this vulnerability by selecting the UEFI Shell boot option during the startup process. According to Cisco PSIRT, this enables the use of shell commands to modify UEFI memory variables. A successful exploit allows the manipulation of the preboot environment and the overwriting of Secure Boot-related memory values. This effectively bypasses validation checks, permitting the execution of unauthorized software on the affected hardware.

Who is exposed

Cisco UCS Servers and UCS-based appliances with UEFI Secure Boot enabled are affected if they run vulnerable BIOS versions. This includes 5000 Series Enterprise Network Compute Systems. Affected appliances also include Application Policy Infrastructure Controller Servers and Secure Firewall Management Center Appliances. For these appliances, physical KVM access is required to exploit the flaw, which significantly reduces the attack vector.

Cost and compliance

Cisco rates the security impact of this vulnerability as high. The advisory assigns a CVSS base score of 7.1. The vulnerability is associated with CWE-749, which relates to improper handling of memory resources. Cisco has confirmed that software updates addressing this issue are available. The company states there are no workarounds that mitigate this vulnerability.

What to watch

Security teams should verify that all Cisco UCS servers and appliances are running the latest BIOS and firmware versions provided by Cisco. Organizations should audit their UEFI boot configurations to ensure Secure Boot is enabled where supported. Monitoring for unusual boot-time activities or unauthorized shell access attempts can help detect exploitation attempts before they succeed.

Signals worth monitoring

Teams usually look for unexpected UEFI Shell sessions in boot logs or firmware telemetry. Check whether preboot execution times deviate from baselines, which may indicate manual intervention or script execution. Monitor for unauthorized changes to boot order or Secure Boot policy flags in system event logs. Verify that firmware integrity measurements match known good states during startup sequences. Containment often involves isolating affected hosts from management networks until firmware updates can be applied safely.

What to do and how to stay safe: Cisco

• Apply the latest BIOS and firmware updates released by Cisco for all affected UCS servers and appliances.
• Restrict physical access to server hardware and disable unnecessary KVM or virtual console access to prevent local exploitation.
• Audit user accounts with admin or user roles to ensure credentials are strong and access is limited to necessary personnel.
• Monitor boot logs for unexpected UEFI Shell activity or changes to Secure Boot configuration settings.

Step-by-step guide: Vulnerability Prioritization: Fix What Actually Matters First

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

Frequently asked questions

What is the CVSS score for this Cisco UEFI vulnerability?

Cisco rates the vulnerability as high with a CVSS base score of 7.1.

Are there any workarounds for CVE-2026-20293?

No, Cisco states there are no workarounds available for this vulnerability.

Which Cisco products are affected by this flaw?

Cisco UCS Servers, 5000 Series ENCS, APIC Servers, and Secure Firewall Management Center Appliances are affected.