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Cisco UCS and UCS-Based Appliances UEFI Shell Secure Boot Bypass Vulnerability
Cisco disclosed a UEFI Secure Boot bypass in UCS servers and UCS-based appliances letting authenticated or physically present attackers execute unauthorized software.
Cisco published an advisory for a vulnerability in the UEFI Shell implementation of UCS servers and UCS-based appliances. Memory write commands remain available in the UEFI Shell while Secure Boot is enabled, allowing an attacker to modify UEFI memory and bypass validation checks to run unauthorized software. Exploitation requires either valid credentials for a user or admin account, or unauthenticated physical access to select the UEFI Shell boot option at boot time. The issue affects firmware boot integrity rather than the running operating system.
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Cisco warns about public exploit code for critical flaws in its 220 Series smart switches
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VU#718077: UEFI Shell module embedded in SPI Flash can be used to bypass Secure Boot
CERT/CC details VU#718077: UEFI Shell embedded in SPI flash lets attackers bypass Secure Boot and execute pre-boot code; patches issued.
CERT/CC's VU#718077, reported by Eclypsium researcher Stas Lyakhov, describes how a UEFI Shell embedded in SPI flash can be abused by attackers who can modify UEFI boot configuration, creating multiple boot entries that bypass controls preventing the Shell from launching under Secure Boot. The Shell's dmem and mm commands allow arbitrary physical memory read/write, letting attackers overwrite Secure Boot values and execute unauthorized pre-boot code that can persist across reboots and OS reinstalls while degrading EDR effectiveness. AMI confirmed its Aptio UEFI BDS module is affected (CVE-2026-33197), and Cisco published an advisory for a variation affecting UCS Servers and UCS-based appliances (CVE-2026-20293). Firmware patches are being rolled out through OEM and IBV BIOS build pipelines.
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Cisco IMC auth bypass vulnerability allows attackers to alter user passwords (CVE-2026-20093)
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Cisco fixes 3 high-severity DoS flaws in NX
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Cisco drops security fixes for Smart Software Manager, security appliances
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