CVE-2026-20293: Cisco UCS UEFI Secure Boot Bypass Vulnerability

CVE-2026-20293 Overview

CVE-2026-20293 is a UEFI Secure Boot bypass vulnerability affecting the Unified Extensible Firmware Interface (UEFI) Shell implementation on Cisco Unified Computing System (UCS) Servers and UCS-based appliances. The flaw permits an authenticated attacker holding valid user or admin credentials, or an unauthenticated attacker with physical access, to circumvent Secure Boot validation checks. Exploitation enables execution of unauthorized software in the preboot environment. The root weakness is classified under CWE-749: Exposed Dangerous Method or Function, specifically the availability of memory write commands inside the UEFI Shell while Secure Boot is enabled.

Critical Impact
Successful exploitation allows an attacker to overwrite UEFI Secure Boot-related memory values and load unsigned or malicious code during the boot process, undermining the platform integrity chain of trust.

Affected Products

Cisco UCS Servers running affected UEFI firmware
Cisco UCS-based appliances with the vulnerable UEFI Shell implementation
Systems where UEFI Shell is exposed as a selectable boot option

Discovery Timeline

2026-09-08 - CVE-2026-20293 published to the National Vulnerability Database
2026-09-11 - Last updated in the NVD database

Technical Details for CVE-2026-20293

Vulnerability Analysis

The vulnerability resides in the UEFI Shell shipped with Cisco UCS platform firmware. The shell exposes memory write commands that remain accessible even when Secure Boot enforcement is active. An attacker who reaches the boot menu can launch the UEFI Shell and use those commands to modify UEFI memory variables that govern Secure Boot policy decisions.

By tampering with these preboot variables, the attacker can neutralize signature verification and load unauthorized bootloaders, drivers, or operating system components. This defeats the platform's chain of trust before any operating system defenses are loaded. Because the compromise occurs below the OS, endpoint security agents running post-boot cannot observe the initial tampering.

The attack requires local access (AV:L). Two exploitation profiles apply: an authenticated administrator or standard user with console or KVM access, and an unauthenticated actor with physical access to the device.

Root Cause

The root cause is an exposed dangerous function [CWE-749]. The UEFI Shell binary bundled by the vendor retains memory manipulation commands that should be disabled or gated when Secure Boot is enforced. No policy check prevents writes to Secure Boot-related NVRAM and memory variables from within the shell.

Attack Vector

Exploitation follows a consistent sequence. The attacker gains console, KVM, or physical access to the target UCS device, interrupts the boot process, and selects the UEFI Shell entry from the boot menu. From the shell prompt, the attacker issues memory write commands to modify Secure Boot policy variables. The attacker then chains the boot process to a malicious payload, which the platform now accepts because signature enforcement has been subverted.

Refer to the Cisco Security Advisory on UEFI Bypass for vendor-specific technical detail.

Detection Methods for CVE-2026-20293

Indicators of Compromise

Unexpected entries or modifications to UEFI boot order and boot variables between reboots
Console or KVM sessions that interrupt boot and select the UEFI Shell option outside change-window activity
Presence of unsigned or unrecognized EFI binaries loaded during boot as reported by firmware measurement logs
Changes to Secure Boot policy variables (PK, KEK, db, dbx) without a corresponding administrative record

Detection Strategies

Collect and review Cisco Integrated Management Controller (CIMC) logs for boot option selections and UEFI Shell launches
Compare TPM PCR measurements against a known-good baseline to detect preboot component changes
Alert on remote KVM sessions initiated during off-hours or by accounts that do not typically perform maintenance
Correlate physical access events from data center badge systems with unscheduled server reboots

Monitoring Recommendations

Forward CIMC syslog and audit events to a central SIEM or data lake for retention and correlation
Enable TPM event log collection on supported UCS platforms and baseline the expected measurement chain
Monitor for firmware and BIOS version drift across the UCS fleet using vendor management tooling

How to Mitigate CVE-2026-20293

Immediate Actions Required

Apply the firmware update referenced in the Cisco Security Advisory on UEFI Bypass as soon as maintenance windows allow
Restrict physical and out-of-band management access to UCS hardware to authorized personnel only
Audit CIMC user accounts and remove or downgrade any user or admin accounts that do not require console redirection
Enforce strong authentication and session logging for all CIMC and KVM access paths

Patch Information

Cisco has published fixed firmware releases in the vendor advisory. Administrators should identify each UCS server model in scope, map it to the corresponding fixed release, and schedule firmware upgrades through Cisco UCS Manager or Intersight. There are no vendor-supplied cryptographic patch identifiers in the enriched data; refer to the advisory for exact version numbers.

Workarounds

Set a strong BIOS or UEFI setup password to prevent unauthorized changes to boot order
Disable the UEFI Shell boot option in system setup where operationally feasible
Remove UEFI Shell entries from the persistent boot order and require signed boot media only
Enable chassis intrusion detection and physical locks on racks hosting UCS equipment

bash

# Configuration example: remove the UEFI Shell entry from the boot order
# Review current boot entries from a trusted OS prior to the vulnerable boot cycle
efibootmgr -v

# Delete the boot entry that references the UEFI Shell (replace XXXX with the entry ID)
efibootmgr -b XXXX -B

# Confirm removal
efibootmgr -v