Firmware Flaws Allow Disabling Secure Boot on Lenovo Laptops

Computer maker Lenovo has started pushing security patches to address three vulnerabilities impacting the UEFI firmware of more than 110 laptop models.

Two of the security flaw — CVE-2021-3972 and CVE-2021-3971 — exist because drivers that should have been used during the manufacturing process only were mistakenly left in production UEFI firmware, potentially exposing devices to attacks.

According to a Lenovo advisory, exploitation of the driver flaws could allow attackers with elevated privileges to either modify the secure boot settings (CVE-2021-3972) or modify the firmware protection region (CVE-2021-3971).

Lenovo credited researchers at anti-malware firm ESET with reporting the vulnerabilities.

ESET notes that the ability to disabling Secure Boot has a significant impact on system security, as it could allow for malicious drivers and applications to be executed during boot. On the other hand, resetting the firmware to factory settings could allow an attacker to deploy vulnerable UEFI applications to exploit in future attacks.

[ READ: High-Severity UEFI Flaws Patched in Dell Laptops ]

ESET notes that CVE-2021-3971 can be exploited to disable the write-protections mechanisms for the SPI flash, the embedded flash memory chip on which the UEFI firmware usually resides.

This could allow an attacker to deploy malicious code directly to the firmware storage, according to documentation shared by ESET.  

Tracked as CVE-2021-3970, the third bug – which exists because of insufficient validation in the SW SMI handler function – could allow a local attacker with elevated privileges to execute arbitrary code.

SW SMI (software System Management Interrupt) is a processor interrupt that needs to be triggered to enter a highly privileged execution mode of x86 processors called System Management Mode (SMM).

“This vulnerability can be exploited from a privileged kernel-mode process by triggering the software SMI interrupt and passing a physical address of a specially crafted buffer as a parameter to the vulnerable SW SMI handler,” ESET added.

The security defects were reported to Lenovo in October 2021. The computer maker has already issued patches for multiple laptop models and targets May 10 for rollout of firmware updates for the remaining products.

Several laptop models that have reached end-of-life (EOL) will remain unpatched, Lenovo said.

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