A GRUB 2 security update has been released for Ubuntu Linux 18.04 LTS, 20.04 LTS, and 20.10.

USN-4992-1: GRUB 2 vulnerabilities

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Ubuntu Security Notice USN-4992-1
June 18, 2021

grub2-signed, grub2-unsigned vulnerabilities
==========================================================================

A security issue affects these releases of Ubuntu and its derivatives:

- Ubuntu 20.10
- Ubuntu 20.04 LTS
- Ubuntu 18.04 LTS

Summary:

Several security issues were fixed in GRUB 2.

Software Description:
- grub2-signed: GRand Unified Bootloader
- grub2-unsigned: GRand Unified Bootloader

Details:

Máté Kukri discovered that the acpi command in GRUB 2 allowed privileged
users to load crafted ACPI tables when secure boot is enabled. An attacker
could use this to bypass UEFI Secure Boot restrictions. (CVE-2020-14372)

Chris Coulson discovered that the rmmod command in GRUB 2 contained a use-
after-free vulnerability. A local attacker could use this to execute
arbitrary code and bypass UEFI Secure Boot restrictions. (CVE-2020-25632)

Chris Coulson discovered that a buffer overflow existed in the command line
parser in GRUB 2. A local attacker could use this to execute arbitrary code
and bypass UEFI Secure Boot restrictions. (CVE-2020-27749)

It was discovered that the cutmem command in GRUB 2 did not honor secure
boot locking. A local attacker could use this to execute arbitrary code and
bypass UEFI Secure Boot restrictions. (CVE-2020-27779)

It was discovered that the option parser in GRUB 2 contained a heap
overflow vulnerability. A local attacker could use this to execute
arbitrary code and bypass UEFI Secure Boot restrictions. (CVE-2021-20225)

It was discovered that the menu rendering implementation in GRUB 2 did not
properly calculate the amount of memory needed in some situations, leading
to out-of-bounds writes. A local attacker could use this to execute
arbitrary code and bypass UEFI Secure Boot restrictions. (CVE-2021-20233)

Update instructions:

The problem can be corrected by updating your system to the following
package versions:

Ubuntu 20.10:
grub-efi-amd64-bin 2.04-1ubuntu44.2
grub-efi-amd64-signed 1.167.2+2.04-1ubuntu44.2
grub-efi-arm64-bin 2.04-1ubuntu44.2
grub-efi-arm64-signed 1.167.2+2.04-1ubuntu44.2

Ubuntu 20.04 LTS:
grub-efi-amd64-bin 2.04-1ubuntu44.2
grub-efi-amd64-signed 1.167.2+2.04-1ubuntu44.2
grub-efi-arm64-bin 2.04-1ubuntu44.2
grub-efi-arm64-signed 1.167.2+2.04-1ubuntu44.2

Ubuntu 18.04 LTS:
grub-efi-amd64-bin 2.04-1ubuntu44.1.2
grub-efi-amd64-signed 1.167~18.04.5+2.04-1ubuntu44.1.2
grub-efi-arm64-bin 2.04-1ubuntu44.1.2
grub-efi-arm64-signed 1.167~18.04.5+2.04-1ubuntu44.1.2

After a standard system update you need to reboot your computer to make
all the necessary changes.

References:
  https://ubuntu.com/security/notices/USN-4992-1
CVE-2020-14372, CVE-2020-25632, CVE-2020-27749,
CVE-2020-27779, CVE-2021-20225, CVE-2021-20233,
  https://wiki.ubuntu.com/SecurityTeam/KnowledgeBase/GRUB2SecureBootBypass2021

Package Information:
  https://launchpad.net/ubuntu/+source/grub2-signed/1.167.2
  https://launchpad.net/ubuntu/+source/grub2-unsigned/2.04-1ubuntu44.2
  https://launchpad.net/ubuntu/+source/grub2-signed/1.167.2
  https://launchpad.net/ubuntu/+source/grub2-unsigned/2.04-1ubuntu44.2
  https://launchpad.net/ubuntu/+source/grub2-signed/1.167~18.04.5
  https://launchpad.net/ubuntu/+source/grub2-unsigned/2.04-1ubuntu44.1.2