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             AUSCERT External Security Bulletin Redistribution

                       USN-4432-2: GRUB2 regression
                               6 August 2020


        AusCERT Security Bulletin Summary

Product:           GRUB2
Publisher:         Ubuntu
Operating System:  Ubuntu
Impact/Access:     Root Compromise                 -- Existing Account
                   Execute Arbitrary Code/Commands -- Existing Account
                   Increased Privileges            -- Existing Account
                   Modify Arbitrary Files          -- Existing Account
                   Denial of Service               -- Existing Account
Resolution:        Patch/Upgrade
CVE Names:         CVE-2020-15707 CVE-2020-15706 CVE-2020-15705
                   CVE-2020-14311 CVE-2020-14310 CVE-2020-14309
                   CVE-2020-14308 CVE-2020-10713 

Reference:         ASB-2020.0136

Original Bulletin: 

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USN-4432-2: GRUB2 regression
04 August 2020

USN-4432-1 introduced a regression in the GRUB2 bootloader.

  o Ubuntu 20.04 LTS
  o Ubuntu 18.04 LTS
  o Ubuntu 16.04 LTS
  o Ubuntu 14.04 ESM


  o grub2 - GRand Unified Bootloader
  o grub2-signed - GRand Unified Bootloader


USN-4432-1 fixed vulnerabilities in GRUB2 affecting Secure Boot
environments. Unfortunately, the update introduced regressions for
some BIOS systems (either pre-UEFI or UEFI configured in Legacy mode),
preventing them from successfully booting. This update addresses
the issue.

Users with BIOS systems that installed GRUB2 versions from USN-4432-1
should verify that their GRUB2 installation has a correct understanding
of their boot device location and installed the boot loader correctly.

We apologize for the inconvenience.

Original advisory details:

Jesse Michael and Mickey Shkatov discovered that the configuration parser
in GRUB2 did not properly exit when errors were discovered, resulting in
heap-based buffer overflows. A local attacker could use this to execute
arbitrary code and bypass UEFI Secure Boot restrictions. (CVE-2020-10713)

Chris Coulson discovered that the GRUB2 function handling code did not
properly handle a function being redefined, leading to a use-after-free
vulnerability. A local attacker could use this to execute arbitrary code
and bypass UEFI Secure Boot restrictions. (CVE-2020-15706)

Chris Coulson discovered that multiple integer overflows existed in GRUB2
when handling certain filesystems or font files, leading to heap-based
buffer overflows. A local attacker could use these to execute arbitrary
code and bypass UEFI Secure Boot restrictions. (CVE-2020-14309,
CVE-2020-14310, CVE-2020-14311)

It was discovered that the memory allocator for GRUB2 did not validate
allocation size, resulting in multiple integer overflows and heap-based
buffer overflows when handling certain filesystems, PNG images or disk
metadata. A local attacker could use this to execute arbitrary code and
bypass UEFI Secure Boot restrictions. (CVE-2020-14308)

Mathieu Trudel-Lapierre discovered that in certain situations, GRUB2
failed to validate kernel signatures. A local attacker could use this
to bypass Secure Boot restrictions. (CVE-2020-15705)

Colin Watson and Chris Coulson discovered that an integer overflow
existed in GRUB2 when handling the initrd command, leading to a heap-based
buffer overflow. A local attacker could use this to execute arbitrary code
and bypass UEFI Secure Boot restrictions. (CVE-2020-15707)

Update instructions

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

Ubuntu 20.04

  o grub-efi-amd64-bin - 2.04-1ubuntu26.2
  o grub-efi-amd64-signed - 1.142.4+2.04-1ubuntu26.2
  o grub-efi-arm-bin - 2.04-1ubuntu26.2
  o grub-efi-arm64-bin - 2.04-1ubuntu26.2
  o grub-efi-arm64-signed - 1.142.4+2.04-1ubuntu26.2
  o grub-efi-ia32-bin - 2.04-1ubuntu26.2

Ubuntu 18.04

  o grub-efi-amd64-bin - 2.02-2ubuntu8.17
  o grub-efi-amd64-signed - 1.93.19+2.02-2ubuntu8.17
  o grub-efi-arm-bin - 2.02-2ubuntu8.17
  o grub-efi-arm64-bin - 2.02-2ubuntu8.17
  o grub-efi-arm64-signed - 1.93.19+2.02-2ubuntu8.17
  o grub-efi-ia32-bin - 2.02-2ubuntu8.17
  o grub-efi-ia64-bin - 2.02-2ubuntu8.17

Ubuntu 16.04

  o grub-efi-amd64-bin - 2.02~beta2-36ubuntu3.27
  o grub-efi-amd64-signed - 1.66.27+2.02~beta2-36ubuntu3.27
  o grub-efi-arm-bin - 2.02~beta2-36ubuntu3.27
  o grub-efi-arm64-bin - 2.02~beta2-36ubuntu3.27
  o grub-efi-arm64-signed - 1.66.27+2.02~beta2-36ubuntu3.27
  o grub-efi-ia32-bin - 2.02~beta2-36ubuntu3.27
  o grub-efi-ia64-bin - 2.02~beta2-36ubuntu3.27

Ubuntu 14.04

  o grub-efi-amd64-bin - 2.02~beta2-9ubuntu1.21
  o grub-efi-amd64-signed - 1.34.20+2.02~beta2-9ubuntu1.21
  o grub-efi-arm-bin - 2.02~beta2-9ubuntu1.21
  o grub-efi-arm64-bin - 2.02~beta2-9ubuntu1.21
  o grub-efi-ia32-bin - 2.02~beta2-9ubuntu1.21
  o grub-efi-ia64-bin - 2.02~beta2-9ubuntu1.21

Fully mitigating these vulnerabilities requires both an updated
GRUB2 boot loader and the application of a UEFI Revocation
List (dbx) to system firmware. Ubuntu will provide a packaged
dbx update at a later time, though system adminstrators may
choose to apply a third party dbx update before then. For more
details on mitigation steps and the risks entailed (especially for
dual/multi-boot scenarios), please see the Knowledge Base article at


  o https://launchpad.net/bugs/1889556
  o https://wiki.ubuntu.com/SecurityTeam/KnowledgeBase/GRUB2SecureBootBypass

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