CVE-2020-15707: 
Alibaba Cloud Linux (Aliyun Linux) vulnerability analysis and mitigation

CVE-2020-15707 is an integer overflow vulnerability discovered in the functions grub_cmd_initrd and grub_initrd_init in the efilinux component of GRUB2, as shipped in Debian, Red Hat, and Ubuntu (the functionality is not included in GRUB2 upstream), leading to a heap-based buffer overflow. The vulnerability was discovered in July 2020 and affects GRUB2 version 2.04 and prior versions. The flaw could be triggered by an extremely large number of arguments to the initrd command on 32-bit architectures, or a crafted filesystem with very large files on any architecture (CVE Mitre).

The vulnerability is rated with a CVSS score of 5.7 (Medium) with vector CVSS:3.1/AV:L/AC:H/PR:H/UI:N/S:U/C:N/I:H/A:H. The issue stems from integer overflows in the efilinux component's initrd handling functions, specifically in grub_cmd_initrd and grub_initrd_init. When processing extremely large arguments to the initrd command or handling very large files, these integer overflows can lead to heap-based buffer overflows (OSS Security).

An attacker could use this vulnerability to execute arbitrary code and bypass UEFI Secure Boot restrictions. This could allow the attacker to gain virtually complete control over the device, its operating system, and its applications and data. The vulnerability is particularly concerning as it affects the boot process, which is fundamental to system security (Eclypsium).

Exploitation of this vulnerability requires local access to the system with elevated privileges. While this somewhat limits the attack surface, it could be particularly dangerous in scenarios where untrusted users can access a machine, such as in classified computing scenarios or computers in public spaces operating in unattended kiosk mode (SUSE KB).

The vulnerability requires patching GRUB2 packages and updating the UEFI Secure Boot DBX (revocation list). Major Linux distributions have released updated packages including Debian, Ubuntu, Red Hat, and SUSE. Full mitigation requires both updating the GRUB2 bootloader and applying a UEFI Revocation List (dbx) to system firmware. Users should ensure all bootable media receives OS updates before applying the dbx update to prevent potential boot failures (Ubuntu Security).

The vulnerability was part of a larger set of GRUB2 vulnerabilities collectively known as 'BootHole'. The discovery prompted a coordinated response from multiple vendors including Microsoft, Oracle, Red Hat, Canonical, VMware, and Debian. The industry response highlighted the complexity of addressing boot-level vulnerabilities, particularly given the need to coordinate across hardware vendors, operating system providers, and the UEFI forum (GRUB Developer List).