CVE-2021-28693: 
NixOS vulnerability analysis and mitigation

CVE-2021-28693 is a vulnerability discovered in Xen's boot module handling on ARM systems. The issue relates to the bootloader's handling of boot modules (such as kernel and initramfs) in temporary areas before they are copied to domain memory. The vulnerability was discovered by Julien Grall of Amazon and affects all versions of Xen since 4.12 (Xen Advisory).

The vulnerability stems from Xen's failure to properly 'scrub' boot modules on ARM systems before handing the page over to the allocator. When the bootloader loads boot modules in a temporary area before copying them to each domain's memory, the system should clean this temporary space to prevent data leakage. However, this scrubbing mechanism was not implemented for ARM systems. The vulnerability has been assigned a CVSS v3.1 base score of 5.5 (MEDIUM) with vector: CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:N/A:N (NVD).

The primary impact of this vulnerability is the potential exposure of sensitive information. After boot, sensitive data from the boot modules might become visible to other domains on the system. This creates a significant confidentiality breach in the system's security model (Xen Advisory).

The vulnerability affects only ARM systems running Xen versions since 4.12. Systems running with 'bootscrub=off' (where boot scrubbing is disabled) are not vulnerable. The local nature of the attack vector and the requirement for specific system conditions make this vulnerability moderately exploitable (Xen Advisory).

No temporary mitigation was available for this vulnerability at the time of discovery. The issue was resolved through security patches that were released for different versions of Xen (4.12.x, 4.14.x, and 4.15.x). Users were advised to upgrade to the patched versions of their respective Xen releases (Gentoo Advisory).