CVE-2023-36811: Archive spoofing vulnerability in borgbackup

A flaw in the cryptographic authentication scheme in borgbackup allowed an attacker to fake archives and potentially indirectly cause backup data loss in the repository.

The attack requires an attacker to be able to

1. insert files (with no additional headers) into backups
2. gain write access to the repository

This vulnerability does not disclose plaintext to the attacker, nor does it affect the authenticity of existing archives.

Creating plausible fake archives may be feasible for empty or small archives, but is unlikely for large archives.

Affected are all borgbackup releases prior to 1.2.5.

Note: CVSS scoring model seemed to badly fit for this case, thus I manually set score to “moderate”.