Home Assistant Core before 2026.6.0 is vulnerable to a path traversal flaw that permits an unauthenticated attacker to upload a deliberately crafted backup archive during the initial onboarding period. By inserting an absolute path in the ‘name’ field of the backup.json embedded in the archive, the system resolves the path in a way that discards the intended backup directory prefix, causing the backup process to write arbitrary files to any location on the host filesystem. If the Home Assistant process runs as root, this results in full control over the underlying system.

The vulnerability affects all installations of Home Assistant Core with a version older than 2026.6.0 that allow backup uploads during onboarding. Any environment where the Home Assistant process executes with elevated privileges is at risk, regardless of the specific hardware or operating system being used.

The CVSS score of 9 indicates very high severity, while the EPSS score of < 1 % suggests a low current worldwide exploitation probability, and the issue is not listed in CISA KEV. However, the flaw requires no authentication, is available during the public onboarding window, and allows arbitrary file writing; an attacker could therefore upload malicious content from any network the instance is reachable on. The resulting arbitrary file write provides a pathway to execute code or modify critical configuration files, leading to complete system compromise if the process runs as root.