A path traversal vulnerability in LXD's instance template processing allows an attacker with container edit permissions, or any user launching a crafted image, to overwrite arbitrary files on the host system as root. When processing target template paths specified in metadata.yaml, LXD validates the path against a confined os.Root directory handle but subsequently opens and creates the file using os.Create with an unconfined string path. This discrepancy between path resolution checks and file creation allows an attacker to escape directory confinement, overwrite root-owned host files, and achieve host root code execution.

A critical path traversal vulnerability exists in LXD's instance template processing engine. Attackers with container edit permissions, or any user launching a crafted image, can overwrite arbitrary files on the host system as root. The flaw lies in a discrepancy between path validation (using a confined os.Root directory handle) and file creation (using os.Create with an unconfined string path) when processing target template paths defined in metadata.yaml. This mismatch allows an attacker to escape directory confinement, overwrite root-owned host files, and ultimately achieve host root code execution. The vulnerability represents a container escape scenario with full host compromise potential. It is tracked as CVE-2026-66897 and has been disclosed via GitHub Security Advisories for the canonical/lxd repository.