The vulnerability stems from atomic_move's file creation logic in basic.py. The code explicitly uses DEFAULT_PERM (0o0666) masked by the umask when creating new files, which could leave files world-readable depending on the environment's umask configuration. The GitHub issue (#67794) directly references this code path, and the CVE description matches this behavior. The lack of a mechanism to specify custom file modes in atomic_move operations is the root cause.