_save_auth() wrote the bearer token with tmp.open('w') — created at
process umask (typically 0o644) — and only chmod'ed to 0o600 after the
write, leaving a window where the token sat world-readable. The temp
name was also fixed and predictable (auth.json.tmp), so it could be
pre-planted (symlink attack).
Create the temp file with os.open(O_WRONLY|O_CREAT|O_EXCL, 0o600) and a
per-process random suffix, fsync before the atomic replace, and clean
the temp file up on failure. Mirrors hermes_cli/auth.py:_save_auth_store
(#19673, #21148), which hardened the same pattern in the core writer.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
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