_write_json (the persistence helper used by HermesTokenStorage for both
tokens and client_info) created the temp file via Path.write_text and
only chmod'd it to 0o600 afterward. Between create and chmod the file
existed on disk at the process umask (commonly 0o644 = world-readable),
briefly exposing MCP OAuth access/refresh tokens to other local users.
Use os.open with O_WRONLY|O_CREAT|O_EXCL and an explicit S_IRUSR|S_IWUSR
mode so the file is created atomically at 0o600, plus tighten the parent
dir to 0o700 so siblings can't traverse to the creds file. The temp name
also gains a per-process random suffix to avoid collisions between
concurrent writers and stale leftovers from a crashed prior write.
Mirrors the fix shipped for agent/google_oauth.py in #19673.
Adds a regression test asserting the resulting file mode is 0o600 and
the parent directory is 0o700 (skipped on Windows where POSIX mode bits
aren't enforced).
Gutslabs
7d36e8346b