security: secret files are born 0600 instead of chmod-after-write

coworker/secrets.py

@@ -88,6 +88,21 @@ def _restrict_to_user(path: Path, *, is_dir: bool) -> None:
     os.chmod(path, 0o700 if is_dir else 0o600)
 
 
+def _write_text_born_private(tmp: Path, content: str) -> None:
+    """Write `content` to a file that is user-only from its first byte.
+
+    `Path.write_text` creates the file umask-wide (typically 0644) and the chmod that
+    follows leaves a window where the content is world-readable — and no protection at
+    all on the paths where that step is best-effort. Passing 0600 at open covers the
+    content for its whole lifetime instead. O_EXCL refuses to write through anything
+    pre-planted at the predictable `.tmp` name (write_text would follow a symlink and
+    pour the secret into its target); a stale tmp left by a crash is removed first."""
+    tmp.unlink(missing_ok=True)
+    fd = os.open(tmp, os.O_WRONLY | os.O_CREAT | os.O_EXCL, 0o600)
+    with os.fdopen(fd, "w", encoding="utf-8") as fh:
+        fh.write(content)
+
+
 def write_private_text(path: str | Path, content: str) -> Path:
     """Atomically write a user-only text file using the SecretStore's OS protections."""
     target = Path(path).expanduser()
@@ -97,8 +112,8 @@ def write_private_text(path: str | Path, content: str) -> Path:
     except OSError:
         pass
     tmp = target.with_name(target.name + ".tmp")
-    tmp.write_text(content, encoding="utf-8")
-    _restrict_to_user(tmp, is_dir=False)
+    _write_text_born_private(tmp, content)
+    _restrict_to_user(tmp, is_dir=False)  # still needed on Windows: ACL, not mode bits
     os.replace(tmp, target)
     return target
 
@@ -188,6 +203,6 @@ def _write(self, store: dict[str, Any]) -> None:
         except OSError:
             pass
         tmp = self.path.with_name(self.path.name + ".tmp")
-        tmp.write_text(json.dumps(store, indent=2), encoding="utf-8")
-        _restrict_to_user(tmp, is_dir=False)
+        _write_text_born_private(tmp, json.dumps(store, indent=2))
+        _restrict_to_user(tmp, is_dir=False)  # still needed on Windows: ACL, not mode bits
         os.replace(tmp, self.path)

tests/test_secrets.py

@@ -85,3 +85,56 @@ def test_delete(tmp_path):
     assert store.delete("x") is True
     assert store.delete("x") is False
     assert store.get("x") is None
+
+
+# -- files must be private from creation, not privatized after the fact ----------
+
+
+def test_secrets_file_private_even_if_restrict_step_does_nothing(tmp_path, monkeypatch):
+    """The 0600 must come from the open itself. If it only arrived via the follow-up
+    _restrict_to_user, the content would sit umask-wide between write and chmod."""
+    if sys.platform == "win32":
+        return  # POSIX mode bits; Windows privacy is the ACL, covered above
+    import coworker.secrets as secrets_mod
+
+    monkeypatch.setattr(secrets_mod, "_restrict_to_user", lambda *a, **k: None)
+    old_umask = os.umask(0)
+    try:
+        store = SecretStore(tmp_path / "secrets.json")
+        store.put("slack:default", {"type": "token", "bot_token": "xoxb-123"})
+    finally:
+        os.umask(old_umask)
+    assert stat.S_IMODE(os.stat(store.path).st_mode) == 0o600
+
+
+def test_write_private_text_private_even_if_restrict_step_does_nothing(
+    tmp_path, monkeypatch
+):
+    if sys.platform == "win32":
+        return  # POSIX mode bits; Windows privacy is the ACL, covered above
+    import coworker.secrets as secrets_mod
+    from coworker.secrets import write_private_text
+
+    monkeypatch.setattr(secrets_mod, "_restrict_to_user", lambda *a, **k: None)
+    old_umask = os.umask(0)
+    try:
+        out = write_private_text(tmp_path / "sidecar-8123.token", "tok\n")
+    finally:
+        os.umask(old_umask)
+    assert stat.S_IMODE(os.stat(out).st_mode) == 0o600
+
+
+def test_write_private_text_does_not_write_through_a_preplanted_tmp(tmp_path):
+    """A symlink parked at the predictable `.tmp` name must not receive the secret.
+    Path.write_text follows it — the secret lands in the symlink's target."""
+    if sys.platform == "win32":
+        return  # symlink creation needs privileges on Windows runners
+    from coworker.secrets import write_private_text
+
+    victim = tmp_path / "victim.txt"
+    victim.write_text("keep me", encoding="utf-8")
+    target = tmp_path / "cred.token"
+    (tmp_path / "cred.token.tmp").symlink_to(victim)
+    out = write_private_text(target, "s3cret")
+    assert victim.read_text(encoding="utf-8") == "keep me"
+    assert out.read_text(encoding="utf-8") == "s3cret"