diff --git a/agent/google_oauth.py b/agent/google_oauth.py
index ede64251e29..6f45c370f6c 100644
--- a/agent/google_oauth.py
+++ b/agent/google_oauth.py
@@ -59,7 +59,7 @@ from dataclasses import dataclass
 from pathlib import Path
 from typing import Any, Dict, Optional, Tuple
 
-from hermes_constants import get_hermes_home
+from hermes_constants import get_hermes_home, secure_parent_dir
 
 logger = logging.getLogger(__name__)
 
@@ -491,10 +491,8 @@ def save_credentials(creds: GoogleCredentials) -> Path:
     path.parent.mkdir(parents=True, exist_ok=True)
     # Tighten parent dir to 0o700 so siblings can't traverse to the creds file.
     # On Windows this is a no-op (POSIX mode bits aren't enforced); ignore failures.
-    try:
-        os.chmod(path.parent, 0o700)
-    except OSError:
-        pass
+    # secure_parent_dir refuses to chmod / or top-level dirs (#25821).
+    secure_parent_dir(path)
     payload = json.dumps(creds.to_dict(), indent=2, sort_keys=True) + "\n"
 
     with _credentials_lock():
diff --git a/hermes_cli/auth.py b/hermes_cli/auth.py
index f21ada7db8b..59daa2c5d40 100644
--- a/hermes_cli/auth.py
+++ b/hermes_cli/auth.py
@@ -48,7 +48,7 @@ import httpx
 import yaml
 
 from hermes_cli.config import get_hermes_home, get_config_path, read_raw_config
-from hermes_constants import OPENROUTER_BASE_URL
+from hermes_constants import OPENROUTER_BASE_URL, secure_parent_dir
 from utils import atomic_replace, atomic_yaml_write, is_truthy_value
 
 logger = logging.getLogger(__name__)
@@ -1030,10 +1030,8 @@ def _save_auth_store(auth_store: Dict[str, Any]) -> Path:
     auth_file.parent.mkdir(parents=True, exist_ok=True)
     # Tighten parent dir to 0o700 so siblings can't traverse to creds.
     # No-op on Windows (POSIX mode bits not enforced); ignore failures.
-    try:
-        os.chmod(auth_file.parent, 0o700)
-    except OSError:
-        pass
+    # secure_parent_dir refuses to chmod / or top-level dirs (#25821).
+    secure_parent_dir(auth_file)
     auth_store["version"] = AUTH_STORE_VERSION
     auth_store["updated_at"] = datetime.now(timezone.utc).isoformat()
     payload = json.dumps(auth_store, indent=2) + "\n"
@@ -1863,10 +1861,8 @@ def _read_qwen_cli_tokens() -> Dict[str, Any]:
 def _save_qwen_cli_tokens(tokens: Dict[str, Any]) -> Path:
     auth_path = _qwen_cli_auth_path()
     auth_path.parent.mkdir(parents=True, exist_ok=True)
-    try:
-        os.chmod(auth_path.parent, 0o700)
-    except OSError:
-        pass
+    # secure_parent_dir refuses to chmod / or top-level dirs (#25821).
+    secure_parent_dir(auth_path)
     # Per-process random temp suffix avoids collisions between concurrent
     # writers and stale leftovers from a crashed prior write.
     tmp_path = auth_path.with_name(f"{auth_path.name}.tmp.{os.getpid()}.{uuid.uuid4().hex}")
@@ -4168,10 +4164,8 @@ def _write_shared_nous_state(state: Dict[str, Any]) -> None:
         with _nous_shared_store_lock():
             path = _nous_shared_store_path()
             path.parent.mkdir(parents=True, exist_ok=True)
-            try:
-                os.chmod(path.parent, 0o700)
-            except OSError:
-                pass
+            # secure_parent_dir refuses to chmod / or top-level dirs (#25821).
+            secure_parent_dir(path)
             tmp = path.with_name(f"{path.name}.tmp.{os.getpid()}.{uuid.uuid4().hex}")
             # Create with 0o600 atomically via os.open(O_EXCL) — closes the TOCTOU
             # window where write_text() + post-write chmod briefly exposed Nous
diff --git a/hermes_constants.py b/hermes_constants.py
index a988fc5fda5..a81441e2080 100644
--- a/hermes_constants.py
+++ b/hermes_constants.py
@@ -235,6 +235,27 @@ def display_hermes_home() -> str:
         return str(home)
 
 
+
+
+def secure_parent_dir(path: Path) -> None:
+    """Chmod ``0o700`` on the parent directory of *path*, but only if safe.
+
+    Refuses to chmod ``/`` or any top-level directory (depth < 2) to
+    prevent catastrophic host bricking when ``HERMES_HOME`` or other
+    path env vars resolve to an unexpected location.
+
+    See https://github.com/NousResearch/hermes-agent/issues/25821.
+    """
+    parent = path.parent.resolve()
+    # Refuse root and its direct children (/usr, /home, /var, /tmp, …).
+    if parent == Path("/") or len(parent.parts) < 3:
+        return
+    try:
+        os.chmod(parent, 0o700)
+    except OSError:
+        pass
+
+
 def get_subprocess_home() -> str | None:
     """Return a per-profile HOME directory for subprocesses, or None.
 
diff --git a/tests/test_hermes_constants.py b/tests/test_hermes_constants.py
index a3ffc0dcc14..edbb4eb7b84 100644
--- a/tests/test_hermes_constants.py
+++ b/tests/test_hermes_constants.py
@@ -12,6 +12,7 @@ from hermes_constants import (
     get_default_hermes_root,
     is_container,
     parse_reasoning_effort,
+    secure_parent_dir,
 )
 
 
@@ -171,3 +172,95 @@ class TestParseReasoningEffort:
         """
         documented = {"minimal", "low", "medium", "high", "xhigh"}
         assert documented.issubset(set(VALID_REASONING_EFFORTS))
+
+
+class TestSecureParentDir:
+    """Tests for secure_parent_dir() — prevents chmod on / or top-level dirs."""
+
+    def test_safe_path_calls_chmod(self, tmp_path, monkeypatch):
+        """Normal nested path (depth >= 3) should call os.chmod."""
+        safe_dir = tmp_path / "home" / "user" / ".hermes"
+        safe_dir.mkdir(parents=True)
+        target = safe_dir / "auth.json"
+        target.touch()
+
+        called_with = []
+        monkeypatch.setattr(os, "chmod", lambda p, m: called_with.append((str(p), m)))
+
+        secure_parent_dir(target)
+        assert len(called_with) == 1
+        assert called_with[0] == (str(safe_dir), 0o700)
+
+    def test_root_dir_skipped(self, monkeypatch):
+        """Parent resolving to / must NOT be chmod'd."""
+        called_with = []
+        monkeypatch.setattr(os, "chmod", lambda p, m: called_with.append((str(p), m)))
+
+        # Path("/foo").parent == Path("/")
+        secure_parent_dir(Path("/foo"))
+        assert called_with == []
+
+    def test_top_level_dir_skipped(self, monkeypatch):
+        """Parent resolving to a top-level dir (depth 2) must NOT be chmod'd."""
+        called_with = []
+        monkeypatch.setattr(os, "chmod", lambda p, m: called_with.append((str(p), m)))
+
+        # Path("/usr/foo").parent == Path("/usr") — depth 2
+        secure_parent_dir(Path("/usr/foo"))
+        assert called_with == []
+
+    def test_two_component_path_skipped(self, monkeypatch):
+        """Parent with < 3 resolved parts must NOT be chmod'd.
+
+        Uses monkeypatch to avoid macOS firmlink resolution of /home.
+        """
+        called_with = []
+        monkeypatch.setattr(os, "chmod", lambda p, m: called_with.append((str(p), m)))
+
+        # Mock Path.resolve to return a short path regardless of OS quirks
+        original_resolve = Path.resolve
+        def mock_resolve(self):
+            if str(self) == "/x/y":
+                return Path("/x")
+            return original_resolve(self)
+        monkeypatch.setattr(Path, "resolve", mock_resolve)
+
+        secure_parent_dir(Path("/x/y"))
+        assert called_with == []
+
+    def test_oserror_suppressed(self, tmp_path, monkeypatch):
+        """OSError from chmod should be silently caught."""
+        safe_dir = tmp_path / "a" / "b" / "c"
+        safe_dir.mkdir(parents=True)
+        target = safe_dir / "file.json"
+        target.touch()
+
+        def raise_oserror(p, m):
+            raise OSError("permission denied")
+
+        monkeypatch.setattr(os, "chmod", raise_oserror)
+        # Should not raise
+        secure_parent_dir(target)
+
+    def test_symlink_resolved(self, tmp_path, monkeypatch):
+        """Symlinks should be resolved before checking depth."""
+        real_dir = tmp_path / "a" / "b"
+        real_dir.mkdir(parents=True)
+        target = real_dir / "file.json"
+        target.touch()
+
+        # Create a symlink with fewer path components
+        link = tmp_path / "link"
+        link.symlink_to(real_dir)
+        link_target = link / "file.json"
+
+        called_with = []
+        monkeypatch.setattr(os, "chmod", lambda p, m: called_with.append((str(p), m)))
+
+        # Even though /tmp/link has only 3 parts, the resolved path has 4
+        # The resolved parent (real_dir) has depth 4, so it should be chmod'd
+        secure_parent_dir(link_target)
+        assert len(called_with) == 1
+        assert called_with[0] == (str(real_dir), 0o700)
+
+
diff --git a/tools/mcp_oauth.py b/tools/mcp_oauth.py
index 8d48eedf0e8..53b4615000f 100644
--- a/tools/mcp_oauth.py
+++ b/tools/mcp_oauth.py
@@ -48,6 +48,7 @@ from http.server import BaseHTTPRequestHandler, HTTPServer
 from pathlib import Path
 from typing import Any
 from urllib.parse import parse_qs, urlparse
+from hermes_constants import secure_parent_dir
 
 logger = logging.getLogger(__name__)
 
@@ -175,10 +176,8 @@ def _write_json(path: Path, data: dict) -> None:
     path.parent.mkdir(parents=True, exist_ok=True)
     # Tighten parent dir to 0o700 so siblings can't traverse to the creds.
     # No-op on Windows (POSIX mode bits aren't enforced); ignore failures.
-    try:
-        os.chmod(path.parent, 0o700)
-    except OSError:
-        pass
+    # secure_parent_dir refuses to chmod / or top-level dirs (#25821).
+    secure_parent_dir(path)
     # Per-process random suffix avoids collisions between concurrent
     # writers and stale leftovers from a prior crashed write.
     tmp = path.with_suffix(f".tmp.{os.getpid()}.{secrets.token_hex(4)}")