fix(security): harden self-hosted SECRET_KEY bootstrap (#36049)

Co-authored-by: EndlessLucky <66432853+EndlessLucky@users.noreply.github.com>

api/configs/feature/__init__.py

@@ -23,9 +23,9 @@ class SecurityConfig(BaseSettings):
     """
 
     SECRET_KEY: str = Field(
-        description="Secret key for secure session cookie signing."
-        "Make sure you are changing this key for your deployment with a strong key."
-        "Generate a strong key using `openssl rand -base64 42` or set via the `SECRET_KEY` environment variable.",
+        description="Secret key for secure session cookie signing. "
+        "Leave empty to let Dify generate a persistent key in the storage directory, "
+        "or set a strong value via the `SECRET_KEY` environment variable.",
         default="",
     )
 

api/configs/secret_key.py

@@ -0,0 +1,38 @@
+"""SECRET_KEY persistence helpers for runtime setup."""
+
+from __future__ import annotations
+
+import secrets
+
+from extensions.ext_storage import storage
+
+GENERATED_SECRET_KEY_FILENAME = ".dify_secret_key"
+
+
+def resolve_secret_key(secret_key: str) -> str:
+    """Return an explicit SECRET_KEY or a generated key persisted in storage."""
+    if secret_key:
+        return secret_key
+
+    return _load_or_create_secret_key()
+
+
+def _load_or_create_secret_key() -> str:
+    try:
+        persisted_key = storage.load_once(GENERATED_SECRET_KEY_FILENAME).decode("utf-8").strip()
+        if persisted_key:
+            return persisted_key
+    except FileNotFoundError:
+        pass
+
+    generated_key = secrets.token_urlsafe(48)
+
+    try:
+        storage.save(GENERATED_SECRET_KEY_FILENAME, f"{generated_key}\n".encode())
+    except Exception as exc:
+        raise ValueError(
+            f"SECRET_KEY is not set and could not be generated at {GENERATED_SECRET_KEY_FILENAME}. "
+            "Set SECRET_KEY explicitly or make storage writable."
+        ) from exc
+
+    return generated_key