Security: Improve overall security in a few ways (#12501)

- Make sure we're always using regex with timeouts for user controlled data
- Adds rate limiting to the token endpoint (configurable)
- Signs the classifier pickle file with the SECRET_KEY and refuse to load one which doesn't verify.
- Require the user to set a secret key, instead of falling back to our old hard coded one

docs/configuration.md

@@ -402,6 +402,12 @@ Defaults to `/usr/share/nltk_data`
 
 : This is where paperless will store the classification model.
 
+    !!! warning
+
+        The classification model uses Python's pickle serialization format.
+        Ensure this file is only writable by the paperless user, as a
+        maliciously crafted model file could execute arbitrary code when loaded.
+
     Defaults to `PAPERLESS_DATA_DIR/classification_model.pickle`.
 
 ## Logging
@@ -422,14 +428,20 @@ Defaults to `/usr/share/nltk_data`
 
 #### [`PAPERLESS_SECRET_KEY=<key>`](#PAPERLESS_SECRET_KEY) {#PAPERLESS_SECRET_KEY}
 
-: Paperless uses this to make session tokens. If you expose paperless
-on the internet, you need to change this, since the default secret
-is well known.
+: **Required.** Paperless uses this to make session tokens and sign
+sensitive data. Paperless will refuse to start if this is not set.
 
     Use any sequence of characters. The more, the better. You don't
-    need to remember this. Just face-roll your keyboard.
+    need to remember this. You can generate a suitable key with:
 
-    Default is listed in the file `src/paperless/settings.py`.
+        python3 -c "import secrets; print(secrets.token_urlsafe(64))"
+
+    !!! warning
+
+        This setting has no default value. You **must** set it before
+        starting Paperless. Existing installations that relied on the
+        previous default value should set `PAPERLESS_SECRET_KEY` to
+        that value to avoid invalidating existing sessions and tokens.
 
 #### [`PAPERLESS_URL=<url>`](#PAPERLESS_URL) {#PAPERLESS_URL}
 
@@ -770,6 +782,14 @@ If both the [PAPERLESS_ACCOUNT_DEFAULT_GROUPS](#PAPERLESS_ACCOUNT_DEFAULT_GROUPS
 
     Defaults to 1209600 (2 weeks)
 
+#### [`PAPERLESS_TOKEN_THROTTLE_RATE=<rate>`](#PAPERLESS_TOKEN_THROTTLE_RATE) {#PAPERLESS_TOKEN_THROTTLE_RATE}
+
+: Rate limit for the API token authentication endpoint (`/api/token/`), used to mitigate brute-force login attempts.
+Uses Django REST Framework's [throttle rate format](https://www.django-rest-framework.org/api-guide/throttling/#setting-the-throttling-policy),
+e.g. `5/min`, `100/hour`, `1000/day`.
+
+    Defaults to `5/min`
+
 ## OCR settings {#ocr}
 
 Paperless uses [OCRmyPDF](https://ocrmypdf.readthedocs.io/en/latest/)