Sign sessions using SECRET_KEY to simplify revocation (#1219)

* Sign sessions using `SECRET_KEY`
* Add `CTFd.utils.security.signing.sign` and `CTFd.utils.security.signing.unsign`

CTFd/utils/security/signing.py

@@ -1,4 +1,5 @@
 from flask import current_app
+from itsdangerous import Signer
 from itsdangerous.url_safe import URLSafeTimedSerializer
 from itsdangerous.exc import (  # noqa: F401
     BadTimeSignature,
@@ -19,3 +20,17 @@ def unserialize(data, secret=None, max_age=432000):
         secret = current_app.config["SECRET_KEY"]
     s = URLSafeTimedSerializer(secret)
     return s.loads(data, max_age=max_age)
+
+
+def sign(data, secret=None):
+    if secret is None:
+        secret = current_app.config["SECRET_KEY"]
+    s = Signer(secret)
+    return s.sign(data)
+
+
+def unsign(data, secret=None):
+    if secret is None:
+        secret = current_app.config["SECRET_KEY"]
+    s = Signer(secret)
+    return s.unsign(data)

CTFd/utils/sessions/__init__.py

@@ -1,8 +1,10 @@
 from flask.sessions import SessionInterface, SessionMixin
 from flask.json.tag import TaggedJSONSerializer
 from werkzeug.datastructures import CallbackDict
+from itsdangerous import BadSignature, want_bytes
 from CTFd.cache import cache
 from CTFd.utils import text_type
+from CTFd.utils.security.signing import sign, unsign
 from uuid import uuid4
 import six
 
@@ -50,7 +52,7 @@ class CachingSessionInterface(SessionInterface):
     def _generate_sid(self):
         return str(uuid4())
 
-    def __init__(self, key_prefix, use_signer=False, permanent=False):
+    def __init__(self, key_prefix, use_signer=True, permanent=False):
         self.key_prefix = key_prefix
         self.use_signer = use_signer
         self.permanent = permanent
@@ -61,6 +63,14 @@ class CachingSessionInterface(SessionInterface):
             sid = self._generate_sid()
             return self.session_class(sid=sid, permanent=self.permanent)
 
+        if self.use_signer:
+            try:
+                sid_as_bytes = unsign(sid)
+                sid = sid_as_bytes.decode()
+            except BadSignature:
+                sid = self._generate_sid()
+                return self.session_class(sid=sid, permanent=self.permanent)
+
         if not six.PY2 and not isinstance(sid, text_type):
             sid = sid.decode("utf-8", "strict")
         val = cache.get(self.key_prefix + sid)
@@ -99,7 +109,12 @@ class CachingSessionInterface(SessionInterface):
                 value=val,
                 timeout=total_seconds(app.permanent_session_lifetime),
             )
-            session_id = session.sid
+
+            if self.use_signer:
+                session_id = sign(want_bytes(session.sid))
+            else:
+                session_id = session.sid
+
             response.set_cookie(
                 app.session_cookie_name,
                 session_id,