Security: prevent timing attack on WEBHOOK_AUTHORIZATION secret

Anymail's webhook validation was vulnerable to a timing attack.
An attacker could have used this to recover your WEBHOOK_AUTHORIZATION
shared secret, potentially allowing them to post fabricated or malicious
email tracking events to your app.

There have not been any reports of attempted exploit in the wild. (The
vulnerability was discovered through code review.) Attempts would be
visible in http logs as a very large number of 400 responses on
Anymail's webhook urls, or in Python error monitoring as a very large
number of AnymailWebhookValidationFailure exceptions.

If you are using Anymail's webhooks, you should upgrade to this release.
In addition, you may want to rotate to a new WEBHOOK_AUTHORIZATION
secret ([docs](http://anymail.readthedocs.io/en/stable/tips/securing_webhooks/#use-a-shared-authorization-secret)),
particularly if your logs indicate attempted exploit.

anymail/webhooks/base.py

@@ -2,6 +2,7 @@
 
 import six
 from django.http import HttpResponse
+from django.utils.crypto import constant_time_compare
 from django.utils.decorators import method_decorator
 from django.views.decorators.csrf import csrf_exempt
 from django.views.generic import View
@@ -40,8 +41,13 @@ def __init__(self, **kwargs):
     def validate_request(self, request):
         """If configured for webhook basic auth, validate request has correct auth."""
         if self.basic_auth:
-            basic_auth = get_request_basic_auth(request)
-            if basic_auth is None or basic_auth not in self.basic_auth:
+            request_auth = get_request_basic_auth(request)
+            # Use constant_time_compare to avoid timing attack on basic auth. (It's OK that any()
+            # can terminate early: we're not trying to protect how many auth strings are allowed,
+            # just the contents of each individual auth string.)
+            auth_ok = any(constant_time_compare(request_auth, allowed_auth)
+                          for allowed_auth in self.basic_auth)
+            if not auth_ok:
                 # noinspection PyUnresolvedReferences
                 raise AnymailWebhookValidationFailure(
                     "Missing or invalid basic auth in Anymail %s webhook" % self.esp_name)
@@ -77,8 +83,11 @@ def validate_request(self, request):
         *All* definitions of this method in the class chain (including mixins)
         will be called. There is no need to chain to the superclass.
         (See self.run_validators and collect_all_methods.)
+
+        Security note: use django.utils.crypto.constant_time_compare for string
+        comparisons, to avoid exposing your validation to a timing attack.
         """
-        # if request.POST['signature'] != expected_signature:
+        # if not constant_time_compare(request.POST['signature'], expected_signature):
         #     raise AnymailWebhookValidationFailure("...message...")
         # (else just do nothing)
         pass