allow 5 minutes of future clock skew when verifying JWTs

fixes CORE-1439

Change-Id: Ia3ed12dd79cee475bedd0879323eacf3a0325476
Reviewed-on: https://gerrit.instructure.com/155617
Tested-by: Jenkins
Reviewed-by: Brent Burgoyne <bburgoyne@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>

lib/canvas/security.rb

@@ -329,27 +329,29 @@ module Canvas::Security
   class << self
     private
     def verify_jwt(body, ignore_expiration: false)
+      verification_time = Time.now.utc
+      if body[:iat].present?
+        iat = timestamp_as_integer(body[:iat])
+        if iat > verification_time.to_i && iat < verification_time.to_i + 300
+          verification_time = iat
+        end
+      end
+
       if body[:exp].present? && !ignore_expiration
-        if timestamp_is_expired?(body[:exp])
+        if timestamp_as_integer(body[:exp]) < verification_time.to_i
           raise Canvas::Security::TokenExpired
         end
       end
 
       if body[:nbf].present?
-        if timestamp_is_future?(body[:nbf])
+        if timestamp_as_integer(body[:nbf]) > verification_time.to_i
           raise Canvas::Security::InvalidToken
         end
       end
     end
 
-    def timestamp_is_expired?(exp_val)
-      now = Time.zone.now
-      (exp_val.is_a?(Time) && exp_val <= now) || exp_val <= now.to_i
-    end
-
-    def timestamp_is_future?(nbf_val)
-      now = Time.zone.now
-      (nbf_val.is_a?(Time) && nbf_val > now) || nbf_val > now.to_i
+    def timestamp_as_integer(timestamp)
+      timestamp.is_a?(Time) ? timestamp.to_i : timestamp
     end
 
     def services_encryption_secret

spec/lib/canvas/security_spec.rb

@@ -136,6 +136,12 @@ describe Canvas::Security do
         )
       end
 
+      it "allows 5 minutes of future clock skew" do
+        back_to_the_future_jwt = test_jwt(exp: 1.hour.from_now, nbf: 1.minutes.from_now, iat: 1.minutes.from_now)
+        body = Canvas::Security.decode_jwt(back_to_the_future_jwt, [ key ])
+        expect(body[:a]).to eq 1
+      end
+
       it "produces an InvalidToken error if string isn't a jwt (even if it looks like one)" do
         # this is an example token which base64_decodes to a thing that looks like a jwt because of the periods
         not_a_jwt = Canvas::Security.base64_decode("1050~LvwezC5Dd3ZK9CR1lusJTRv24dN0263txia3KF3mU6pDjOv5PaoX8Jv4ikdcvoiy")