add checks to initialize References() when missing, prevent empty TTP…

…s tags. closes #366

stix/common/information_source.py

@@ -47,6 +47,8 @@ def add_contributing_source(self, value):
     def add_reference(self, value):
         if not value:
             return
+        if self.references is None:
+            self.references = References()
         # TODO: Check if it's a valid URI?
         self.references.append(value)
 

stix/core/stix_package.py

@@ -99,7 +99,7 @@ def __init__(self, id_=None, idref=None, timestamp=None, stix_header=None,
         self.indicators = indicators or Indicators()
         self.incidents = incidents or Incidents()
         self.threat_actors = threat_actors or ThreatActors()
-        self.ttps = ttps or TTPs()
+        self.ttps = ttps
         self.related_packages = related_packages
         self.timestamp = timestamp
 

stix/core/ttps.py

@@ -26,3 +26,8 @@ def __init__(self, ttps=None):
 
     def add_ttp(self, ttp):
         self.ttp.append(ttp)
+
+    def add_kill_chain(self, kc):
+        if self.kill_chains is None:
+            self.kill_chains = KillChains()
+        self.kill_chains.kill_chain.append(kc)

stix/exploit_target/vulnerability.py

@@ -37,18 +37,19 @@ class Vulnerability(stix.Entity):
     affected_software = fields.TypedField("Affected_Software", "stix.exploit_target.vulnerability.AffectedSoftware")
     references = fields.TypedField("References", References)
 
-    def __init__(self, title=None, description=None, short_description=None):
+    def __init__(self, title=None, description=None, short_description=None, references=None):
         super(Vulnerability, self).__init__()
 
         self.title = title
         self.description = description
         self.short_description = short_description
-        self.references = []
+        self.references = references
 
     def add_reference(self, reference):
         if not reference:
             return
-
+        if self.references is None:
+            self.references = References()
         self.references.append(reference)