diff --git a/content/base/src/CSPUtils.jsm b/content/base/src/CSPUtils.jsm
index 9d17d5de7bd..d7bac0c9de7 100644
--- a/content/base/src/CSPUtils.jsm
+++ b/content/base/src/CSPUtils.jsm
@@ -838,17 +838,27 @@ CSPRep.prototype = {
   function cspsd_makeExplicit() {
     let SD = this._specCompliant ? CSPRep.SRC_DIRECTIVES_NEW : CSPRep.SRC_DIRECTIVES_OLD;
 
-    var defaultSrcDir = this._directives[SD.DEFAULT_SRC];
-
     // It's ok for a 1.0 spec compliant policy to not have a default source,
     // in this case it should use default-src *
     // However, our original CSP implementation required a default src
     // or an allow directive.
-    if (!defaultSrcDir && !this._specCompliant) {
-      this.log(WARN_FLAG, CSPLocalizer.getStr("allowOrDefaultSrcRequired"));
-      return false;
+    if (!this._directives[SD.DEFAULT_SRC]) {
+      if(!this._specCompliant) {
+        this.warn(CSPLocalizer.getStr("allowOrDefaultSrcRequired"));
+        return false;
+      }
+
+      // bug 780978 will remove these lines and needs to make sure that
+      // CSPRep.permits() enforces this logic: prefixed headers default to
+      // "disallowed" and unprefixed headers default to "allowed" when
+      // neither specific -src nor default-src directive is provided.
+      this._directives[SD.DEFAULT_SRC]
+            = CSPSourceList.fromString("*", this, this._self, true);
+      this._directives[SD.DEFAULT_SRC]._isImplicit = true;
     }
 
+    var defaultSrcDir = this._directives[SD.DEFAULT_SRC];
+
     for (var dir in SD) {
       var dirv = SD[dir];
       if (dirv === SD.DEFAULT_SRC) continue;
diff --git a/content/base/test/unit/test_csputils.js b/content/base/test/unit/test_csputils.js
index 2ae292745eb..7e55c8df15f 100644
--- a/content/base/test/unit/test_csputils.js
+++ b/content/base/test/unit/test_csputils.js
@@ -699,7 +699,7 @@ test(function test_FrameAncestor_ignores_userpass_bug779918() {
       // construct fake ancestry with CSP applied to the child.
       // [aChildUri] -> [aParentUri] -> (root/top)
       // and then test "permitsAncestry" on the child/self docshell.
-      function testPermits(aChildUri, aParentUri, aContext) {
+      function testPermits(aChildUri, aParentUri, aContentType) {
         let cspObj = Cc["@mozilla.org/contentsecuritypolicy;1"]
                        .createInstance(Ci.nsIContentSecurityPolicy);
         cspObj.refinePolicy(testPolicy, aChildUri, false);
@@ -940,8 +940,8 @@ test(
                      .createInstance(Ci.nsIContentSecurityPolicy);
       var selfURI = URI("http://self.com/");
 
-      function testPermits(aUri, aContext) {
-        return cspObj.shouldLoad(aContext, aUri, null, null, null, null)
+      function testPermits(aUri, aContentType) {
+        return cspObj.shouldLoad(aContentType, aUri, null, null, null, null)
                == Ci.nsIContentPolicy.ACCEPT;
       };
 
@@ -982,6 +982,57 @@ test(
                      Ci.nsIContentPolicy.TYPE_IMAGE));
     });
 
+test(
+    function test_bug764937_defaultSrcMissing() {
+      var cspObjSpecCompliant = Cc["@mozilla.org/contentsecuritypolicy;1"]
+                     .createInstance(Ci.nsIContentSecurityPolicy);
+      var cspObjOld = Cc["@mozilla.org/contentsecuritypolicy;1"]
+                     .createInstance(Ci.nsIContentSecurityPolicy);
+      var selfURI = URI("http://self.com/");
+
+      function testPermits(cspObj, aUri, aContentType) {
+        return cspObj.shouldLoad(aContentType, aUri, null, null, null, null)
+               == Ci.nsIContentPolicy.ACCEPT;
+      };
+
+      const policy = "script-src 'self'";
+      cspObjSpecCompliant.refinePolicy(policy, selfURI, true);
+
+      // Spec-Compliant policy default-src defaults to *.
+      // This means all images are allowed, and only 'self'
+      // script is allowed.
+      do_check_true(testPermits(cspObjSpecCompliant,
+                                URI("http://bar.com/foo.png"),
+                                Ci.nsIContentPolicy.TYPE_IMAGE));
+      do_check_true(testPermits(cspObjSpecCompliant,
+                                URI("http://self.com/foo.png"),
+                                Ci.nsIContentPolicy.TYPE_IMAGE));
+      do_check_true(testPermits(cspObjSpecCompliant,
+                                URI("http://self.com/foo.js"),
+                                Ci.nsIContentPolicy.TYPE_SCRIPT));
+      do_check_false(testPermits(cspObjSpecCompliant,
+                                 URI("http://bar.com/foo.js"),
+                                 Ci.nsIContentPolicy.TYPE_SCRIPT));
+
+      cspObjOld.refinePolicy(policy, selfURI, false);
+
+      // non-Spec-Compliant policy default-src defaults to 'none'
+      // This means all images are blocked, and so are all scripts (because the
+      // policy is ignored and fails closed).
+      do_check_false(testPermits(cspObjOld,
+                                URI("http://bar.com/foo.png"),
+                                Ci.nsIContentPolicy.TYPE_IMAGE));
+      do_check_false(testPermits(cspObjOld,
+                                URI("http://self.com/foo.png"),
+                                Ci.nsIContentPolicy.TYPE_IMAGE));
+      do_check_false(testPermits(cspObjOld,
+                                URI("http://self.com/foo.js"),
+                                Ci.nsIContentPolicy.TYPE_SCRIPT));
+      do_check_false(testPermits(cspObjOld,
+                                 URI("http://bar.com/foo.js"),
+                                 Ci.nsIContentPolicy.TYPE_SCRIPT));
+
+    });
 
 /*