diff --git a/js/src/jswrapper.h b/js/src/jswrapper.h
index 8bebe9768bc8..41a794cba373 100644
--- a/js/src/jswrapper.h
+++ b/js/src/jswrapper.h
@@ -100,8 +100,35 @@ class JS_FRIEND_API(Wrapper) : public ProxyHandler
 
     virtual void trace(JSTracer *trc, JSObject *wrapper) MOZ_OVERRIDE;
 
-    /* Policy enforcement traps. */
-    enum Action { GET, SET, CALL };
+    /* Policy enforcement traps.
+     *
+     * enter() allows the policy to specify whether the caller may perform |act|
+     * on the underlying object's |id| property. In the case when |act| is CALL,
+     * |id| is generally JSID_VOID.
+     *
+     * leave() allows the policy to undo various scoped state changes taken in
+     * enter(). If enter() succeeds, leave() must be called upon completion of
+     * the approved action.
+     *
+     * The |act| parameter to enter() specifies the action being performed. GET,
+     * SET, and CALL are self-explanatory, but PUNCTURE requires more explanation:
+     *
+     * GET and SET allow for a very fine-grained security membrane, through
+     * which access can be granted or denied on a per-property, per-object, and
+     * per-action basis. Sometimes though, we just want to asks if we can access
+     * _everything_ behind the wrapper barrier. For example, when the structured
+     * clone algorithm runs up against a cross-compartment wrapper, it needs to
+     * know whether it can enter the compartment and keep cloning, or whether it
+     * should throw. This is the role of PUNCTURE.
+     *
+     * PUNCTURE allows the policy to specify whether the wrapper barrier may
+     * be lifted - that is to say, whether the caller is allowed to access
+     * anything that the wrapped object could access. This is a very powerful
+     * permission, and thus should generally be denied for security wrappers
+     * except under very special circumstances. When |act| is PUNCTURE, |id|
+     * should be JSID_VOID.
+     * */
+    enum Action { GET, SET, CALL, PUNCTURE };
     virtual bool enter(JSContext *cx, JSObject *wrapper, jsid id, Action act, bool *bp);
     virtual void leave(JSContext *cx, JSObject *wrapper);
 
diff --git a/js/xpconnect/wrappers/AccessCheck.cpp b/js/xpconnect/wrappers/AccessCheck.cpp
index 007b6535b146..bc72afc7b00d 100644
--- a/js/xpconnect/wrappers/AccessCheck.cpp
+++ b/js/xpconnect/wrappers/AccessCheck.cpp
@@ -306,6 +306,15 @@ AccessCheck::isCrossOriginAccessPermitted(JSContext *cx, JSObject *wrapper, jsid
 
     JSObject *obj = Wrapper::wrappedObject(wrapper);
 
+    // LocationPolicy checks PUNCTURE first, so we should never get here for
+    // Location wrappers. For all other wrappers interested in cross-origin
+    // semantics, we want to allow puncturing only for the same-origin
+    // document.domain case.
+    if (act == Wrapper::PUNCTURE) {
+        MOZ_ASSERT(!WrapperFactory::IsLocationObject(obj));
+        return documentDomainMakesSameOrigin(cx, obj);
+    }
+
     const char *name;
     js::Class *clasp = js::GetObjectClass(obj);
     NS_ASSERTION(Jsvalify(clasp) != &XrayUtils::HolderClass, "shouldn't have a holder here");
@@ -498,6 +507,10 @@ ExposedPropertiesOnly::check(JSContext *cx, JSObject *wrapper, jsid id, Wrapper:
         perm = PermitObjectAccess;
         return true;
     }
+    if (act == Wrapper::PUNCTURE) {
+        perm = DenyAccess;
+        return false;
+    }
 
     perm = DenyAccess;
 
diff --git a/js/xpconnect/wrappers/AccessCheck.h b/js/xpconnect/wrappers/AccessCheck.h
index 065821dd9883..43e8d3f5c7ca 100644
--- a/js/xpconnect/wrappers/AccessCheck.h
+++ b/js/xpconnect/wrappers/AccessCheck.h
@@ -151,12 +151,18 @@ struct LocationPolicy : public Policy {
         // We should only be dealing with Location objects here.
         MOZ_ASSERT(WrapperFactory::IsLocationObject(js::UnwrapObject(wrapper)));
 
+        // Default to deny.
+        perm = DenyAccess;
+
+        // Location object security is complicated enough. Don't allow punctures.
+        if (act == js::Wrapper::PUNCTURE)
+            return false;
+
         if (AccessCheck::isCrossOriginAccessPermitted(cx, wrapper, id, act) ||
             AccessCheck::isLocationObjectSameOrigin(cx, wrapper)) {
             perm = PermitPropertyAccess;
             return true;
         }
-        perm = DenyAccess;
         JSAutoEnterCompartment ac;
         if (!ac.enter(cx, wrapper))
             return false;