diff --git a/Lib/test/test_dict.py b/Lib/test/test_dict.py
index 4474c9b..6b2db34 100644
--- a/Lib/test/test_dict.py
+++ b/Lib/test/test_dict.py
@@ -690,6 +690,97 @@
         test_support.check_free_after_iterating(self, lambda d: iter(d.viewvalues()), dict)
         test_support.check_free_after_iterating(self, lambda d: iter(d.viewitems()), dict)
 
+    def test_equal_operator_modifying_operand(self):
+        # test fix for seg fault reported in issue 27945 part 3.
+        class X(object):
+            def __del__(self):
+                dict_b.clear()
+
+            def __eq__(self, other):
+                dict_a.clear()
+                return True
+
+            def __hash__(self):
+                return 13
+
+        dict_a = {X(): 0}
+        dict_b = {X(): X()}
+        self.assertTrue(dict_a == dict_b)
+
+    def test_fromkeys_operator_modifying_dict_operand(self):
+        # test fix for seg fault reported in issue 27945 part 4a.
+        class X(int):
+            def __hash__(self):
+                return 13
+
+            def __eq__(self, other):
+                if len(d) > 1:
+                    d.clear()
+                return False
+
+        d = {}  # this is required to exist so that d can be constructed!
+        d = {X(1): 1, X(2): 2}
+        try:
+            dict.fromkeys(d)  # shouldn't crash
+        except RuntimeError:  # implementation defined
+            pass
+
+    def test_fromkeys_operator_modifying_set_operand(self):
+        # test fix for seg fault reported in issue 27945 part 4b.
+        class X(int):
+            def __hash__(self):
+                return 13
+
+            def __eq__(self, other):
+                if len(d) > 1:
+                    d.clear()
+                return False
+
+        d = {}  # this is required to exist so that d can be constructed!
+        d = {X(1), X(2)}
+        try:
+            dict.fromkeys(d)  # shouldn't crash
+        except RuntimeError:  # implementation defined
+            pass
+
+    def test_dictitems_contains_use_after_free(self):
+        class X(object):
+            def __eq__(self, other):
+                d.clear()
+                return NotImplemented
+
+            __hash__ = object.__hash__  # silence Py3k warning
+
+        d = {0: set()}
+        try:
+            (0, X()) in d.iteritems()  # shouldn't crash
+        except RuntimeError:  # implementation defined
+            pass
+
+    def test_init_use_after_free(self):
+        class X(object):
+            def __hash__(self):
+                pair[:] = []
+                return 13
+
+        pair = [X(), 123]
+        dict([pair])
+
+    def test_oob_indexing_dictiter_iternextitem(self):
+        class X(int):
+            def __del__(self):
+                d.clear()
+
+        d = {i: X(i) for i in range(8)}
+
+        def iter_and_mutate():
+            for result in d.iteritems():
+                if result[0] == 2:
+                    d[2] = None # free d[2] --> X(2).__del__ was called
+
+        self.assertRaises(RuntimeError, iter_and_mutate)
+
+
 from test import mapping_tests
 
 class GeneralMappingTests(mapping_tests.BasicTestMappingProtocol):
diff --git a/Misc/ACKS b/Misc/ACKS
index 01bcd3b..10b5d7c 100644
--- a/Misc/ACKS
+++ b/Misc/ACKS
@@ -500,6 +500,7 @@
 Tim Graham
 Nathaniel Gray
 Eddy De Greef
+Duane Griffin
 Grant Griffin
 Andrea Griffini
 Duncan Grisby
diff --git a/Misc/NEWS b/Misc/NEWS
index ebe9d2c..254bb52 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -10,6 +10,10 @@
 Core and Builtins
 -----------------
 
+- bpo-27945: Fixed various segfaults with dict when input collections are
+  mutated during searching, inserting or comparing.  Based on patches by
+  Duane Griffin and Tim Mitchell.
+
 - bpo-25794: Fixed type.__setattr__() and type.__delattr__() for
   non-interned or unicode attribute names.  Based on patch by Eryk Sun.
 
diff --git a/Objects/dictobject.c b/Objects/dictobject.c
index 870923d..9506067 100644
--- a/Objects/dictobject.c
+++ b/Objects/dictobject.c
@@ -1595,11 +1595,18 @@
         /* Update/merge with this (key, value) pair. */
         key = PySequence_Fast_GET_ITEM(fast, 0);
         value = PySequence_Fast_GET_ITEM(fast, 1);
+        Py_INCREF(key);
+        Py_INCREF(value);
         if (override || PyDict_GetItem(d, key) == NULL) {
             int status = PyDict_SetItem(d, key, value);
-            if (status < 0)
+            if (status < 0) {
+                Py_DECREF(key);
+                Py_DECREF(value);
                 goto Fail;
+            }
         }
+        Py_DECREF(key);
+        Py_DECREF(value);
         Py_DECREF(fast);
         Py_DECREF(item);
     }
@@ -1938,12 +1945,13 @@
             /* ditto for key */
             Py_INCREF(key);
             bval = PyDict_GetItem((PyObject *)b, key);
-            Py_DECREF(key);
             if (bval == NULL) {
+                Py_DECREF(key);
                 Py_DECREF(aval);
                 return 0;
             }
             cmp = PyObject_RichCompareBool(aval, bval, Py_EQ);
+            Py_DECREF(key);
             Py_DECREF(aval);
             if (cmp <= 0)  /* error or not equal */
                 return cmp;
@@ -2743,7 +2751,7 @@
 
 static PyObject *dictiter_iternextitem(dictiterobject *di)
 {
-    PyObject *key, *value, *result = di->di_result;
+    PyObject *key, *value, *result;
     register Py_ssize_t i, mask;
     register PyDictEntry *ep;
     PyDictObject *d = di->di_dict;
@@ -2770,22 +2778,27 @@
     if (i > mask)
         goto fail;
 
-    if (result->ob_refcnt == 1) {
-        Py_INCREF(result);
-        Py_DECREF(PyTuple_GET_ITEM(result, 0));
-        Py_DECREF(PyTuple_GET_ITEM(result, 1));
-    } else {
-        result = PyTuple_New(2);
-        if (result == NULL)
-            return NULL;
-    }
     di->len--;
     key = ep[i].me_key;
     value = ep[i].me_value;
     Py_INCREF(key);
     Py_INCREF(value);
-    PyTuple_SET_ITEM(result, 0, key);
-    PyTuple_SET_ITEM(result, 1, value);
+    result = di->di_result;
+    if (Py_REFCNT(result) == 1) {
+        PyObject *oldkey = PyTuple_GET_ITEM(result, 0);
+        PyObject *oldvalue = PyTuple_GET_ITEM(result, 1);
+        PyTuple_SET_ITEM(result, 0, key);  /* steals reference */
+        PyTuple_SET_ITEM(result, 1, value);  /* steals reference */
+        Py_INCREF(result);
+        Py_DECREF(oldkey);
+        Py_DECREF(oldvalue);
+    } else {
+        result = PyTuple_New(2);
+        if (result == NULL)
+            return NULL;
+        PyTuple_SET_ITEM(result, 0, key);  /* steals reference */
+        PyTuple_SET_ITEM(result, 1, value);  /* steals reference */
+    }
     return result;
 
 fail:
@@ -3186,6 +3199,7 @@
 static int
 dictitems_contains(dictviewobject *dv, PyObject *obj)
 {
+    int result;
     PyObject *key, *value, *found;
     if (dv->dv_dict == NULL)
         return 0;
@@ -3199,7 +3213,10 @@
             return -1;
         return 0;
     }
-    return PyObject_RichCompareBool(value, found, Py_EQ);
+    Py_INCREF(found);
+    result = PyObject_RichCompareBool(value, found, Py_EQ);
+    Py_DECREF(found);
+    return result;
 }
 
 static PySequenceMethods dictitems_as_sequence = {