diff --git a/Lib/test/test_dict.py b/Lib/test/test_dict.py
index 832bb9c..8013f37 100644
--- a/Lib/test/test_dict.py
+++ b/Lib/test/test_dict.py
@@ -1085,6 +1085,91 @@
         support.check_free_after_iterating(self, lambda d: iter(d.values()), dict)
         support.check_free_after_iterating(self, lambda d: iter(d.items()), dict)
 
+    def test_equal_operator_modifying_operand(self):
+        # test fix for seg fault reported in issue 27945 part 3.
+        class X():
+            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:
+            def __eq__(self, other):
+                d.clear()
+                return NotImplemented
+
+        d = {0: set()}
+        (0, X()) in d.items()
+
+    def test_init_use_after_free(self):
+        class X:
+            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.items():
+                if result[0] == 2:
+                    d[2] = None # free d[2] --> X(2).__del__ was called
+
+        self.assertRaises(RuntimeError, iter_and_mutate)
+
 
 class CAPITest(unittest.TestCase):
 
diff --git a/Misc/ACKS b/Misc/ACKS
index d00fa79..098c801 100644
--- a/Misc/ACKS
+++ b/Misc/ACKS
@@ -546,6 +546,7 @@
 Kim Gräsman
 Nathaniel Gray
 Eddy De Greef
+Duane Griffin
 Grant Griffin
 Andrea Griffini
 Duncan Grisby
diff --git a/Misc/NEWS b/Misc/NEWS
index 9aa982e..74f7922 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 attribute names.  Based on patch by Eryk Sun.
 
diff --git a/Objects/dictobject.c b/Objects/dictobject.c
index 2543118..808f548 100644
--- a/Objects/dictobject.c
+++ b/Objects/dictobject.c
@@ -1107,18 +1107,18 @@
     PyDictKeyEntry *ep;
     Py_ssize_t hashpos, ix;
 
+    Py_INCREF(key);
+    Py_INCREF(value);
     if (mp->ma_values != NULL && !PyUnicode_CheckExact(key)) {
         if (insertion_resize(mp) < 0)
-            return -1;
+            goto Fail;
     }
 
     ix = mp->ma_keys->dk_lookup(mp, key, hash, &old_value, &hashpos);
-    if (ix == DKIX_ERROR) {
-        return -1;
-    }
+    if (ix == DKIX_ERROR)
+        goto Fail;
 
     assert(PyUnicode_CheckExact(key) || mp->ma_keys->dk_lookup == lookdict);
-    Py_INCREF(value);
     MAINTAIN_TRACKING(mp, key, value);
 
     /* When insertion order is different from shared key, we can't share
@@ -1127,10 +1127,8 @@
     if (_PyDict_HasSplitTable(mp) &&
         ((ix >= 0 && old_value == NULL && mp->ma_used != ix) ||
          (ix == DKIX_EMPTY && mp->ma_used != mp->ma_keys->dk_nentries))) {
-        if (insertion_resize(mp) < 0) {
-            Py_DECREF(value);
-            return -1;
-        }
+        if (insertion_resize(mp) < 0)
+            goto Fail;
         hashpos = find_empty_slot(mp->ma_keys, key, hash);
         ix = DKIX_EMPTY;
     }
@@ -1140,15 +1138,12 @@
         assert(old_value == NULL);
         if (mp->ma_keys->dk_usable <= 0) {
             /* Need to resize. */
-            if (insertion_resize(mp) < 0) {
-                Py_DECREF(value);
-                return -1;
-            }
+            if (insertion_resize(mp) < 0)
+                goto Fail;
             hashpos = find_empty_slot(mp->ma_keys, key, hash);
         }
         ep = &DK_ENTRIES(mp->ma_keys)[mp->ma_keys->dk_nentries];
         dk_set_index(mp->ma_keys, hashpos, mp->ma_keys->dk_nentries);
-        Py_INCREF(key);
         ep->me_key = key;
         ep->me_hash = hash;
         if (mp->ma_values) {
@@ -1183,7 +1178,13 @@
     mp->ma_version_tag = DICT_NEXT_VERSION();
     Py_XDECREF(old_value); /* which **CAN** re-enter (see issue #22653) */
     assert(_PyDict_CheckConsistency(mp));
+    Py_DECREF(key);
     return 0;
+
+Fail:
+    Py_DECREF(value);
+    Py_DECREF(key);
+    return -1;
 }
 
 /*
@@ -2419,11 +2420,18 @@
         /* 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);
     }
@@ -2720,14 +2728,15 @@
             Py_INCREF(key);
             /* reuse the known hash value */
             b->ma_keys->dk_lookup(b, key, ep->me_hash, &bval, NULL);
-            Py_DECREF(key);
             if (bval == NULL) {
+                Py_DECREF(key);
                 Py_DECREF(aval);
                 if (PyErr_Occurred())
                     return -1;
                 return 0;
             }
             cmp = PyObject_RichCompareBool(aval, bval, Py_EQ);
+            Py_DECREF(key);
             Py_DECREF(aval);
             if (cmp <= 0)  /* error or not equal */
                 return cmp;
@@ -3612,7 +3621,7 @@
 static PyObject *
 dictiter_iternextitem(dictiterobject *di)
 {
-    PyObject *key, *value, *result = di->di_result;
+    PyObject *key, *value, *result;
     Py_ssize_t i;
     PyDictObject *d = di->di_dict;
 
@@ -3650,20 +3659,25 @@
     }
     di->di_pos = i+1;
     di->len--;
-    if (result->ob_refcnt == 1) {
+    Py_INCREF(key);
+    Py_INCREF(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(PyTuple_GET_ITEM(result, 0));
-        Py_DECREF(PyTuple_GET_ITEM(result, 1));
+        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 */
     }
-    Py_INCREF(key);
-    Py_INCREF(value);
-    PyTuple_SET_ITEM(result, 0, key);  /* steals reference */
-    PyTuple_SET_ITEM(result, 1, value);  /* steals reference */
     return result;
 
 fail:
@@ -4156,6 +4170,7 @@
 static int
 dictitems_contains(_PyDictViewObject *dv, PyObject *obj)
 {
+    int result;
     PyObject *key, *value, *found;
     if (dv->dv_dict == NULL)
         return 0;
@@ -4169,7 +4184,10 @@
             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 = {