diff -r cca2ed4e8b41 Doc/c-api/exceptions.rst
--- a/Doc/c-api/exceptions.rst	Thu Dec 17 10:35:05 2015 +0000
+++ b/Doc/c-api/exceptions.rst	Thu Dec 17 17:51:01 2015 -0500
@@ -570,6 +570,10 @@
    it.  There is no type check to make sure that *ctx* is an exception instance.
    This steals a reference to *ctx*.
 
+   .. versionchanged:: 3.5.2
+      If setting *ctx* introduces an object cycle, the context will be
+      set to *NULL*.
+
 
 .. c:function:: PyObject* PyException_GetCause(PyObject *ex)
 
diff -r cca2ed4e8b41 Doc/library/exceptions.rst
--- a/Doc/library/exceptions.rst	Thu Dec 17 10:35:05 2015 +0000
+++ b/Doc/library/exceptions.rst	Thu Dec 17 17:51:01 2015 -0500
@@ -66,6 +66,18 @@
 exceptions so that the final line of the traceback always shows the last
 exception that was raised.
 
+.. versionchanged:: 3.5.2
+
+   An attempt to introduce a object cycle when setting
+   :attr:`__context__` will trigger a :exc:`RuntimeError`
+   exception::
+
+       >>> exc = Exception()
+       >>> exc.__context__ = exc
+       Traceback (most recent call last):
+         File "<stdin>", line 1, in <module>
+       RuntimeError: cycle in exception context chain
+
 
 Base classes
 ------------
diff -r cca2ed4e8b41 Lib/test/test_exceptions.py
--- a/Lib/test/test_exceptions.py	Thu Dec 17 10:35:05 2015 +0000
+++ b/Lib/test/test_exceptions.py	Thu Dec 17 17:51:01 2015 -0500
@@ -625,6 +625,21 @@
         obj = wr()
         self.assertTrue(obj is None, "%s" % obj)
 
+    def testExceptionChainedContext(self):
+        class Err1(Exception): pass
+        class Err2(Exception): pass
+
+        err1 = Err1()
+        with self.assertRaisesRegex(RuntimeError, 'cycle in exception context'):
+            err1.__context__ = err1
+        self.assertIsNone(err1.__context__)
+
+        err2 = Err2()
+        err2.__context__ = err1
+        with self.assertRaisesRegex(RuntimeError, 'cycle in exception context'):
+            err1.__context__ = err2
+        self.assertIsNone(err1.__context__)
+
     def test_exception_target_in_nested_scope(self):
         # issue 4617: This used to raise a SyntaxError
         # "can not delete variable 'e' referenced in nested scope"
diff -r cca2ed4e8b41 Objects/exceptions.c
--- a/Objects/exceptions.c	Thu Dec 17 10:35:05 2015 +0000
+++ b/Objects/exceptions.c	Thu Dec 17 17:51:01 2015 -0500
@@ -265,6 +265,10 @@
         Py_INCREF(arg);
     }
     PyException_SetContext(self, arg);
+    if (((PyBaseExceptionObject *)self)->context != arg) {
+        PyErr_SetString(PyExc_RuntimeError, "cycle in exception context chain");
+        return -1;
+    }
     return 0;
 }
 
@@ -292,6 +296,10 @@
         Py_INCREF(arg);
     }
     PyException_SetCause(self, arg);
+    if (arg != NULL && ((PyBaseExceptionObject *)self)->cause == NULL) {
+        PyErr_SetString(PyExc_RuntimeError, "cycle in exception cause chain");
+        return -1;
+    }
     return 0;
 }
 
@@ -331,6 +339,7 @@
 /* Steals a reference to cause */
 void
 PyException_SetCause(PyObject *self, PyObject *cause) {
+    assert(cause == NULL || PyExceptionInstance_Check(cause));
     PyObject *old_cause = ((PyBaseExceptionObject *)self)->cause;
     ((PyBaseExceptionObject *)self)->cause = cause;
     ((PyBaseExceptionObject *)self)->suppress_context = 1;
@@ -346,7 +355,18 @@
 
 /* Steals a reference to context */
 void
-PyException_SetContext(PyObject *self, PyObject *context) {
+PyException_SetContext(PyObject *self, PyObject *context)
+{
+    PyObject *exc = context;
+    while (exc != NULL) {
+        if (exc == self) {
+            context = NULL;
+            break;
+        }
+        assert(PyExceptionInstance_Check(exc));
+        exc = ((PyBaseExceptionObject *)exc)->context;
+    }
+
     PyObject *old_context = ((PyBaseExceptionObject *)self)->context;
     ((PyBaseExceptionObject *)self)->context = context;
     Py_XDECREF(old_context);