diff --git a/Lib/test/test_mmap.py b/Lib/test/test_mmap.py
index 8b7e3e2..59b2374 100644
--- a/Lib/test/test_mmap.py
+++ b/Lib/test/test_mmap.py
@@ -466,6 +466,44 @@
         self.assert_(issubclass(mmap.error, EnvironmentError))
         self.assert_("mmap.error" in str(mmap.error))
 
+    def test_io_methods(self):
+        data = b"0123456789"
+        open(TESTFN, "wb").write(b"x"*len(data))
+        f = open(TESTFN, "r+b")
+        m = mmap.mmap(f.fileno(), len(data))
+        f.close()
+        # Test write_byte()
+        for i in range(len(data)):
+            self.assertEquals(m.tell(), i)
+            m.write_byte(data[i:i+1])
+            self.assertEquals(m.tell(), i+1)
+        self.assertRaises(ValueError, m.write_byte, b"x")
+        self.assertEquals(m[:], data)
+        # Test read_byte()
+        m.seek(0)
+        for i in range(len(data)):
+            self.assertEquals(m.tell(), i)
+            # XXX: Disable this test for now because it's not clear
+            # which type of object m.read_byte returns. Currently, it
+            # returns 1-length str (unicode).
+            if 0:
+                self.assertEquals(m.read_byte(), data[i:i+1])
+            else:
+                m.read_byte()
+            self.assertEquals(m.tell(), i+1)
+        self.assertRaises(ValueError, m.read_byte)
+        # Test read()
+        m.seek(3)
+        self.assertEquals(m.read(3), b"345")
+        self.assertEquals(m.tell(), 6)
+        # Test write()
+        m.seek(3)
+        m.write(b"bar")
+        self.assertEquals(m.tell(), 6)
+        self.assertEquals(m[:], b"012bar6789")
+        m.seek(8)
+        self.assertRaises(ValueError, m.write, b"bar")
+
 
 def test_main():
     run_unittest(MmapTests)
diff --git a/Misc/NEWS b/Misc/NEWS
index d354be8..ff271b9 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -173,6 +173,9 @@
 Library
 -------
 
+- Issue #5386: mmap.write_byte didn't check map size, so it could cause buffer
+  overrun.
+
 - Issue #1533164: Installed but not listed *.pyo was breaking Distutils
   bdist_rpm command.
 
diff --git a/Modules/mmapmodule.c b/Modules/mmapmodule.c
index 7fa1037..db94406 100644
--- a/Modules/mmapmodule.c
+++ b/Modules/mmapmodule.c
@@ -376,10 +376,17 @@
 
 	if (!is_writable(self))
 		return NULL;
-	*(self->data+self->pos) = value;
-	self->pos += 1;
-	Py_INCREF(Py_None);
-	return Py_None;
+
+	if (self->pos < self->size) {
+		*(self->data+self->pos) = value;
+		self->pos += 1;
+		Py_INCREF(Py_None);
+		return Py_None;
+	}
+	else {
+		PyErr_SetString(PyExc_ValueError, "write byte out of range");
+		return NULL;
+	}
 }
 
 static PyObject *