Index: Lib/ConfigParser.py
===================================================================
--- Lib/ConfigParser.py	(revision 53450)
+++ Lib/ConfigParser.py	(working copy)
@@ -588,6 +588,10 @@
 
 class SafeConfigParser(ConfigParser):
 
+    def __init__(self, defaults=None, dict_type=dict):
+        ConfigParser.__init__(self, defaults, dict_type)
+        self._value_pattern = re.compile(r"%[^%(]|%$")
+
     def _interpolate(self, section, option, rawval, vars):
         # do the string interpolation
         L = []
@@ -638,4 +642,9 @@
         """Set an option.  Extend ConfigParser.set: check for string values."""
         if not isinstance(value, basestring):
             raise TypeError("option values must be strings")
-        ConfigParser.set(self, section, option, value)
+        ConfigParser.set(self, section, option, self.validate_value(value))
+
+    def validate_value(self, value):
+        if self._value_pattern.search(value):
+            raise ValueError("invalid interpolation syntax: %s" % value)
+        return value
Index: Lib/test/test_cfgparser.py
===================================================================
--- Lib/test/test_cfgparser.py	(revision 53450)
+++ Lib/test/test_cfgparser.py	(working copy)
@@ -422,6 +422,18 @@
         self.assertEqual(cf.get("section", "ok"), "xxx/%s")
         self.assertEqual(cf.get("section", "not_ok"), "xxx/xxx/%s")
 
+    def test_set_malformatted_interpolation(self):
+        cf = self.fromstring("[sect]\n"
+                             "option1=foo\n")
+
+        self.assertEqual(cf.get('sect', "option1"), "foo")
+
+        self.assertRaises(ValueError, cf.set, "sect", "option1", "%foo")
+        self.assertRaises(ValueError, cf.set, "sect", "option1", "foo%")
+        self.assertRaises(ValueError, cf.set, "sect", "option1", "f%oo")
+
+        self.assertEqual(cf.get('sect', "option1"), "foo")
+
     def test_set_nonstring_types(self):
         cf = self.fromstring("[sect]\n"
                              "option1=foo\n")