Issue #24917: time_strftime() Buffer Over-read. Patch by John Leitch.

Lib/test/test_time.py

@@ -177,6 +177,12 @@ class TimeTestCase(unittest.TestCase):
     def test_strftime_bounding_check(self):
         self._bounds_checking(lambda tup: time.strftime('', tup))
 
+    def test_strftime_format_check(self):
+        for x in [ '', 'A', '%A', '%AA' ]:
+            for y in range(0x0, 0x10):
+                for z in [ '%', 'A%', 'AA%', '%A%', 'A%A%', '%#' ]:
+                    self.assertRaises(ValueError, time.strftime, x * y + z)
+
     def test_default_values_for_zero(self):
         # Make sure that using all zeros uses the proper default
         # values.  No test for daylight savings since strftime() does

Misc/NEWS

@@ -179,6 +179,8 @@ Core and Builtins
 Library
 -------
 
+- Issue #24917: time_strftime() Buffer Over-read. Patch by John Leitch.
+
 - Issue #24635: Fixed a bug in typing.py where isinstance([], typing.Iterable)
   would return True once, then False on subsequent calls.
 

Modules/timemodule.c

@@ -623,6 +623,12 @@ time_strftime(PyObject *self, PyObject *args)
             Py_DECREF(format);
             return NULL;
         }
+        else if (outbuf[1] == '\0')
+        {
+            PyErr_SetString(PyExc_ValueError, "Incomplete format string");
+            Py_DECREF(format);
+            return NULL;
+        }
     }
 #elif (defined(_AIX) || defined(sun)) && defined(HAVE_WCSFTIME)
     for(outbuf = wcschr(fmt, '%');
@@ -636,6 +642,12 @@ time_strftime(PyObject *self, PyObject *args)
                             "format %y requires year >= 1900 on AIX");
             return NULL;
         }
+        else if (outbuf[1] == '\0')
+        {
+            PyErr_SetString(PyExc_ValueError, "Incomplete format string");
+            Py_DECREF(format);
+            return NULL;
+        }
     }
 #endif