diff -r 63c3a3f28b42 Lib/test/test_str.py
--- a/Lib/test/test_str.py	Sat Oct 06 18:26:40 2012 -0400
+++ b/Lib/test/test_str.py	Sun Oct 07 12:16:13 2012 +0100
@@ -35,6 +35,16 @@
         string_tests.MixinStrUnicodeUserStringTest.test_formatting(self)
         self.assertRaises(OverflowError, '%c'.__mod__, 0x1234)
 
+    def test_formatting_huge_precision(self):
+        format_string = "%.{}f".format(sys.maxint + 1)
+        with self.assertRaises(ValueError):
+            result = format_string % 2.34
+
+    def test_formatting_huge_width(self):
+        format_string = "%{}f".format(sys.maxsize + 1)
+        with self.assertRaises(ValueError):
+            result = format_string % 2.34
+
     def test_conversion(self):
         # Make sure __str__() behaves properly
         class Foo0:
@@ -371,6 +381,21 @@
         self.assertRaises(ValueError, format, "", "-")
         self.assertRaises(ValueError, "{0:=s}".format, '')
 
+    def test_format_huge_precision(self):
+        format_string = ".{}f".format(sys.maxsize + 1)
+        with self.assertRaises(ValueError):
+            result = format(2.34, format_string)
+
+    def test_format_huge_width(self):
+        format_string = "{}f".format(sys.maxsize + 1)
+        with self.assertRaises(ValueError):
+            result = format(2.34, format_string)
+
+    def test_format_huge_item_number(self):
+        format_string = "{{{}:.6f}}".format(sys.maxsize + 1)
+        with self.assertRaises(ValueError):
+            result = format_string.format(2.34)
+
     def test_format_auto_numbering(self):
         class C:
             def __init__(self, x=100):
diff -r 63c3a3f28b42 Lib/test/test_unicode.py
--- a/Lib/test/test_unicode.py	Sat Oct 06 18:26:40 2012 -0400
+++ b/Lib/test/test_unicode.py	Sun Oct 07 12:16:13 2012 +0100
@@ -644,6 +644,16 @@
                 return u'\u1234'
         self.assertEqual('%s' % Wrapper(), u'\u1234')
 
+    def test_formatting_huge_precision(self):
+        format_string = u"%.{}f".format(sys.maxint + 1)
+        with self.assertRaises(ValueError):
+            result = format_string % 2.34
+
+    def test_formatting_huge_width(self):
+        format_string = u"%{}f".format(sys.maxsize + 1)
+        with self.assertRaises(ValueError):
+            result = format_string % 2.34
+
     def test_startswith_endswith_errors(self):
         for meth in (u'foo'.startswith, u'foo'.endswith):
             with self.assertRaises(UnicodeDecodeError):
@@ -1556,6 +1566,21 @@
         #  will fail
         self.assertRaises(UnicodeEncodeError, "foo{0}".format, u'\u1000bar')
 
+    def test_format_huge_precision(self):
+        format_string = u".{}f".format(sys.maxsize + 1)
+        with self.assertRaises(ValueError):
+            result = format(2.34, format_string)
+
+    def test_format_huge_width(self):
+        format_string = u"{}f".format(sys.maxsize + 1)
+        with self.assertRaises(ValueError):
+            result = format(2.34, format_string)
+
+    def test_format_huge_item_number(self):
+        format_string = u"{{{}:.6f}}".format(sys.maxsize + 1)
+        with self.assertRaises(ValueError):
+            result = format_string.format(2.34)
+
     def test_format_auto_numbering(self):
         class C:
             def __init__(self, x=100):
diff -r 63c3a3f28b42 Objects/stringlib/formatter.h
--- a/Objects/stringlib/formatter.h	Sat Oct 06 18:26:40 2012 -0400
+++ b/Objects/stringlib/formatter.h	Sun Oct 07 12:16:13 2012 +0100
@@ -73,7 +73,7 @@
 get_integer(STRINGLIB_CHAR **ptr, STRINGLIB_CHAR *end,
                   Py_ssize_t *result)
 {
-    Py_ssize_t accumulator, digitval, oldaccumulator;
+    Py_ssize_t accumulator, digitval;
     int numdigits;
     accumulator = numdigits = 0;
     for (;;(*ptr)++, numdigits++) {
@@ -83,19 +83,17 @@
         if (digitval < 0)
             break;
         /*
-           This trick was copied from old Unicode format code.  It's cute,
-           but would really suck on an old machine with a slow divide
-           implementation.  Fortunately, in the normal case we do not
-           expect too many digits.
+           Detect possible overflow before it happens:
+
+              accumulator * 10 + digitval > PY_SSIZE_T_MAX if and only if
+              accumulator > (PY_SSIZE_T_MAX - digitval) / 10.
         */
-        oldaccumulator = accumulator;
-        accumulator *= 10;
-        if ((accumulator+10)/10 != oldaccumulator+1) {
+        if (accumulator > (PY_SSIZE_T_MAX - digitval) / 10) {
             PyErr_Format(PyExc_ValueError,
                          "Too many decimal digits in format string");
             return -1;
         }
-        accumulator += digitval;
+        accumulator = accumulator * 10 + digitval;
     }
     *result = accumulator;
     return numdigits;
diff -r 63c3a3f28b42 Objects/stringlib/string_format.h
--- a/Objects/stringlib/string_format.h	Sat Oct 06 18:26:40 2012 -0400
+++ b/Objects/stringlib/string_format.h	Sun Oct 07 12:16:13 2012 +0100
@@ -197,7 +197,6 @@
 {
     Py_ssize_t accumulator = 0;
     Py_ssize_t digitval;
-    Py_ssize_t oldaccumulator;
     STRINGLIB_CHAR *p;
 
     /* empty string is an error */
@@ -209,19 +208,17 @@
         if (digitval < 0)
             return -1;
         /*
-           This trick was copied from old Unicode format code.  It's cute,
-           but would really suck on an old machine with a slow divide
-           implementation.  Fortunately, in the normal case we do not
-           expect too many digits.
+           Detect possible overflow before it happens:
+
+              accumulator * 10 + digitval > PY_SSIZE_T_MAX if and only if
+              accumulator > (PY_SSIZE_T_MAX - digitval) / 10.
         */
-        oldaccumulator = accumulator;
-        accumulator *= 10;
-        if ((accumulator+10)/10 != oldaccumulator+1) {
+        if (accumulator > (PY_SSIZE_T_MAX - digitval) / 10) {
             PyErr_Format(PyExc_ValueError,
                          "Too many decimal digits in format string");
             return -1;
         }
-        accumulator += digitval;
+        accumulator = accumulator * 10 + digitval;
     }
     return accumulator;
 }
diff -r 63c3a3f28b42 Objects/stringobject.c
--- a/Objects/stringobject.c	Sat Oct 06 18:26:40 2012 -0400
+++ b/Objects/stringobject.c	Sun Oct 07 12:16:13 2012 +0100
@@ -4369,7 +4369,7 @@
                     c = Py_CHARMASK(*fmt++);
                     if (!isdigit(c))
                         break;
-                    if ((width*10) / 10 != width) {
+                    if (width > (PY_SSIZE_T_MAX - ((int)c - '0')) / 10) {
                         PyErr_SetString(
                             PyExc_ValueError,
                             "width too big");
@@ -4404,7 +4404,7 @@
                         c = Py_CHARMASK(*fmt++);
                         if (!isdigit(c))
                             break;
-                        if ((prec*10) / 10 != prec) {
+                        if (prec > (INT_MAX - ((int)c - '0')) / 10) {
                             PyErr_SetString(
                                 PyExc_ValueError,
                                 "prec too big");
diff -r 63c3a3f28b42 Objects/unicodeobject.c
--- a/Objects/unicodeobject.c	Sat Oct 06 18:26:40 2012 -0400
+++ b/Objects/unicodeobject.c	Sun Oct 07 12:16:13 2012 +0100
@@ -8394,7 +8394,7 @@
                     c = *fmt++;
                     if (c < '0' || c > '9')
                         break;
-                    if ((width*10) / 10 != width) {
+                    if (width > (PY_SSIZE_T_MAX - ((int)c - '0')) / 10) {
                         PyErr_SetString(PyExc_ValueError,
                                         "width too big");
                         goto onError;
@@ -8427,7 +8427,7 @@
                         c = *fmt++;
                         if (c < '0' || c > '9')
                             break;
-                        if ((prec*10) / 10 != prec) {
+                        if (prec > (INT_MAX - ((int)c - '0')) / 10) {
                             PyErr_SetString(PyExc_ValueError,
                                             "prec too big");
                             goto onError;