1 files changed, 9 insertions, 1 deletions
diff --git a/django/contrib/auth/forms.py b/django/contrib/auth/forms.py
index 9d6a9918bb..1d23bcc57a 100644
--- a/django/contrib/auth/forms.py
+++ b/django/contrib/auth/forms.py
@@ -71,7 +71,15 @@ class ReadOnlyPasswordHashField(forms.Field):
 
 class UsernameField(forms.CharField):
     def to_python(self, value):
-        return unicodedata.normalize("NFKC", super().to_python(value))
+        value = super().to_python(value)
+        if self.max_length is not None and len(value) > self.max_length:
+            # Normalization can increase the string length (e.g.
+            # "ﬀ" -> "ff", "½" -> "1⁄2") but cannot reduce it, so there is no
+            # point in normalizing invalid data. Moreover, Unicode
+            # normalization is very slow on Windows and can be a DoS attack
+            # vector.
+            return value
+        return unicodedata.normalize("NFKC", value)
 
     def widget_attrs(self, widget):
         return {