Fixed CVE-2023-36053 -- Prevented potential ReDoS in EmailValidator and URLValidator.

Thanks Seokchan Yoon for reports.

docs/ref/forms/fields.txt

@@ -596,7 +596,12 @@ For each field, we describe the default widget used if you don't specify
     * Error message keys: ``required``, ``invalid``
 
     Has the optional arguments ``max_length``, ``min_length``, and
-    ``empty_value`` which work just as they do for :class:`CharField`.
+    ``empty_value`` which work just as they do for :class:`CharField`. The
+    ``max_length`` argument defaults to 320 (see :rfc:`3696#section-3`).
+
+    .. versionchanged:: 3.2.20
+
+        The default value for ``max_length`` was changed to 320 characters.
 
 ``FileField``
 -------------

docs/ref/validators.txt

@@ -133,6 +133,11 @@ to, or in lieu of custom ``field.clean()`` methods.
     :param code: If not ``None``, overrides :attr:`code`.
     :param allowlist: If not ``None``, overrides :attr:`allowlist`.
 
+    An :class:`EmailValidator` ensures that a value looks like an email, and
+    raises a :exc:`~django.core.exceptions.ValidationError` with
+    :attr:`message` and :attr:`code` if it doesn't. Values longer than 320
+    characters are always considered invalid.
+
     .. attribute:: message
 
         The error message used by
@@ -154,13 +159,19 @@ to, or in lieu of custom ``field.clean()`` methods.
         validation, so you'd need to add them to the ``allowlist`` as
         necessary.
 
+    .. versionchanged:: 3.2.20
+
+        In older versions, values longer than 320 characters could be
+        considered valid.
+
 ``URLValidator``
 ----------------
 
 .. class:: URLValidator(schemes=None, regex=None, message=None, code=None)
 
     A :class:`RegexValidator` subclass that ensures a value looks like a URL,
-    and raises an error code of ``'invalid'`` if it doesn't.
+    and raises an error code of ``'invalid'`` if it doesn't. Values longer than
+    :attr:`max_length` characters are always considered invalid.
 
     Loopback addresses and reserved IP spaces are considered valid. Literal
     IPv6 addresses (:rfc:`3986#section-3.2.2`) and Unicode domains are both
@@ -177,6 +188,18 @@ to, or in lieu of custom ``field.clean()`` methods.
 
         .. _valid URI schemes: https://www.iana.org/assignments/uri-schemes/uri-schemes.xhtml
 
+    .. attribute:: max_length
+
+        .. versionadded:: 3.2.20
+
+        The maximum length of values that could be considered valid. Defaults
+        to 2048 characters.
+
+    .. versionchanged:: 3.2.20
+
+        In older versions, values longer than 2048 characters could be
+        considered valid.
+
 ``validate_email``
 ------------------