diff --git a/django/core/validators.py b/django/core/validators.py
index d32b54f68c..2da0688e28 100644
--- a/django/core/validators.py
+++ b/django/core/validators.py
@@ -75,7 +75,7 @@ class URLValidator(RegexValidator):
     ul = '\u00a1-\uffff'  # unicode letters range (must not be a raw string)
 
     # IP patterns
-    ipv4_re = r'(?:25[0-5]|2[0-4]\d|[0-1]?\d?\d)(?:\.(?:25[0-5]|2[0-4]\d|[0-1]?\d?\d)){3}'
+    ipv4_re = r'(?:0|25[0-5]|2[0-4]\d|1\d?\d?|[1-9]\d?)(?:\.(?:0|25[0-5]|2[0-4]\d|1\d?\d?|[1-9]\d?)){3}'
     ipv6_re = r'\[[0-9a-f:\.]+\]'  # (simple regex, validated later)
 
     # Host patterns
@@ -256,6 +256,18 @@ def validate_ipv4_address(value):
         ipaddress.IPv4Address(value)
     except ValueError:
         raise ValidationError(_('Enter a valid IPv4 address.'), code='invalid')
+    else:
+        # Leading zeros are forbidden to avoid ambiguity with the octal
+        # notation. This restriction is included in Python 3.9.5+.
+        # TODO: Remove when dropping support for PY39.
+        if any(
+            octet != '0' and octet[0] == '0'
+            for octet in value.split('.')
+        ):
+            raise ValidationError(
+                _('Enter a valid IPv4 address.'),
+                code='invalid',
+            )
 
 
 def validate_ipv6_address(value):
diff --git a/docs/releases/2.2.24.txt b/docs/releases/2.2.24.txt
index 9bcf7037c4..1064fc53a0 100644
--- a/docs/releases/2.2.24.txt
+++ b/docs/releases/2.2.24.txt
@@ -17,3 +17,16 @@ the existence but also the file contents would have been exposed.
 
 As a mitigation, path sanitation is now applied and only files within the
 template root directories can be loaded.
+
+CVE-2021-33571: Possible indeterminate SSRF, RFI, and LFI attacks since validators accepted leading zeros in IPv4 addresses
+===========================================================================================================================
+
+:class:`~django.core.validators.URLValidator`,
+:func:`~django.core.validators.validate_ipv4_address`, and
+:func:`~django.core.validators.validate_ipv46_address` didn't prohibit leading
+zeros in octal literals. If you used such values you could suffer from
+indeterminate SSRF, RFI, and LFI attacks.
+
+:func:`~django.core.validators.validate_ipv4_address` and
+:func:`~django.core.validators.validate_ipv46_address` validators were not
+affected on Python 3.9.5+.
diff --git a/tests/validators/invalid_urls.txt b/tests/validators/invalid_urls.txt
index 4a092034ff..a5a41ba845 100644
--- a/tests/validators/invalid_urls.txt
+++ b/tests/validators/invalid_urls.txt
@@ -46,6 +46,14 @@ http://1.1.1.1.1
 http://123.123.123
 http://3628126748
 http://123
+http://000.000.000.000
+http://016.016.016.016
+http://192.168.000.001
+http://01.2.3.4
+http://01.2.3.4
+http://1.02.3.4
+http://1.2.03.4
+http://1.2.3.04
 http://.www.foo.bar/
 http://.www.foo.bar./
 http://[::1:2::3]:8080/
diff --git a/tests/validators/tests.py b/tests/validators/tests.py
index 012b098f4e..1f09fb53fc 100644
--- a/tests/validators/tests.py
+++ b/tests/validators/tests.py
@@ -135,6 +135,16 @@ TEST_DATA = [
     (validate_ipv4_address, '1.1.1.1\n', ValidationError),
     (validate_ipv4_address, '٧.2٥.3٣.243', ValidationError),
 
+    # Leading zeros are forbidden to avoid ambiguity with the octal notation.
+    (validate_ipv4_address, '000.000.000.000', ValidationError),
+    (validate_ipv4_address, '016.016.016.016', ValidationError),
+    (validate_ipv4_address, '192.168.000.001', ValidationError),
+    (validate_ipv4_address, '01.2.3.4', ValidationError),
+    (validate_ipv4_address, '01.2.3.4', ValidationError),
+    (validate_ipv4_address, '1.02.3.4', ValidationError),
+    (validate_ipv4_address, '1.2.03.4', ValidationError),
+    (validate_ipv4_address, '1.2.3.04', ValidationError),
+
     # validate_ipv6_address uses django.utils.ipv6, which
     # is tested in much greater detail in its own testcase
     (validate_ipv6_address, 'fe80::1', None),
@@ -160,6 +170,16 @@ TEST_DATA = [
     (validate_ipv46_address, '::zzz', ValidationError),
     (validate_ipv46_address, '12345::', ValidationError),
 
+    # Leading zeros are forbidden to avoid ambiguity with the octal notation.
+    (validate_ipv46_address, '000.000.000.000', ValidationError),
+    (validate_ipv46_address, '016.016.016.016', ValidationError),
+    (validate_ipv46_address, '192.168.000.001', ValidationError),
+    (validate_ipv46_address, '01.2.3.4', ValidationError),
+    (validate_ipv46_address, '01.2.3.4', ValidationError),
+    (validate_ipv46_address, '1.02.3.4', ValidationError),
+    (validate_ipv46_address, '1.2.03.4', ValidationError),
+    (validate_ipv46_address, '1.2.3.04', ValidationError),
+
     (validate_comma_separated_integer_list, '1', None),
     (validate_comma_separated_integer_list, '12', None),
     (validate_comma_separated_integer_list, '1,2', None),
diff --git a/tests/validators/valid_urls.txt b/tests/validators/valid_urls.txt
index f79f948142..ef9e563f8e 100644
--- a/tests/validators/valid_urls.txt
+++ b/tests/validators/valid_urls.txt
@@ -63,6 +63,12 @@ http://0.0.0.0/
 http://255.255.255.255
 http://224.0.0.0
 http://224.1.1.1
+http://111.112.113.114/
+http://88.88.88.88/
+http://11.12.13.14/
+http://10.20.30.40/
+http://1.2.3.4/
+http://127.0.01.09.home.lan
 http://aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa.example.com
 http://example.aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa.com
 http://example.aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa