[4.2.x] Fixed CVE-2024-56374 -- Mitigated potential DoS in IPv6 validation.

Thanks Saravana Kumar for the report, and Sarah Boyce and Mariusz
Felisiak for the reviews.

Co-authored-by: Natalia <124304+nessita@users.noreply.github.com>

2 files changed, 23 insertions, 2 deletions

diff --git a/docs/ref/forms/fields.txt b/docs/ref/forms/fields.txt
index 1a7274e0d1..76b4587e21 100644
--- a/docs/ref/forms/fields.txt
+++ b/docs/ref/forms/fields.txt
@@ -719,7 +719,7 @@ For each field, we describe the default widget used if you don't specify
     * Empty value: ``''`` (an empty string)
     * Normalizes to: A string. IPv6 addresses are normalized as described below.
     * Validates that the given value is a valid IP address.
-    * Error message keys: ``required``, ``invalid``
+    * Error message keys: ``required``, ``invalid``, ``max_length``
 
     The IPv6 address normalization follows :rfc:`4291#section-2.2` section 2.2,
     including using the IPv4 format suggested in paragraph 3 of that section, like
@@ -727,7 +727,7 @@ For each field, we describe the default widget used if you don't specify
     ``2001::1``, and ``::ffff:0a0a:0a0a`` to ``::ffff:10.10.10.10``. All characters
     are converted to lowercase.
 
-    Takes two optional arguments:
+    Takes three optional arguments:
 
     .. attribute:: protocol
 
@@ -742,6 +742,15 @@ For each field, we describe the default widget used if you don't specify
         ``192.0.2.1``. Default is disabled. Can only be used
         when ``protocol`` is set to ``'both'``.
 
+    .. attribute:: max_length
+
+        Defaults to 39, and behaves the same way as it does for
+        :class:`CharField`.
+
+    .. versionchanged:: 4.2.18
+
+        The default value for ``max_length`` was set to 39 characters.
+
 ``ImageField``
 --------------
 
diff --git a/docs/releases/4.2.18.txt b/docs/releases/4.2.18.txt
index ae0c4e3334..fef91d9150 100644
--- a/docs/releases/4.2.18.txt
+++ b/docs/releases/4.2.18.txt
@@ -5,3 +5,15 @@ Django 4.2.18 release notes
 *January 14, 2025*
 
 Django 4.2.18 fixes a security issue with severity "moderate" in 4.2.17.
+
+CVE-2024-56374: Potential denial-of-service vulnerability in IPv6 validation
+============================================================================
+
+Lack of upper bound limit enforcement in strings passed when performing IPv6
+validation could lead to a potential denial-of-service attack. The undocumented
+and private functions ``clean_ipv6_address`` and ``is_valid_ipv6_address`` were
+vulnerable, as was the  :class:`django.forms.GenericIPAddressField` form field,
+which has now been updated to define a ``max_length`` of 39 characters.
+
+The :class:`django.db.models.GenericIPAddressField` model field was not
+affected.