[4.2.x] Fixed CVE-2024-39614 -- Mitigated potential DoS in get_supported_language_variant().

Language codes are now parsed with a maximum length limit of 500 chars.

Thanks to MProgrammer for the report.

django/utils/translation/trans_real.py

@@ -31,9 +31,10 @@ _default = None
 CONTEXT_SEPARATOR = "\x04"
 
 # Maximum number of characters that will be parsed from the Accept-Language
-# header to prevent possible denial of service or memory exhaustion attacks.
-# About 10x longer than the longest value shown on MDN’s Accept-Language page.
-ACCEPT_LANGUAGE_HEADER_MAX_LENGTH = 500
+# header or cookie to prevent possible denial of service or memory exhaustion
+# attacks. About 10x longer than the longest value shown on MDN’s
+# Accept-Language page.
+LANGUAGE_CODE_MAX_LENGTH = 500
 
 # Format of Accept-Language header values. From RFC 9110 Sections 12.4.2 and
 # 12.5.4, and RFC 5646 Section 2.1.
@@ -497,11 +498,25 @@ def get_supported_language_variant(lang_code, strict=False):
     If `strict` is False (the default), look for a country-specific variant
     when neither the language code nor its generic variant is found.
 
+    The language code is truncated to a maximum length to avoid potential
+    denial of service attacks.
+
     lru_cache should have a maxsize to prevent from memory exhaustion attacks,
     as the provided language codes are taken from the HTTP request. See also
     <https://www.djangoproject.com/weblog/2007/oct/26/security-fix/>.
     """
     if lang_code:
+        # Truncate the language code to a maximum length to avoid potential
+        # denial of service attacks.
+        if len(lang_code) > LANGUAGE_CODE_MAX_LENGTH:
+            if (
+                not strict
+                and (index := lang_code.rfind("-", 0, LANGUAGE_CODE_MAX_LENGTH)) > 0
+            ):
+                # There is a generic variant under the maximum length accepted length.
+                lang_code = lang_code[:index]
+            else:
+                raise ValueError("'lang_code' exceeds the maximum accepted length")
         # If 'zh-hant-tw' is not supported, try special fallback or subsequent
         # language codes i.e. 'zh-hant' and 'zh'.
         possible_lang_codes = [lang_code]
@@ -625,13 +640,13 @@ def parse_accept_lang_header(lang_string):
     functools.lru_cache() to avoid repetitive parsing of common header values.
     """
     # If the header value doesn't exceed the maximum allowed length, parse it.
-    if len(lang_string) <= ACCEPT_LANGUAGE_HEADER_MAX_LENGTH:
+    if len(lang_string) <= LANGUAGE_CODE_MAX_LENGTH:
         return _parse_accept_lang_header(lang_string)
 
     # If there is at least one comma in the value, parse up to the last comma
     # before the max length, skipping any truncated parts at the end of the
     # header value.
-    if (index := lang_string.rfind(",", 0, ACCEPT_LANGUAGE_HEADER_MAX_LENGTH)) > 0:
+    if (index := lang_string.rfind(",", 0, LANGUAGE_CODE_MAX_LENGTH)) > 0:
         return _parse_accept_lang_header(lang_string[:index])
 
     # Don't attempt to parse if there is only one language-range value which is

docs/ref/utils.txt

@@ -1155,6 +1155,11 @@ For a complete discussion on the usage of the following see the
     ``lang_code`` is ``'es-ar'`` and ``'es'`` is in :setting:`LANGUAGES` but
     ``'es-ar'`` isn't.
 
+    ``lang_code`` has a maximum accepted length of 500 characters. A
+    :exc:`ValueError` is raised if ``lang_code`` exceeds this limit and
+    ``strict`` is ``True``, or if there is no generic variant and ``strict``
+    is ``False``.
+
     If ``strict`` is ``False`` (the default), a country-specific variant may
     be returned when neither the language code nor its generic variant is found.
     For example, if only ``'es-co'`` is in :setting:`LANGUAGES`, that's
@@ -1163,6 +1168,11 @@ For a complete discussion on the usage of the following see the
 
     Raises :exc:`LookupError` if nothing is found.
 
+    .. versionchanged:: 4.2.14
+
+        In older versions, ``lang_code`` values over 500 characters were
+        processed without raising a :exc:`ValueError`.
+
 .. function:: to_locale(language)
 
     Turns a language name (en-us) into a locale name (en_US).

docs/releases/4.2.14.txt

@@ -32,3 +32,18 @@ directory-traversal via certain inputs when calling :meth:`save()
 <django.core.files.storage.Storage.save()>`.
 
 Built-in ``Storage`` sub-classes were not affected by this vulnerability.
+
+CVE-2024-39614: Potential denial-of-service vulnerability in ``get_supported_language_variant()``
+=================================================================================================
+
+:meth:`~django.utils.translation.get_supported_language_variant` was subject to
+a potential denial-of-service attack when used with very long strings
+containing specific characters.
+
+To mitigate this vulnerability, the language code provided to
+:meth:`~django.utils.translation.get_supported_language_variant` is now parsed
+up to a maximum length of 500 characters.
+
+When the language code is over 500 characters, a :exc:`ValueError` will now be
+raised if ``strict`` is ``True``, or if there is no generic variant and
+``strict`` is ``False``.

tests/i18n/tests.py

@@ -65,6 +65,7 @@ from django.utils.translation.reloader import (
     translation_file_changed,
     watch_for_translation_changes,
 )
+from django.utils.translation.trans_real import LANGUAGE_CODE_MAX_LENGTH
 
 from .forms import CompanyForm, I18nForm, SelectDateForm
 from .models import Company, TestModel
@@ -1888,6 +1889,16 @@ class MiscTests(SimpleTestCase):
             g("xyz")
         with self.assertRaises(LookupError):
             g("xy-zz")
+        msg = "'lang_code' exceeds the maximum accepted length"
+        with self.assertRaises(LookupError):
+            g("x" * LANGUAGE_CODE_MAX_LENGTH)
+        with self.assertRaisesMessage(ValueError, msg):
+            g("x" * (LANGUAGE_CODE_MAX_LENGTH + 1))
+        # 167 * 3 = 501 which is LANGUAGE_CODE_MAX_LENGTH + 1.
+        self.assertEqual(g("en-" * 167), "en")
+        with self.assertRaisesMessage(ValueError, msg):
+            g("en-" * 167, strict=True)
+        self.assertEqual(g("en-" * 30000), "en")  # catastrophic test
 
     def test_get_supported_language_variant_null(self):
         g = trans_null.get_supported_language_variant