Fixed CVE-2025-27556 -- Mitigated potential DoS in url_has_allowed_host_and_scheme() on Windows.

Thank you sw0rd1ight for the report.

2 files changed, 6 insertions, 3 deletions

diff --git a/django/utils/html.py b/django/utils/html.py
index a9d029165f..8676ae3092 100644
--- a/django/utils/html.py
+++ b/django/utils/html.py
@@ -13,7 +13,7 @@ from django.core.exceptions import SuspiciousOperation, ValidationError
 from django.core.validators import EmailValidator
 from django.utils.deprecation import RemovedInDjango70Warning
 from django.utils.functional import Promise, cached_property, keep_lazy, keep_lazy_text
-from django.utils.http import RFC3986_GENDELIMS, RFC3986_SUBDELIMS
+from django.utils.http import MAX_URL_LENGTH, RFC3986_GENDELIMS, RFC3986_SUBDELIMS
 from django.utils.regex_helper import _lazy_re_compile
 from django.utils.safestring import SafeData, SafeString, mark_safe
 from django.utils.text import normalize_newlines
@@ -41,7 +41,6 @@ VOID_ELEMENTS = frozenset(
     )
 )
 
-MAX_URL_LENGTH = 2048
 MAX_STRIP_TAGS_DEPTH = 50
 
 
diff --git a/django/utils/http.py b/django/utils/http.py
index 1f9adeb707..1c7aec7141 100644
--- a/django/utils/http.py
+++ b/django/utils/http.py
@@ -39,6 +39,7 @@ ASCTIME_DATE = _lazy_re_compile(r"^\w{3} %s %s %s %s$" % (__M, __D2, __T, __Y))
 
 RFC3986_GENDELIMS = ":/?#[]@"
 RFC3986_SUBDELIMS = "!$&'()*+,;="
+MAX_URL_LENGTH = 2048
 
 
 def urlencode(query, doseq=False):
@@ -274,7 +275,10 @@ def url_has_allowed_host_and_scheme(url, allowed_hosts, require_https=False):
 def _url_has_allowed_host_and_scheme(url, allowed_hosts, require_https=False):
     # Chrome considers any URL with more than two slashes to be absolute, but
     # urlsplit is not so flexible. Treat any url with three slashes as unsafe.
-    if url.startswith("///"):
+    if url.startswith("///") or len(url) > MAX_URL_LENGTH:
+        # urlsplit does not perform validation of inputs. Unicode normalization
+        # is very slow on Windows and can be a DoS attack vector.
+        # https://docs.python.org/3/library/urllib.parse.html#url-parsing-security
         return False
     try:
         url_info = urlsplit(url)