[4.2.x] Fixed CVE-2025-13473 -- Standardized timing of check_password() in mod_wsgi auth handler.

Refs CVE-2024-39329, #20760.

Thanks Stackered for the report, and Jacob Walls and Markus Holtermann
for the reviews.

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

Backport of 3eb814e02a4c336866d4189fa0c24fd1875863ed from main.

1 files changed, 30 insertions, 7 deletions

diff --git a/django/contrib/auth/handlers/modwsgi.py b/django/contrib/auth/handlers/modwsgi.py
index 591ec72cb4..086db89fc8 100644
--- a/django/contrib/auth/handlers/modwsgi.py
+++ b/django/contrib/auth/handlers/modwsgi.py
@@ -4,24 +4,47 @@ from django.contrib import auth
 UserModel = auth.get_user_model()
 
 
+def _get_user(username):
+    """
+    Return the UserModel instance for `username`.
+
+    If no matching user exists, or if the user is inactive, return None, in
+    which case the default password hasher is run to mitigate timing attacks.
+    """
+    try:
+        user = UserModel._default_manager.get_by_natural_key(username)
+    except UserModel.DoesNotExist:
+        user = None
+    else:
+        if not user.is_active:
+            user = None
+
+    if user is None:
+        # Run the default password hasher once to reduce the timing difference
+        # between existing/active and nonexistent/inactive users (#20760).
+        UserModel().set_password("")
+
+    return user
+
+
 def check_password(environ, username, password):
     """
     Authenticate against Django's auth database.
 
     mod_wsgi docs specify None, True, False as return value depending
     on whether the user exists and authenticates.
+
+    Return None if the user does not exist, return False if the user exists but
+    password is not correct, and return True otherwise.
+
     """
     # db connection state is managed similarly to the wsgi handler
     # as mod_wsgi may call these functions outside of a request/response cycle
     db.reset_queries()
     try:
-        try:
-            user = UserModel._default_manager.get_by_natural_key(username)
-        except UserModel.DoesNotExist:
-            return None
-        if not user.is_active:
-            return None
-        return user.check_password(password)
+        user = _get_user(username)
+        if user:
+            return user.check_password(password)
     finally:
         db.close_old_connections()