[1.4.x] Fixed queries that may return unexpected results on MySQL due to typecasting.

This is a security fix. Disclosure will follow shortly.

Backport of 75c0d4ea3ae48970f788c482ee0bd6b29a7f1307 from master

2 files changed, 26 insertions, 0 deletions

diff --git a/docs/ref/databases.txt b/docs/ref/databases.txt
index 4c18658304..269197946e 100644
--- a/docs/ref/databases.txt
+++ b/docs/ref/databases.txt
@@ -432,6 +432,22 @@ MySQL does not support the ``NOWAIT`` option to the ``SELECT ... FOR UPDATE``
 statement. If ``select_for_update()`` is used with ``nowait=True`` then a
 ``DatabaseError`` will be raised.
 
+Automatic typecasting can cause unexpected results
+--------------------------------------------------
+
+When performing a query on a string type, but with an integer value, MySQL will
+coerce the types of all values in the table to an integer before performing the
+comparison. If your table contains the values ``'abc'``, ``'def'`` and you
+query for ``WHERE mycolumn=0``, both rows will match. Similarly, ``WHERE mycolumn=1``
+will match the value ``'abc1'``. Therefore, string type fields included in Django
+will always cast the value to a string before using it in a query.
+
+If you implement custom model fields that inherit from :class:`~django.db.models.Field`
+directly, are overriding :meth:`~django.db.models.Field.get_prep_value`, or use
+:meth:`extra() <django.db.models.query.QuerySet.extra>` or
+:meth:`raw() <django.db.models.Manager.raw>`, you should ensure that you
+perform the appropriate typecasting.
+
 .. _sqlite-notes:
 
 SQLite notes
diff --git a/docs/ref/models/querysets.txt b/docs/ref/models/querysets.txt
index 022a251e5c..2decddbc28 100644
--- a/docs/ref/models/querysets.txt
+++ b/docs/ref/models/querysets.txt
@@ -1041,6 +1041,16 @@ of the arguments is required, but you should use at least one of them.
 
       Entry.objects.extra(where=['headline=%s'], params=['Lennon'])
 
+.. warning::
+
+    If you are performing queries on MySQL, note that MySQL's silent type coercion
+    may cause unexpected results when mixing types. If you query on a string
+    type column, but with an integer value, MySQL will coerce the types of all values
+    in the table to an integer before performing the comparison. For example, if your
+    table contains the values ``'abc'``, ``'def'`` and you query for ``WHERE mycolumn=0``,
+    both rows will match. To prevent this, perform the correct typecasting
+    before using the value in a query.
+
 defer
 ~~~~~