[1.6.x] Prevented arbitrary file inclusion with {% ssi %} tag and relative paths.

Thanks Rainer Koirikivi for the report and draft patch.

This is a security fix; disclosure to follow shortly.

Backport of 7fe5b656c9 from master

django/template/defaulttags.py

@@ -1,6 +1,7 @@
 """Default tags used by the template system, available to all templates."""
 from __future__ import unicode_literals
 
+import os
 import sys
 import re
 from datetime import datetime
@@ -332,6 +333,7 @@ class RegroupNode(Node):
         return ''
 
 def include_is_allowed(filepath):
+    filepath = os.path.abspath(filepath)
     for root in settings.ALLOWED_INCLUDE_ROOTS:
         if filepath.startswith(root):
             return True

tests/template_tests/tests.py

@@ -1832,3 +1832,34 @@ class RequestContextTests(unittest.TestCase):
             template.Template('{% include "child" only %}').render(ctx),
             'none'
         )
+
+
+class SSITests(TestCase):
+    def setUp(self):
+        self.this_dir = os.path.dirname(os.path.abspath(upath(__file__)))
+        self.ssi_dir = os.path.join(self.this_dir, "templates", "first")
+
+    def render_ssi(self, path):
+        # the path must exist for the test to be reliable
+        self.assertTrue(os.path.exists(path))
+        return template.Template('{%% ssi "%s" %%}' % path).render(Context())
+
+    def test_allowed_paths(self):
+        acceptable_path = os.path.join(self.ssi_dir, "..", "first", "test.html")
+        with override_settings(ALLOWED_INCLUDE_ROOTS=(self.ssi_dir,)):
+            self.assertEqual(self.render_ssi(acceptable_path), 'First template\n')
+
+    def test_relative_include_exploit(self):
+        """
+        May not bypass ALLOWED_INCLUDE_ROOTS with relative paths
+
+        e.g. if ALLOWED_INCLUDE_ROOTS = ("/var/www",), it should not be
+        possible to do {% ssi "/var/www/../../etc/passwd" %}
+        """
+        disallowed_paths = [
+            os.path.join(self.ssi_dir, "..", "ssi_include.html"),
+            os.path.join(self.ssi_dir, "..", "second", "test.html"),
+        ]
+        with override_settings(ALLOWED_INCLUDE_ROOTS=(self.ssi_dir,)):
+            for path in disallowed_paths:
+                self.assertEqual(self.render_ssi(path), '')