Merge pull request #5177 from edx/usman/fix-xml-imports

Moved imports to startup.py.

cms/startup.py

 """
 Module with code executed during Studio startup
 """
+
 from django.conf import settings
 
 # Force settings to run so that the python path is modified
@@ -14,6 +15,10 @@ def run():
     """
     Executed during django startup
     """
+    # Patch the xml libs.
+    from safe_lxml import defuse_xml_libs
+    defuse_xml_libs()
+
     django_utils_translation.patch()
 
     autostartup()

common/lib/safe_lxml/safe_lxml/__init__.py

+"""
+Defuse vulnerabilities in XML packages.
+"""
+
+
+def defuse_xml_libs():
+    """
+    Monkey patch and defuse all stdlib xml packages and lxml.
+    """
+    from defusedxml import defuse_stdlib
+    defuse_stdlib()
+
+    import lxml
+    import lxml.etree
+    from . import etree as safe_etree
+
+    lxml.etree = safe_etree

common/lib/safe_lxml/safe_lxml/etree.py

+"""
+Safer version of lxml.etree.
+
+It overrides some unsafe functions from lxml.etree with safer versions from defusedxml.
+It also includes a safer XMLParser.
+
+For processing xml always prefer this over using lxml.etree directly.
+"""
+
+from lxml.etree import *  # pylint: disable=wildcard-import, unused-wildcard-import
+from lxml.etree import XMLParser as _XMLParser
+
+# This should be imported after lxml.etree so that it overrides the following attributes.
+from defusedxml.lxml import parse, fromstring, XML
+
+
+class XMLParser(_XMLParser):  # pylint: disable=function-redefined
+    """
+    A safer version of XMLParser which by default disables entity resolution.
+    """
+
+    def __init__(self, *args, **kwargs):
+        if "resolve_entities" not in kwargs:
+            kwargs["resolve_entities"] = False
+        super(XMLParser, self).__init__(*args, **kwargs)

common/lib/safe_lxml/setup.py

+"""
+Setup.py for safe_lxml.
+"""
+
+from setuptools import setup
+
+setup(
+    name="safe_lxml",
+    version="1.0",
+    packages=["safe_lxml"],
+    install_requires=[
+        "lxml",
+        "defusedxml"
+    ],
+)

lms/startup.py

@@ -20,6 +20,11 @@ def run():
     """
     Executed during django startup
     """
+
+    # Patch the xml libs.
+    from safe_lxml import defuse_xml_libs
+    defuse_xml_libs()
+
     django_utils_translation.patch()
 
     autostartup()

requirements/edx/base.txt

@@ -12,6 +12,7 @@ boto==2.13.3
 celery==3.0.19
 cssselect==0.9.1
 dealer==0.2.3
+defusedxml==0.4.1
 distribute>=0.6.28, <0.7
 django-babel-underscore==0.1.0
 django-celery==3.0.17

requirements/edx/local.txt

@@ -3,6 +3,7 @@
 -e common/lib/calc
 -e common/lib/capa
 -e common/lib/chem
+-e common/lib/safe_lxml
 -e common/lib/sandbox-packages
 -e common/lib/symmath
 -e common/lib/xmodule