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Secure GAE Scaffold for Python 2

Introduction

Please note: this is not an official Google product.

This scaffold is for users of App Engine's Python 2.7 runtime. For websites deployed to the Python 3 runtime, please see Secure Scaffold for Python 3.

This contains a boilerplate AppEngine application meant to provide a secure base on which to build additional functionality. Structure:

  • / - top level directory for common files, e.g. app.yaml
  • /js - directory for uncompiled Javascript resources.
  • /src - directory for all source code
  • /static - directory for static content
  • /templates - directory for Django/Jinja2 templates your app renders.
  • /templates/soy - directory for Closure Templates your application uses.

Javascript resources for your application can be written using Closure, and compiled by Google's Closure Compiler (detailed below in the dependencies section).

The scaffold provides the following basic security guarantees by default through a set of base classes found in src/base/handlers.py. These handlers:

  1. Set assorted security headers (Strict-Transport-Security, X-Frame-Options, X-XSS-Protection, X-Content-Type-Options, Content-Security-Policy) with strong default values to help avoid attacks like Cross-Site Scripting (XSS) and Cross-Site Script Inclusion. See _SetCommonResponseHeaders() and SetAjaxResponseHeaders().
  2. Prevent the XSS-prone construction of HTML via string concatenation by forcing the use of a template system (Django/Jinja2 supported). The template systems have non-contextual autoescaping enabled by default. See the render(), render_json() methods in BaseHandler and BaseAjaxHandler. For contextual autoescaping, you should use Closure Templates in strict mode (https://developers.google.com/closure/templates/docs/security).
  3. Test for the presence of headers that guarantee requests to Cron or Task endpoints are made by the AppEngine serving environment or an application administrator. See the dispatch() method in BaseCronHandler and BaseTaskHandler.
  4. Verify XSRF tokens by default on authenticated requests using any verb other that GET, HEAD, or OPTIONS. See the _RequestContainsValidXsrfToken() method for more information.

In addition to the protections above, the scaffold monkey patches assorted APIs that use insecure or dangerous defaults (see src/base/api_fixer.py).

Obviously no framework is perfect, and the flexibility of Python offers many ways for a motivated developer to circumvent the protections offered. Under the assumption that developers are not malicious, using the scaffold should centralize many security mechanisms, provide safe defaults, and structure the code in a way that facilitates security review.

Sample implementations can be found in src/handlers.py. These demonstrate basic functionality, and should be removed / replaced by code specific to your application.

Prerequisites

These instructions have been tested with the following software:

  • node.js >= 0.8.0
    • 0.8.0 is the minimum required to build with Grunt.
  • git
  • curl

Dependency Setup

From the root of the repository:

  1. git submodule init
  2. git submodule update
  3. cd closure-compiler - refer to closure-compiler/README.md on how to build the compiler. Feel free to use this GWT-skipping variant: mvn -pl externs/pom.xml,pom-main.xml,pom-main-shaded.xml
  4. cd ../closure-templates && mvn && cd ..
  5. npm install
  6. mkdir $HOME/bin; cd $HOME/bin
  7. npm install grunt-cli
    • Alternatively, sudo npm install -g grunt-cli will install system-wide and you may skip the next step.
  8. export PATH=$HOME/bin/node_modules/grunt-cli/bin:$PATH
    • It is advisable to add this to login profile scripts (.bashrc, etc.).
  9. Visit https://cloud.google.com/appengine/docs/python/download, and choose the alternative option to "download the original App Engine SDK for Python." Choose the "Linux" platform (even if you use OS X). Unzip the file, such that $HOME/bin/google_appengine/ is populated with the contents of the .zip.

To install dependencies for unit testing:

  1. sudo easy_install pip
  2. sudo pip install unittest2

Scaffold Development

Testing

To run unit tests:

python run_tests.py ~/bin/google_appengine src

Local Development

To run the development appserver locally:

  1. grunt clean
  2. grunt
  3. grunt appengine:run:app

Note that the development appserver will be running on a snapshot of code at the time you run it. If you make changes, you can run the various Grunt tasks in order to propagate them to the local appserver. For instance, grunt copy will refresh the source code (local and third party), static files, and templates. grunt closureSoys and/or grunt closureBuilder will rebuild the templates or your provided Javascript and the updated versions will be written in the output directory.

Deployment

To deploy to AppEngine:

  1. grunt clean
  2. grunt --appid=<appid>
  3. grunt appengine:update:app --appid=<appid>

Specifying --appid= will override any value set in config.json. You may modify the config.json file to avoid having to pass this parameter on every invocation.

Notes

Files in js/ are compiled by the Closure Compiler (if available) and placed in out/static/app.js. Included in this compilation pass is the the output of the closureSoys:js task (intermediate artifacts: out/generated/js/*.js).

Closure Templates that you provide are also compiled using the Python backend, and are available using the constants.CLOSURE template strategy (the default). The generated source code is stored in out/generated/*.py. To use them, pass the callable template as the first argument to render(), and a dictionary containing the template values as the second argument, e.g.:

from generated import helloworld

[...]

self.render(helloworld.helloWorld, { 'name': 'first last' })

The /static and /template directories are replicated in out/, and the files in src/ are rebased into out/ (so src/base/foo.py becomes out/base/foo.py).