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Shiny
Let’s Be Bad Guys!
Exploiting and Mitigating the
Top 1
0 Web App Vulnerabilities
Mike Pirnat - @mpirnat
PyCon 2016
Setup: https://github.com/mpirnat/lets-be-bad-guys
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Announcements
• Feedback: http://tiny.cc/phxoby
• This session will run 9:00 AM – 12:20 PM
• 15-minute break 10:00 – 10:15 AM
• Lunch upstairs in Oregon Ballroom
201-204
• Dietary needs? Let a server know!
• Share power outlets
Setup: https://github.com/mpirnat/lets-be-bad-guys
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Shiny
Let’s Be Bad Guys!
Exploiting and Mitigating the
Top 1
0 Web App Vulnerabilities
Mike Pirnat - @mpirnat
PyCon 2016
Setup: https://github.com/mpirnat/lets-be-bad-guys
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Who here makes
web apps?
Setup: https://github.com/mpirnat/lets-be-bad-guys
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Who here has
vulnerable apps?
Setup: https://github.com/mpirnat/lets-be-bad-guys
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Why it Matters
• Your users
• Your data
• Your business
Setup: https://github.com/mpirnat/lets-be-bad-guys
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OWASP
• http://www.owasp.org
• Open Web Application Security Project
• Non-profit focused on improving
software security
• Documentation and tools to help learn
about security and protect your apps
Setup: https://github.com/mpirnat/lets-be-bad-guys
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OWASP Top Ten
• Based on risk data from 8 firms
• Over 500,000 vulnerabilities, hundreds of
orgs, thousands of apps
• Selected & prioritized by prevalence data
combined with estimates of
exploitability, detectability, and impact
• Updated in 2013
Setup: https://github.com/mpirnat/lets-be-bad-guys
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Today
• Background on a type of vulnerability
• Exploit it!
• Discuss prevention
• Django & Flask specific advice where
possible
• Light examples where we have time
Setup: https://github.com/mpirnat/lets-be-bad-guys
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Disclaimer
Setup: https://github.com/mpirnat/lets-be-bad-guys
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About Django
Setup: https://github.com/mpirnat/lets-be-bad-guys
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About Flask
Setup: https://github.com/mpirnat/lets-be-bad-guys
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Setup: 1
Make & activate a virtualenv:
# Python 3...
$ pyvenv badguys
# Python 2...
$ virtualenv badguys
$ cd badguys
$ source bin/activate
Setup: https://github.com/mpirnat/lets-be-bad-guys
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Setup: 2
Clone our repository:
$ git clone https://github.com/
mpirnat/lets-be-bad-guys src
Or pull the latest changes:
$ cd src
$ git pull
Setup: https://github.com/mpirnat/lets-be-bad-guys
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Setup: 3
Install dependencies:
$ cd src
$ pip install -r requirements.txt
Setup: https://github.com/mpirnat/lets-be-bad-guys
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Setup: 4
Start up the app:
$ python manage.py runserver
Setup: https://github.com/mpirnat/lets-be-bad-guys
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Find a Partner
Setup: https://github.com/mpirnat/lets-be-bad-guys
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Injection
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Injection Attacks
• When an application sends untrusted
data to an interpreter
• Can result in data loss/corruption, lack of
accountability, denial of access
• Can lead to complete host takeover
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Who can you
trust?
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Trust No One
• External users
• Internal users
• Administrators
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Attack Vectors
• GET parameters
• POST parameters
• PATH_INFO
• Some HTTP headers: Cookie, Host
• Uploaded Files
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Possible
Consequences
• Creation of malicious SQL (or other
queries)
• Accessing private files on disk
• Arbitrary code execution
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Real-World
Examples
• Sony Playstation Network
• Ruby on Rails
• HBGary
• MySQL
• Many, many others…
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SQL Injection
• Unescaped user input causes the premature
end of a SQL query and allows a malicious
query to be executed...
"""
select * from users where username='%s';
"""
• Use 2 dashes to start a SQL comment: --
• http://localhost:8000/injection/sql
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Accessing
Private Files
• File system access + unvalidated user
input allows attackers to navigate the
file system
• http://localhost:8000/injection/file-
access
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Arbitrary Code
Execution
• Unsafe input is dynamically evaluated
or executed
• http://localhost:8000/injection/code-
execution
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Prevention
• Validate ALL user input
• Sign cookies, don’t accept if signature
is bogus/missing
• Use ORMs or bind variables when
talking to the database
• Don’t use eval or exec, beware of pickle,
user-supplied YAML, etc.
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Django Advice
• Make sure data types for your model are
tight
• Use Forms instead of ModelForms for
stronger validation
• Make new validators as needed for your
application
• Make sure your URL regexes for dynamic
URLs are tight
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Django Advice
• Use the ORM when you can
• When you can’t, use extreme caution!
• Use bind variables/parameters
• No string concatenation/formatting of
anything that came from the client
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Without the ORM
# If it's a basic select:
MyModel.objects.raw("SELECT ... WHERE foo = %s",
params={'foo': ...})
# If it's more complicated:
from django.db import connection
cursor = connection.cursor()
cursor.execute("UPDATE bar set bar = 1 WHERE foo < %s",
[foo])
row = cursor.fetchall()
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Flask Advice
• Use Flask-WTF to validate form input
• Use SQLAlchemy
• Bind variables if you don’t
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Broken
Authentication
& Session
Management
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Broken Auth &
Session Management
• Attacker uses leaks or flaws in
authentication or session management
to impersonate users
• Roll-your-own solutions contribute to
the difficulty of finding these flaws
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Possible
Consequences
• Compromised user accounts
• Compromised administrative
accounts
• Unauthorized use of privileged
functionality
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Prevention
• Hash or encrypt passwords
• Don’t let credentials be easily overwritten
• Don’t put session IDs in URLs
• Allow session IDs to timeout/log out
• Rotate session IDs after successful login
• TLS connections for passwords, session
IDs
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Django Advice
• Use django.contrib.auth
• Consider https://github.com/
yourlabs/django-session-security
middleware for timing out sessions
• We’ll talk about transport layer security
later on...
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Flask Advice
• Use flask-security
• Read https://pythonhosted.org/Flask-
Security/
• Avoid overuse of cookies for storing
session state
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Cross-Site
Scripting (XSS)
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XSS Attacks
• Cross-Site Scripting (XSS)
• The most prevalent web app security
flaw
• App includes user-supplied data in
content sent to the browser without
properly validating or sanitizing it
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XSS Attacks
• Stored: injected code permanently stored in
database, message forum, comment, etc.
• Reflected: injected code in live request to
server, reflected back in error message or
search result
• DOM: injected code in browser DOM
environment that causes scripts to run in
unexpected ways (eg, reading from URL)
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Possible
Consequences
• Execute scripts in a victim’s browser
• Hijack sessions
• Deface sites
• Insert hostile content
• Redirect users
• Hijack browser (install malware)
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Most Often Seen...
• Places where user-created text is
displayed to other users (comments,
messages)
• Form inputs where value is populated
with user-supplied data
• Script tags where user-supplied data is
populated into script variables
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Text
Real-World
Example
• http://es.pn/Z0jnoi
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XSS in
Dynamic URLs
• Part of the URL path is variable, isn’t
validated, and gets included into the
page
• http://localhost:8000/cross-site-
scripting/path-matching/your-path-
here
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XSS in Query
String Parameters
• Unvalidated user input from a query
string parameter is included in the
page
• http://localhost:8000/cross-site-
scripting/query-params?
qs=awesome
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XSS in
Form Fields
• The value part of an input is
prematurely terminated, allowing
Javascript to be injected into the element
(eg, adding an onclick)
• http://localhost:8000/cross-site-
scripting/form-field
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Can you trust the
database?
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Prevention
• Escape all untrusted data based on the
HTML context the data will be placed
into
• Whitelist input validation
• Consider auto-sanitization libraries for
rich content (eg, OWASP’s AntiSamy)
• Update your parents’/in-laws’ browsers!
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Django Advice
• Be careful with the safe filter,
django.utils.safestring, etc.
• Use form.as_p, form.as_table,
form.as_ul when displaying a form in a
template
• Be careful with your own template tags;
django.utils.html.escape is your friend!
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django.utils.html
cleaned = escape(unsafe_value)
cleaned = escapejs(unsafe_value)
cleaned = strip_tags(unsafe_value)
cleaned = remove_tags(unsafe_value, ['script', ...])
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Flask Advice
• Don’t disable autoescaping
• Be very careful with anything you
bless via Markup objects, the safe filter,
or blocks where autoescape is disabled
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Insecure
Direct Object
References
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Insecure Direct
Object Reference
• Expose a reference to an internal
implementation object without verifying
authorization
• Attacker changes URL, GET/POST
parameters, cookies
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Possible
Consequences
• Compromise of all data that can be
referenced by the vulnerable parameter
• Unless the namespace is sparse, an
attacker can easily access all available
data of that type
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Exercises
• Manipulate parameters in the URL to
access data that doesn’t belong to you
• http://localhost:8000/direct-object-
references
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Prevention
• Implement access controls on any
direct references to restricted resources
• Implement per-user or per-session
indirect object references
• This can be as much about URL design
as about access control!
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Django + Flask
• Lock down views:
• Use Django’s permissions architecture
• Use Flask-Security or Flask-Login
• Customize Django queryset for looking
up objects that involve user ownership
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Custom Queryset
# In models.py...
class ThingyManager(models.Manager):
def for_user(self, user):
return self.get_query_set().filter(user=user)
class Thingy(models.Model):
objects = ThingyManager()
# In views.py...
class ThingyUpdateView(UpdateView):
def get_queryset(self):
return Thingy.objects.for_user(
self.request.user)
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Security
Misconfiguration
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Security
Misconfiguration
• Insecure application settings
• Unpatched flaws
• Unused pages
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Possible
Consequences
• Unauthorized access to some system
data or functionality
• Potential complete system
compromise
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Exercises
• Demos and discussion
• http://localhost:8000/
misconfiguration
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Text
This Really
Happens!
• Patreon compromised in 2015 via
exposed Werkzeug Debugger
• https://labs.detectify.com/2015/10/02/
how-patreon-got-hacked-publicly-
exposed-werkzeug-debugger/
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Prevention
• Have a repeatable hardening process
• Have a process for keeping on top of
updates and patches
• Architecture that provides secure
separation between components
• Periodic scans and audits
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Django Advice
• Don’t run in debug mode in production
• Keep your SECRET_KEY secret!
• Keep Python code out of webserver’s root
• Don’t run admin publicly
• Don’t use the built-in admin for normal
user admin tasks
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Flask Advice
• Read http://flask.pocoo.org/docs/
security
• Set app.debug = False to turn off
debugging
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Gateway to Social
Engineering?
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Sensitive
Data Exposure
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Sensitive Data
Exposure
• Failure to properly protect credit
cards, tax IDs, authentication credentials,
etc.
• Sensitive data deserves extra
protection such as encryption at rest or
in transit, special precautions when
exchanged with the browser
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Insecure
Cryptographic Storage
• Not encrypting worthy data
• Unsafe key generation & storage, failure
to rotate keys
• Weak algorithms
• Weak or unsalted hashes
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Insufficient Transport
Layer Protection
• May not authenticate, encrypt, and
protect the confidentiality and integrity of
sensitive network traffic
• May use weak algorithms
• May use expired or invalid certificates
• May use certificates incorrectly
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Possible
Consequences
• Compromise of all data that should
have been encrypted
• This can be highly sensitive
information: credentials, credit cards,
personal data, health records, etc.
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Possible
Consequences
• Expose individual users’ data
• Account theft
• Compromise an admin account?!
• Poor SSL setup can facilitate phishing
and man-in-the-middle attacks
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Attack Vectors
• Attacker monitors network traffic of
your users
• Maybe in public places (Starbucks,
conference wi-fi, etc.)
• Maybe back end connections
• Maybe inside your network (!!!)
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http://cdn.ttgtmedia.com/digitalguide/images/Misc/WiresharkSS3_lg.png
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http://echeng.com/journal/images/misc/firesheep.png
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Prevention
• Encrypt sensitive data at rest
• Encrypt offsite backups; manage keys
separately
• Use strong standard algorithms, strong keys
• Hash passwords with strong standard
algorithm & use appropriate salt
• Protect passwords & keys from unauthorized
access
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Prevention
• Require SSL for all sensitive pages;
redirect non-SSL requests to SSL
• Set the “secure” flag on sensitive cookies
• Use only strong SSL algorithms
• Ensure your cert is valid, not expired, not
revoked, and matches your domain
• SSL/encryption on the back end too
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Django Advice
• Use django.contrib.auth for proper password salting
and hashing
• Use bcrypt; see https://docs.djangoproject.com/
en/1.8/topics/auth/passwords/
• Require SSL in Apache or Nginx
• Require SSL using middleware:
• Configure Django SecurityMiddleware–new in Django 1.8!
• http://django-secure.readthedocs.org/en/v0.1.2/
• https://github.com/rdegges/django-sslify
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Secure Cookies
# In your settings.py...
SESSION_COOKIE_SECURE = True
CSRF_COOKIE_SECURE = True
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Flask Advice
• Salt passwords using werkzeug.security:
• http://flask.pocoo.org/snippets/54/
• Require SSL:
• https://github.com/kennethreitz/flask-sslify
• https://github.com/jacobian/wsgi-sslify
• Use bcrypt; see https://pythonhosted.org/
passlib/
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Missing
Function Level
Access Control
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Missing
Function Level
Access Control
• Application doesn’t protect its functions
properly
• Misconfiguration
• Forgot proper code checks
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Attack Vectors
• Authorized user changes a URL or
parameter to a privileged function
• Anonymous users could access private
functions that aren’t protected
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Possible
Consequences
• Compromised user accounts
• Compromised administrative
accounts
• Unauthorized use of privileged
functionality
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Exercises
• Manipulate the URL to access
privileged functionality
• http://localhost:8000/missing-
access-control
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Prevention
• Consider every page; public or private?
• If authentication is required, make sure
that checks are in place
• If additional authorization is required,
make sure that checks are in place
• Deny all by default; explicitly grant
access to users or roles
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Django + Flask
• Lock down views:
• Use Django’s permissions architecture
• Use Flask-Security or Flask-Login
• Don’t use Django’s built-in admin for
normal user admin tasks
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Cross-Site
Request Forgery
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CSRF Attacks
• Cross-Site Request Forgery (CSRF)
• Attacker tricks victim into submitting
forged HTTP requests
• Attack succeeds if user is authorized/
authenticated
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Attack Vectors
• Image tags
• Cross-Site Scripting (XSS)
• Fake buttons
• Phishing forms
• Other techniques
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Possible
Consequences
• Cause victim to change any data the
victim is allowed to change
• Cause victim to perform any function
the victim is authorized to use
• Impact varies based on victim’s role
• Think of some possibilities...
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Real-World
Examples
• Facebook:
http://amolnaik4.blogspot.com/
2012/08/facebook-csrf-worth-
usd-5000.html
• Google/Gmail:
http://cryptogasm.com/2012/02/
does-google-understand-csrf/
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What if...
http://example.com/transferFunds?amount=…&destinationAccount=…
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What if...
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CSRF via Image
• Craft an “image” link that triggers some
site functionality
• http://localhost:8000/csrf/image
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CSRF via
Form Post
• Create an innocuous-looking form
that POSTs to a vulnerable location
• http://localhost:8000/csrf/third-
party-site
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Prevention
• Don’t “do” things on a GET
• Include a unique token in a hidden field
(often used in concert with a cookie)
• Validate token to make sure the
request is from on-site
• Avoid putting the token into a query
string
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Django Advice
• Don’t change the built-in settings!
• Do use the CSRF middleware and template
tag in forms
• Be VERY CAREFUL about deactivating it
(csrf_exempt decorator)
• Be careful about APIs (Tastypie, oauth);
http://codrspace.com/vote539/csrf-
protection-in-django-tastypie/
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Flask Advice
Various CSRF solutions...
• DIY/naïve: http://flask.pocoo.org/snippets/3/
• Flask-WTF: http://flask-wtf.readthedocs.org/
en/latest/csrf.html
• Flask-SeaSurf: https://flask-
seasurf.readthedocs.org/en/latest/
• Flask-csrf: http://sjl.bitbucket.org/flask-csrf/
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Using Known
Vulnerable
Components
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Components
with Known Vulnerabilities
• Libraries, frameworks, and other modules
almost always run with full privilege
• Hard to stay up to date on everything
• Do you even know all the components in
use, let alone their versions?
• Components with known problems can be
identified & exploited with automated tools
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Attack Vectors
• Attacker identifies a weak component
through scanning or manual analysis
• Customize exploit as needed
• More difficult the deeper the component
is in the application
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Possible
Consequences
• Full range of weaknesses are possible
• Impact could be minimal, or...
• Complete host takeover!
• Data compromise!
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Prevention
• Don’t use components you don’t write
(unrealistic)
• Keep components up to date
• Identify all components and versions
• Monitor security of these components
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Python Advice
• Use pip to find old packages:
pip list --outdated
• Use requires.io to monitor dependencies
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Django + Flask
• Keep an eye on the mailing lists:
• https://groups.google.com/group/
django-announce
• http://flask.pocoo.org/mailinglist
• Follow @djangoproject on Twitter
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Unvalidated
Redirects &
Forwards
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Redirection Abuse
• Attacker tricks user into visiting a URL
that redirects or forwards the request
without validating the redirect location
• Users prone to click because the link is
to a legitimate site
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Possible
Consequences
• Install malware
• Phishing/information disclosure
• Bypass access controls
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External
Redirection
• Use a redirection URL to redirect to an
external location
• http://localhost:8000/redirects-and-
forwards/redirects
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Forwards
• Manipulate a forward parameter to
gain access to privileged functionality
• http://localhost:8000/redirects-and-
forwards/forwards
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Prevention
• Don’t use redirects or forwards
• Don’t involve user-supplied data to build
the redirect location
• Ensure the supplied value is valid and
authorized for the user
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Django Advice
• Use django.utils.http.is_safe_url to
check redirect URLs
• Used by django.contrib.auth internally
• Consider wrapping is_safe_url if you
have to allow other off-domain URLs
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Checking
Redirects
from django.shortcuts import redirect
from django.utils.http import is_safe_url
def my_view(request):
...
url = 'http://www.example.com/foo/bar'
if is_safe_url(url, host='www.example.com'):
return redirect(url)
else:
...
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Allowing Multiple
Safe Hosts
from django.utils.http import is_safe_url
def is_whitelisted_url(url, hosts):
for host in hosts:
if is_safe_url(url, host=host):
return True
return False
>>> url = 'http://us.pycon.org'
>>> whitelist = ['us.pycon.org', 'pycon.org', ...]
>>> is_whitelisted_url(url, whitelist)
True
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Flask Advice
• Redirect back: http://flask.pocoo.org/
snippets/62/
• Redirect back, more selectively with
whitelist, using cookies for previous URL:
http://flask.pocoo.org/snippets/120/
• Can modify these approaches to suit,
as with the Django example
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Who here has
vulnerable apps?
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Who is ready to
do something
about it?
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Parting Thoughts
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Think Like
a Bad Guy
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Don’t Stop at Ten
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Constant Change
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Think Positive
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Further
Exploration
• http://www.owasp.org
• https://www.owasp.org/index.php/
Category:OWASP_Top_Ten_Project
• https://docs.djangoproject.com/en/dev/topics/security/
• http://flask.pocoo.org/docs/security
• OSCON 2016 & PyCon 2016: Security on a Shoestring
• https://www.fullstackpython.com/web-application-
security.html
• https://github.com/mpirnat/lets-be-bad-guys
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Contact Me
Mike Pirnat
http://mike.pirnat.com
@mpirnat
Tutorial feedback: http://tiny.cc/phxoby
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