Use CSRF tokens to avoid CSRF attacks

Cross-Site Request Forgery (CSRF) a.k.a. session riding occurs when sensitive web services have no protection to prevent attackers arbitrarily submitting data and commands on a website a user trusts. For example, an attacker may be able to cause an authorized user to submit form data to a web service which performs administrative functionality, or modifies personal settings.

Incorrect

In a typical attack scenario a victim with an existing session of a legitimate site is tricked into visiting a malicious site that leverages this session to trigger some action. The snippet below provides an example of how an attacker might execute such an attack on a site that doesn’t have CSRF token protection in place. The code below would be hosted on a attackers site, but execute a transfer on a legitimate users site.

<!-- bank.evil.com site -->
<html>
    <head>
        <title>Malicious website</title>
    </head>
    <body>
        <!-- hehe i'm gonna be rich -->
        <h1>Enter your name to find out your funny nickname:</h1>
        <form action="http://legitimate-site.com/transferfunds" method="POST">
            Your name: <input type="text"><br/>
            <input type="submit">
            <input type="hidden" name="amount" value="10000">
            <input type="hidden" name="recipient" value="evil_hacker">
        </form>
    </body>
</html>

To protect against this issue, a cryptographically secure nonce or hash must be included with each request, which must be verified prior to performing sensitive functionality.

Correct

Most web frameworks will provide middleware that is capable of generating secure CSRF tokens for you. Some of the major ones are summarized below.

Django

1. Add ‘django.middleware.csrf.CsrfViewMiddleware’ to MIDDLEWARE_CLASSES in your configuration BEFORE any other middleware that assumes that CSRF attacks have been dealt with.

2. In any template that user can submit POST data you need to add a special csrf_token tag.

<form action="." method="POST">
    {% csrf_token %}
    <!-- other form fields -->
</form>

3. Ensure that the csrf context is being passed into any views being rendered. Is you are not using a RequestContext you will need to do this manually.

Refer to the Django documentation <https://docs.djangoproject.com/en/1.8/ref/csrf/#using-csrf> for more detailed instructions.

Flask-WTF

Without any configuration, a flask_wtf.Form will be a session secure form with csrf protection.

Given the following form definition:

# form.py
from flask_wtf import Form
from wtforms import StringField
from wtforms.validators import DataRequired

class PersonForm(Form):
    name = StringField('name', validators=[DataRequired()])

You need only include the form.hidden_tag() in your template definition:

<!-- view.html -->
<form method="POST" action="{{ url_for('submit') }}">
    {{ form.hidden_tag() }}
    {{ form.name.label }} {{ form.name(size=20 }}
    <input type="submit" value="ok">
</form>

The CSRF token will be validated automatically in any place you are validating the form submission.

# view.py
from form import PersonForm

@app.route("/submit", methods=("GET", "POST"))
def submit():
    f = PersonForm()
    if form.validate_on_submit():
        # csrf token also validated
        return redirect("/")
    return render_template("view.html", form=f)

Consequences

  • Legitimate user sessions can be hijacked
  • Privileged services and functionality can be accessed
  • Protected data can be modified

References

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