Form validation fails due missing CSRF

A few days ago I have reset my local flask environment without having captured the dependencies via a pip freeze before I deleted it. Hence I had to re-install the latest version of the entire stack.

Now out of the blue I am no longer able to validate with forms. Flask claims CSRF would be missing.

def register():
    form = RegisterForm()
    if form.validate_on_submit():
       ...
    return make_response("register.html", form=form, error=form.errors)

The first time I send a Get I retrieve an empty form.errors as expected. Now I fill out the form and submit it and form.errors is showing: {'csrf_token': [u'CSRF token missing']}

This is so strange. I wonder if Flask-WTF has changed and I am using it wrongly.

I can clearly see the form.CSRF_token exists, so why is it claiming it was missing?

CSRFTokenField: <input id="csrf_token" name="csrf_token" type="hidden" value="1391278044.35##3f90ec8062a9e91707e70c2edb919f7e8236ddb5">

I never touched the working template, but I post it here nonetheless:

{% from "_formhelpers.html" import render_field %}
{% extends "base.html" %}
{% block body %}
<div class="center simpleform">
    <h2>Register</h2>
    {% if error %}<p class=error><strong>Error:</strong> {{ error }}{% endif %}
    <form class="form-signin" action="{{ url_for('register') }}" method=post>
        {{form.hidden_tag()}}
        <dl>
            {{ render_field(form.name) }}
            {{ render_field(form.email) }}
            {{ render_field(form.password) }}
            {{ render_field(form.confirm) }}
            <dd><input type=submit value=Register class='btn btn-primary'>
        </dl>
    </form>
</div>
{% endblock %}

Is this a new bug?

UPDATE:

I have reinstalled everything and the problem persists.

As Martijn suggested, I am debugging into the the following method in flask_wtf :

def validate_csrf_token(self, field):
        if not self.csrf_enabled:
            return True
        if hasattr(request, 'csrf_valid') and request.csrf_valid:
            # this is validated by CsrfProtect
            return True
        if not validate_csrf(field.data, self.SECRET_KEY, self.TIME_LIMIT):
            raise ValidationError(field.gettext('CSRF token missing'))

The last condition is raising the validation error.

field.data = "1391296243.8##1b02e325eb0cd0c15436d0384f981f06c06147ec"
self.SECRET_KEY = None (? Is this the problem)
self.TIME_LIMIT = 3600

And you were right the HMAC comparison fails....both values are in every time different.

return hmac_compare == hmac_csrf

I have both SECRET_KEY and CSRF_SESSION_KEY in my config defined.

Solution

The Flask-WTF CSRF infrastructure rejects a token if:

the token is missing. Not the case here, you can see the token in the form.
it is too old (default expiration is set to 3600 seconds, or an hour). Set the TIME_LIMIT attribute on forms to override this. Probably not the case here.
if no 'csrf_token' key is found in the current session. You can apparently see the session token, so that's out too.
If the HMAC signature doesn't match; the signature is based on the random value set in the session under the 'csrf_token' key, the server-side secret, and the expiry timestamp in the token.

Having eliminated the first three possibilities, you need to verify why the 4th step fails. You can debug the validation in flask_wtf/csrf.py file, in the validate_csrf() function.

For your setup, you need to verify that the session setup is correct (especially if you don't use the default session configuration), and that you are using the correct server-side secret. The form itself could have a SECRET_KEY attribute set but is not stable across requests, or the app WTF_CSRF_SECRET_KEY key has changed (the latter defaults to the app.secret_key value).

The CSRF support was added in version 0.9.0, do check out the specific CSRF protection documentation if you upgraded. The standard Flask-WTF Form class includes the CSRF token as a hidden field, rendering the hidden fields is enough to include it:

{{ form.hidden_tag() }}