Security Considerations

Web applications usually face all kinds of security problems and it’s very hard to get everything right. Flask tries to solve a few of these things for you, but there are a couple more you have to take care of yourself.

Cross-Site Scripting (XSS)

Cross site scripting is the concept of injecting arbitrary HTML (and with it JavaScript) into the context of a website. To remedy this, developers have to properly escape text so that it cannot include arbitrary HTML tags. For more information on that have a look at the Wikipedia article on Cross-Site Scripting.

Flask configures Jinja2 to automatically escape all values unless explicitly told otherwise. This should rule out all XSS problems caused in templates, but there are still other places where you have to be careful:

  • generating HTML without the help of Jinja2
  • calling Markup on data submitted by users
  • sending out HTML from uploaded files, never do that, use the Content-Disposition: attachment header to prevent that problem.
  • sending out textfiles from uploaded files. Some browsers are using content-type guessing based on the first few bytes so users could trick a browser to execute HTML.

Another thing that is very important are unquoted attributes. While Jinja2 can protect you from XSS issues by escaping HTML, there is one thing it cannot protect you from: XSS by attribute injection. To counter this possible attack vector, be sure to always quote your attributes with either double or single quotes when using Jinja expressions in them:

<a href="{{ href }}">the text</a>

Why is this necessary? Because if you would not be doing that, an attacker could easily inject custom JavaScript handlers. For example an attacker could inject this piece of HTML+JavaScript:


When the user would then move with the mouse over the link, the cookie would be presented to the user in an alert window. But instead of showing the cookie to the user, a good attacker might also execute any other JavaScript code. In combination with CSS injections the attacker might even make the element fill out the entire page so that the user would just have to have the mouse anywhere on the page to trigger the attack.

Cross-Site Request Forgery (CSRF)

Another big problem is CSRF. This is a very complex topic and I won’t outline it here in detail just mention what it is and how to theoretically prevent it.

If your authentication information is stored in cookies, you have implicit state management. The state of “being logged in” is controlled by a cookie, and that cookie is sent with each request to a page. Unfortunately that includes requests triggered by 3rd party sites. If you don’t keep that in mind, some people might be able to trick your application’s users with social engineering to do stupid things without them knowing.

Say you have a specific URL that, when you sent POST requests to will delete a user’s profile (say If an attacker now creates a page that sends a post request to that page with some JavaScript they just has to trick some users to load that page and their profiles will end up being deleted.

Imagine you were to run Facebook with millions of concurrent users and someone would send out links to images of little kittens. When users would go to that page, their profiles would get deleted while they are looking at images of fluffy cats.

How can you prevent that? Basically for each request that modifies content on the server you would have to either use a one-time token and store that in the cookie and also transmit it with the form data. After receiving the data on the server again, you would then have to compare the two tokens and ensure they are equal.

Why does Flask not do that for you? The ideal place for this to happen is the form validation framework, which does not exist in Flask.

JSON Security

ECMAScript 5 Changes

Starting with ECMAScript 5 the behavior of literals changed. Now they are not constructed with the constructor of Array and others, but with the builtin constructor of Array which closes this particular attack vector.

JSON itself is a high-level serialization format, so there is barely anything that could cause security problems, right? You can’t declare recursive structures that could cause problems and the only thing that could possibly break are very large responses that can cause some kind of denial of service at the receiver’s side.

However there is a catch. Due to how browsers work the CSRF issue comes up with JSON unfortunately. Fortunately there is also a weird part of the JavaScript specification that can be used to solve that problem easily and Flask is kinda doing that for you by preventing you from doing dangerous stuff. Unfortunately that protection is only there for jsonify() so you are still at risk when using other ways to generate JSON.

So what is the issue and how to avoid it? The problem are arrays at top-level in JSON. Imagine you send the following data out in a JSON request. Say that’s exporting the names and email addresses of all your friends for a part of the user interface that is written in JavaScript. Not very uncommon:

    {"username": "admin",
     "email": "admin@localhost"}

And it is doing that of course only as long as you are logged in and only for you. And it is doing that for all GET requests to a certain URL, say the URL for that request is

So now what happens if a clever hacker is embedding this to his website and social engineers a victim to visiting his site:

<script type=text/javascript>
var captured = [];
var oldArray = Array;
function Array() {
  var obj = this, id = 0, capture = function(value) {
    obj.__defineSetter__(id++, capture);
    if (value)
<script type=text/javascript
<script type=text/javascript>
Array = oldArray;
// now we have all the data in the captured array.

If you know a bit of JavaScript internals you might know that it’s possible to patch constructors and register callbacks for setters. An attacker can use this (like above) to get all the data you exported in your JSON file. The browser will totally ignore the application/json mimetype if text/javascript is defined as content type in the script tag and evaluate that as JavaScript. Because top-level array elements are allowed (albeit useless) and we hooked in our own constructor, after that page loaded the data from the JSON response is in the captured array.

Because it is a syntax error in JavaScript to have an object literal ({...}) toplevel an attacker could not just do a request to an external URL with the script tag to load up the data. So what Flask does is to only allow objects as toplevel elements when using jsonify(). Make sure to do the same when using an ordinary JSON generate function.