Escape user input to prevent XSS attacks

These days, almost every service we create has some form of web interface, be it for administration, monitoring or for the core functionality of the service. These interfaces are becoming ever more complex and dynamic, and increasingly interactive. There is a risk however, when increasing interactivity of these web services, that we inadvertently allow a user to supply data which can corrupt, or disrupt the normal running of that service.

Cross-Site Scripting (XSS) is a class of vulnerability whereby an attacker is able to present active web content to a web service, which is subsequently echoed back to a user and executed by the browser. This content can be as seemingly benign as an embarrassing image or text, or as malign as browser based exploits intended to steal and utilize the user’s web session, or even compromise the user’s web browser and take control of their client system.

There are three main classes of XSS issue: Persistent, Reflected and DOM-Based. Persistent XSS issues are those where user input is stored by the server, either in a database or server files, which is later presented to any user visiting the affected web page. Reflected XSS issues are those where user input in a request is immediately reflected to the user without sanitization.

DOM-Based issues are less common, and are present in web applications with rich client-side JavaScript clients which generate dynamic code or web content using user controllable data (i.e. URL parameters).

When developing web applications, we must be extremely careful to protect against all these classes of issue. To do so, we must never trust any data that originates from, or can be controlled by, the client. All data must be sanitized in a way suitable for how that data is going to be used. To do so, many languages provide built-in functionality to make sure any potentially dangerous control characters are encoded in a way to render them inactive. The following is a PHP example of this.


The following is a contrived example of how a reflected XSS exploit may occur. If an attacker were to submit a request to ‘<script>alert(1)</script>’ then any user viewing that url would have the javascript executed within the context of their browser. This can be used for malicious purposes.

# flask example
def hello():
    name = request.args.get('name')
    return "Hello %s" % name

Most modern Python web frameworks will escape any input that is rendered via templates which mitigates the majority of these types of attacks. However there are ways that this can be disabled.

<!– by default flask will html escape var –> <p>{{ var }}</p>

<!– in this instance it will not! –> <p>{{ var | safe }}</p>


The correct way to prevent XSS attacks is to validate user input and ensure that data rendered by templates is escaped. Using templates in the way they are intended is preferable:

# flask example
def hello():
    name = request.args.get('name')
    return render('hello.html', name=name)

# where hello.html is:
# <html>Hello {{ name }}</html>

Any HTML content that is generated directly within a request handler should use the appropriate escaping function:

from flask import escape
def hello():
    name = request.args.get('name')
    return "Hello %s" % escape(name)

Allowing certain special characters

The issue is made more complex when we encounter situations where we need to allow a specific set of special characters, such as the ability to post content containing HTML tags. In this situation we can either accept only known good data, or we can deny all known bad data. Both approaches have pros and cons, with the specific choice of implementation being dependent on the given application. In general however, the following should be the list of priorities:

  1. Encoding - Replace ALL control characters with known safe alternatives

  2. Positive validation (whitelist) - Only allow a specific set of values

  3. Negative validation (blacklist) - Block a specified list of dangerous values

In cases where positive validation is used, it should also be coupled with additional sanitization. For example, when allowing certain HTML tags, certain attributes of those tags should be removed, such as event handlers. e.g.:

<img src='someimage.jpg' onload='do_evil()'/>

Again, the preferable approach is to only allow known safe attributes, and sanitize the content of those attribute values. If the content is not sanitized, the following vulnerable code could occur:

function add_image(link) {
  document.write('<img src="' + link + '"'></img>'');

If the preceding JavaScript function is called with the link parameter containing the following value, the function can be exploited to execute arbitrary code:

x" onerror="do_evil()

A more secure implementation of the above would be:

function add_image(link) {
  clean = link.replace(/"/g, '&quot;');
  document.write('<img src="' + clean + '"'></img>'');

Note, this is a very specific example for illustration. A more comprehensive approach to sanitization should be taken for larger applications.


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