Front-end Security

There are attacks on security that concern the front-end programmer as much as the server-side programmer. These are attacks where the attack vector is the user's web browser. They involve the execution of malicious Javascript or sending malicious requests. Careful coding practices on behalf of both front-end and back-end developers are needed to mitigate these attacks.

Cross Site Scripting (XSS)

Given the restrictions on what a script is allowed to access, to mount an attack on a site via Javascript the attacker needs to arrange for their code to be loaded into the target page and executed. If that is possible, then they will have access to all of the context of that page and further attacks can be carried out.

One way to insert content into a page you have not written is to exploit a weakness in a site that allows users to post content that will be displayed to other users. Any social media site would be a good candidate for this kind of attack as they allow users to post messages that are then shown to other users. Imagine a social media site that allows me to post a message. This message is stored in the database and when another user requests a list of the latest messages, my message will be shown on their page. If I include some HTML markup in my message and the site is not checking for this, the markup might be inserted directly into the page delivered to other users.

For example, say I write the following message:

    Happy to visit my family today! Had 
    a <span onload="alert('hello')">great</span> day.

The simple social media site just inserts my message into the page delivered to the next user:

<div class="message">
    <div class="author">Steve</div>
    <p>Nailed it! LOL</p>
<div class="message">
    <div class="author">Jane</div>
        Happy to visit my family today! Had 
        a <span onload="alert('hello')">great</span> day.
<div class="message">
    <div class="author">Jai</div>
    <p>Thanks for everything!</p>

Now, when another user loads this page, the Javascript attached to the span element will be executed (in response to the onload event) and the user will see an alert box. Of course, I could have included any kind of Javascript in this attribute; the point is that I have arranged for my Javascript code to be run in the context of another user's web browser.

This is an example of a Cross Site Scripting (XSS) attack, one of the most common kinds of security vulnerability on the web.

Reflected XSS

Another possible route for an XSS attack is with a site that makes use of some part of the requesting URL in generating the page that is returned for the response. For example, I might have a site that shows various product categories with URLs like, or In each case I have content in the page like <h3>Products in category <i>womens</i></h3> where the product category comes from the URL. I could attack this site by constructing a URL like<span onmouseover="alert('XSS')">mens</span>. This might force the site to include this markup in the resulting page and my Javascript would be executed. Now I just need to arrange for my victim to click on this link and I have an attack; this can be done by sending this link in an email or using it in a forum post. A naive user sees the domain name of a site they now and doesn't understand all of the other stuff in the URL and so might be fooled by this.

Data taken from the URL is just as dangerous as data taken from form submissions when it comes to trusting input to your site. We often use very general patterns for our URL routes and often don't check that the values we get from them are reasonable. We assume that we are the only ones constructing URLs pointing to our site, but attackers can and will exploit this if they can.

Protecting against XSS

XSS relies on a web application reproducing the text of my message in the page that is sent to other users. The main defence against XSS then is to not do this and assume that any text that is entered by users is a potential attack vector.

The strongest defence against XSS is to replace any special HTML characters in the message with entity references so that they will not be interpreted as HTML by the browser. In this case my sample message would be converted to:

  Happy to visit my family today! Had 
  a &lt;span onload=&quot;alert(&#x27;hello&#x27;)&quot;&lt;/span&gt;great&lt;/span&gt; day.

When this is inserted into the page, the user will see HTML text in my message because the browser will display &lt; as < etc. This is known as escaping the HTML content for inclusion in the page.

This method is so commonly useful that most HTML templating systems will by default carry out this conversion on any data that is inserted into the HTML page. For example in a Handlebars template"

<script id="book-template" type="text/x-handlebars-template">
    <div class="message">
        <div class="author">{{author}}</div>

This will have the effect of escaping the content of the author and text fields before inserting them into the page and will therefore prevent XSS attacks. For example:

let template = get_template("book-template");
let book = {
    'author': "Mary & Bob",
    'text': "Text with <em>markup</em>"
content = template(book)

The result will be:

    <div class="message">
        <div class="author">Mary &amp; Bob</div>
        <p>Text with &lt;em&gt;markup&lt;/em&gt;</p>

I actually need to take a further step to prevent Handlebars from escaping the content by adding an extra set of curly braces: {{{text}}}.

However, it may be that I want to allow my users to add HTML markup to their messages. After all it's nice to be able to format them nicely, add emphasis, bullet points etc. In that case, I might want to accept messages in HTMl and insert them directly into the page. To avoid the possibility of XSS attacks, I need to make sure that the HTML that I am inserting does not contain a payload of Javascript code. There are a few approaches to doing this.

Markdown I could disallow HTML markup but allow something like Markdown which is a kind of markup that can be converted to HTML. This allows users to format their content but prevents them writing any kind of HTML that might have an exploit.

Whitelisting I can define a subset of HTML tags that I will allow in messages and then scan the message to remove anything other than these. So I might allow <p>, <strong>, <ul> and <li> tags and remove anything else including any attributes on any of these tags. If I do this then it will be reasonably safe to insert the HTML content into the page.

Filtering I can try to remove any scripts from the HTML that is entered in the message. There are only so many places that scripts can go so I can just look in those places and remove anything that looks suspicious. The danger here is that the attacker is cleverer than you - or at least has more time to think up devilish ways to defeat your filters. For example, what if you thought images were ok:

<IMG SRC="javascript:alert('XSS');">

ah, maybe we look for javascript:

<IMG SRC=JaVaScRiPt:alert('XSS')>

ok, I can make it case sensitive:

<IMG SRC="jav ascript:alert('XSS');">

(this has an embedded tab but still works). Ok I can get rid of whitespace before I scan.

<IMG SRC=&#x6A&#x61&#x76&#x61&#x73&#x63&#x72&#x69&#x70&#x74&#x3A&#x61&#x6C&#x65&#x72&#x74&#x28&#x27&#x58&#x53&#x53&#x27&#x29>

(the code is written as escaped characters). Give in yet? (For more ways that can be used to defeat filters see the OWASP XSS Filter Evasion Cheat Sheet).

So while it is possible to filter HTML to sanitise user input it is very difficult to pick up all of the many ways that Javascript code can be included in a page. All it takes is one way in and the attacker has full control of the page on another user's system.

Attacks with XSS

Once I am able to mount an XSS attack and load some Javascript into the page, what can I do that is truly evil?

Capture cookies Javascript has access to cookies for the page it is running in, if that is something like Facebook or Ebay there is potential value in being able to impersonate a user and so having their cookies would allow me to do that. I can use Javascript to construct a URL like ''+document.cookie and then set document.location to this URL, the browser will immediately request this URL and I would be able to capture their cookies for the site I had infected.

Password Capture since Javascript can modify the entire page, I could replace the current page with HTML designed to look like the login page for the site but with the login form directed to The user thinks they have just been logged out, enters their login details (the URL still says they are on the social media site) and they are sent to my site. My site redirects back to the original page and the user doesn't know they've lost their details. I can then login as them at leisure.

Make purchases if this is a commerce site and I've infected the reviews section of the site, then the user might be authenticated on the site and if I'm lucky have a credit card registered for purchases. My Javascript code could make a request to purchase something from the site by submitting the right combination of forms. It might be possible to have them delivered to my address or I might just want to annoy the user by racking up large purchases on their credit card.

Requests to other sites while there are restrictions on what requests Javascript can make, I might be able to arrange to send malicious requests to other sites by adding elements to the page. I could for example add a link to the page with a URL like and hope that the user clicks on this link. If they are logged in to their bank at the time then this request would be sent with their current session cookie and if I'm lucky the money will be transferred to me. (This is a simplistic example but many sites support some kind of transactional requests via URLs like this and could be exploited in this way). This would be an example of cross-site request forgery (see below).

XSS Auditing

Modern browsers will make some attempt to stop you being attacked via an XSS vector. In particular there is a component called the XSS Auditor that checks for some obvious reflection attacks. If you make a request with special characters in a URL like this one described earlier:<span onmouseover='alert('XSS')>mens</span>

and the resulting page contains the same tags as in the URL, the XSS Auditor will block the display of the page in case this is an attack vector. This can block a number of the more obvious attacks.

Cross Site Request Forgery (CSRF)

In a CSRF attack, the goal is to get a user carry out a transaction on a site they are logged in to without their knowledge.

When you are logged into a web application, you typically have a cookie that is validated for that site and send that cookie back to the site to maintain the authenticated status. Any form that you submit, for example, to post a social media message, will have that cookie sent along to validate the request and ensure that the user is authenticated.

An attacker may try to get you to visit a page at that has an HTML form embedded in it who's action is to send a POST request to The content of the form might not be visible, but the submit button is disguised as part of a game so gets clicked by the user, triggering a POST request.

Since the user is logged in to, the request will carry a valid session cookie and so be authenticated by that site. It will create a post on behalf of that user.

This same technique could be used to purchase goods on an online shop or even transfer funds into a target bank account.

Protection - CSRF Token

The defence against this attack is to use what is called a CSRF Token. This is generated by the server and inserted into any valid form in the main application as a hidden value. The value of the token could just be a random string of characters, but it could also be a cryptographic hash of some user-specific value. Now, when the form is submitted, the CSRF token is also sent along and can be validated on the server - by looking up in a table or re-computing the hash value. Only if this token can be validated will the transaction be carried out.

Read more about CSRF Attacks on OWASP.