HTML 5 Has Vulnerabilities Too

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In our last post, we talked about the imminent demise of Flash and how it’s eventually going to be replaced by HTML 5. One of the reasons why Flash is getting axed is its propensity for vulnerabilities. But before you start letting your guard down, you should know that HTML 5 isn’t totally secure either. Today, we’ll talk about some of the HTML 5 vulnerabilities you need to be aware of.

PostMessage vulnerabilities

One of the major vulnerabilities found in HMTL 5 affects cross-origin communications. These types of vulnerabilities can enable hackers to carry out cross-site scripting attacks or lead to unintentional, unauthorized disclosures.

One problem lies in postMessage; an HTML 5 API that allows data to be exchanged between two pages even if they have different origins (i.e. they don’t use the same protocol, port, and hostname). Normally, cross-origin communications aren’t allowed by the web application security model’s Same Origin Policy (SOP). In theory, postMessage is supposed to provide a controlled way of circumventing the SOP. However, if not implemented properly, this API can expose web applications to critical vulnerabilities.

A web application can be exposed to cross-site scripting if the developer of the receiving page allows the page to receive data sent via the postMessage() method but fails to validate the origin of that message. If the message comes from a malicious source and the receiving page processes the message, the receiving page can be compromised.

On the other hand, unauthorized disclosures can happen if the developer of the sending page uses the postMessage() method but doesn’t specify a particular target/receiver for that message. That is, the developer might simply use the wildcard “*”, which, in effect, allows the message to be sent to any receiver regardless of origin. If the message contains sensitive information, that information could end up in a malicious receiving page.

CORS (Cross-Origin Resource Sharing) vulnerabilities

Another way to circumvent the Same-Origin Policy and enable cross-domain requests (i.e. between different origins) is through the use of CORS. CORS enables cross-domain requests (typically, XMLHttpRequest AJAX requests through JavaScript) in a controlled manner. It does this by employing the following headers, among others:

● Access-Control-Allow-Origin
● Access-Control-Allow-Credentials
● Access-Control-Allow-Methods

There are instances, however, when the Access-Control-Allow-Origin and Access-Control-Allow-Credentials headers are used insecurely.

The Access-Control-Allow-Origin header specifies which origins are allowed to make requests to and read responses from a CORS-enabled server. When that server receives a request, it checks the value of the request’s Origin header and then validates it against a list of allowed domains/origins.

The validation process can vary. Some processes look for a particular string, others use regular expressions, and so on. If the submitted Origin value passes the validation process, the server replies with an Access-Control-Allow-Origin header that contains the submitted value. This is where the problem lies. If the validation process is flawed, the server could end up unintentionally allowing access to a malicious domain.

Just like in the case of postMessage(), vulnerabilities can also occur if the server returns an asterisk (*) for the Access-Control-Allow-Origin header, which basically means the server allows all domains to read the response.

The Access-Control-Allow-Credentials header, on the other hand, defines whether cookies will be sent. Cookies will only be sent if this header’s value is set to True.

Almost all cases of CORS-based attacks require that the Access-Control-Allow-Credentials be set to True. That’s because attackers will need cookies to redirect sensitive information from legitimate sites to the sites they (the attackers) own.

Web Storage Vulnerabilities

HTML 5 supports a nifty performance-enhancing feature known as Web Storage. This feature, which also goes by the name “Local Storage” or “Offline Storage”, allows web applications to take advantage of a client-side database. The web application can, for example, use the database to (persistently) store and access often-used data in order to speed up certain processes. Unfortunately, this local storage, which is accessed via JavaScript, can be subjected to XSS or injection attacks.

For example, if a web application is vulnerable to XSS, like when entries in a text field are not properly sanitized, an attacker may inject malicious script through the text field and that script will be stored in web storage. That malicious code would then be executed each time the user loads the browser and accesses the same site.

What’s more, the contents of localStorage can always be viewed by anyone who has access the same browser. So if you’re using a shared computer, access to any sensitive data stored in localStorage will not be restricted to you.

Because of this vulnerability, it’s never safe to store sensitive information in these local storage databases. This includes login credentials (i.e. usernames and passwords), credit card information, personally identifiable information, and others. In other words, any type of information that you wouldn’t normally store or transmit in plaintext should never be stored in web storage.

WebSockets Vulnerabilities

HTML 5’s WebSocket protocol enables persistent, bidirectional (i.e. full duplex) connections between a client and a server. This capability allows developers to create applications that require real-time data exchanges between client and server. The usual applications include online games, live streaming, chat/messaging, and reporting systems.

Here’s one issue that’s been dubbed the Cross-Site WebSocket Hijacking (CSWSH) vulnerability. The WebSockets protocol doesn’t have any built-in capability for authentication and authorization during handshake. When a client requests a WebSocket connection, the server can carry out client authentication through typical HTTP authentication mechanisms, including cookies, HTTP authentication, or TLS authentication.

The problem is that WebSockets is not governed by the Same Origin Policy. So if an end user is logged in to a WebSockets-enabled application that’s running on the same web browser as a malicious site, that malicious site can potentially take advantage of the authentication credentials (e.g. a session cookie) and then send them along with a handshake request to the WebSocket URL of the application.

Once authenticated, the malicious site will have established a separate WebSocket connection with the same level of privileges as the original connection. Meaning, the malicious site could potentially have read/write capabilities.

Other areas in HTML 5 that are also afflicted with vulnerabilities include geolocation, web workers, sandboxed frames, and offline applications. Of course, most – if not all – of those vulnerabilities can be avoided through secure coding practices.

For instance, in the case of CORS and postMessage vulnerabilities, the developer simply has to be careful in crafting origin validation. But until all web developers are able to adhere to those practices, there will always be HTML 5 web sites that can be exploited.
The rise of HTML 5 as the de facto standard for multimedia and rich web applications is inevitable. Major browsers are starting to ditch programs like Flash and Silverlight in favor of it. The same reception is expected from end users and IT admins, who will no longer have to install and maintain crash-prone third party plug-ins.

As more and more developers write web applications in HTML 5, people must bear in mind that – like all other technologies – HTML 5 also has its fair share of vulnerabilities and that the bad guys are bound to exploit them whenever they can.

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