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Hacking WebSocket

With Cross-Site WebSocket Hijacking attacks

Nov 28 · 5 min read
Photo by Thomas Kolnowski on Unsplash

The Same-Origin Policy (SOP) is one of the fundamental defences deployed in modern web applications. It restricts how a script from one origin can interact with the resources of a different origin. Last time, we talked about the Same-Origin Policy and how it protects a site’s data from unauthorized access:

However, there are some edge cases that the SOP does not cover, and these can often be exploited by attackers to steal private information. Today, we are going to talk about one of these edge cases: WebSocket connections, and how attackers can hijack WebSocket connections to leak private data. Let’s dive in!

What is WebSocket?

WebSocket is, like HTTP, a communications protocol that enables interaction between a browser and a web server.

The WebSocket protocol allows both servers and browsers to send messages to each other using a single TCP connection. This is very useful when trying to create real-time applications such as online games and live chat. For example, Slack’s web app uses WebSocket connections to sync messages in its chat functionality.

In order for a web application to sync in real-time, web servers need to be able to actively push data to its clients. And this is where WebSocket comes in.

Traditionally, HTTP only supports client-initiated communications. This means that every time the real-time application needs to be synced (for example, an online game updating its live leaderboard), the client’s browser would need to send an HTTP request to retrieve the data from the server. When an application is constantly doing this type of update, this traditional method incurs a lot of unnecessary overhead and ultimately slows down the application.

Whereas the WebSocket protocol solves this problem by creating a persistent connection between the client and the server that allows both client and server-initiated data transfers. During the lifetime of a WebSocket connection, the client and the server are free to exchange any amount of data without incurring the overhead and latency of using traditional HTTP requests.

How WebSocket connections are created

A WebSocket connection between a client and a server is established through a WebSocket handshake.

This process is initiated by the client sending a normal HTTP or HTTPS request to the server with the special header: “Upgrade: websocket”. If the server supports WebSocket connections, it will respond with a 101 status code (Switching Protocols). From that point on, the handshake is complete and both parties are free to send data to the other.

Side note: WebSocket uses the ws:// URL scheme, and the wss:// URL scheme for secure connections.

The problem with WebSocket

As I mentioned in the Same-Origin Policy article, the SOP is a way of preventing unwanted data access from malicious domains. However, the Same-Origin Policy does not apply to WebSocket connections and modern browsers would not prevent data reads on a WebSocket connection across origins.

This means that if an attacker can create a WebSocket connection using a victim’s credentials, that connection would have the same access as a legitimate connection, regardless of where the connection is coming from.

Cross-Site WebSocket Hijacking (CSWSH)

A Cross-Site WebSocket Hijacking attack is essentially a CSRF on a WebSocket handshake.

When a user is logged into victim.com in her browser and opens attacker.com in the same browser, attacker.com can try to establish a WebSocket connection to the server of victim.com. Since the user’s browser would automatically send over her credentials with any HTTP/ HTTPS request to victim.com, the WebSocket handshake request initiated by attacker.com would contain the user’s legitimate credentials. This means the resulting the WebSocket connection (created by attacker.com) would have the same level of access as if it originated from vicitm.com.

After the WebSocket connection is established, attacker.com can communicate directly to victim.com as a legitimate user.

Structure of an attack

To carry out the attack, an attacker would create a script that will initiate the WebSocket connection to the victim server. She can then embed that script on a malicious page and trick a user into accessing the page.

When the victim accesses the malicious page, her browser will automatically include her cookies into the WebSocket handshake request (since it’s a regular HTTP request). The malicious script crafted by the attacker will now have access to a WebSocket connection created using the victim’s credentials.

The impact of Cross-Site WebSocket Hijacking

Using a hijacked WebSocket connection, the attacker can now achieve a lot of things:

  1. WebSocket CSRF: If the WebSocket communication is used to carry out sensitive, state-changing actions, attackers can use this connection to forge actions on behalf of the user. For example, attackers can post fake messages onto a user’s chat groups.
  2. Private data retrieval: If the WebSocket communication can be used to retrieve sensitive information via a client request, attackers can initiate fake requests to retrieve sensitive data belonging to the user.
  3. Private data leaks via server messages: Attackers can also simply listen in on server messages and passively collect information leaked from these messages. For example, an attacker can use the connection to eavesdrop on a user’s incoming notifications.

How to prevent Cross-Site WebSocket Hijacking

In order to prevent Cross-Site WebSocket Hijacking, an application would need to deny WebSocket handshake requests from unknown origins. There are two ways this can be achieved:

  1. Check the Origin header: browsers would automatically include an Origin header. This can be used to validate where the handshake request is coming from. When validating the Origin of the request, be sure to use a whitelist of URLs instead of a blacklist, and use a strict and rigorously tested regex expression.
  2. Use CSRF tokens for the WebSocket handshake request: applications could also use a randomized token on a WebSocket handshake request and validate it server-side before establishing a WebSocket connection. This way, if an attacker cannot leak or predict the random token, she will not be able to establish the connection.

WebSocket is a big part of many modern applications. However, it is often overlooked as a potential attack vector.

It is important for developers and pentesters to be aware of this common pitfall and look out for these vulnerabilities before they are exploited in the wild.

As always, thanks for reading!

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Vickie Li

Written by

Basically a nerd. Studies web security. Stalks great hackers. Creates god awful infographics. https://twitter.com/vickieli7

Sursa: https://medium.com/swlh/hacking-websocket-25d3cba6a4b9

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