Skip to main content

Content Security Policy of Firefox

Firefox support for Content Security Policy (CSP) has been in the news and is now available in test builds for web developers to try. Support for CSP isn’t slated for Firefox 3.6 but is likely to be included in the release after 3.6, mostly likely called 3.7.

This post is targeted at web developers and gives a quick overview of the three kinds of attacks that CSP helps to mitigate and also gives some quick examples so developers can get a sense of how it will work for them.

In case you don’t know what our Content Security Policy code is – and based on anecdotal evidence a lot of people don’t – it’s a set of easy to use tools that allow a web site owner to tell the browser where it should or should not load resources from. In particular it aims to prevent three different classes of common attacks we see on the web today: cross-site scripting, clickjacking and packet sniffing attacks.

Cross-site scripting attacks are largely the result of a mistake made on backend web servers where someone fails to escape data that’s submitted by users. When that happens it’s possible to inject a tag to load JavaScript code from another web site. That code could be harmless but it could also contain something dangerous, like malware. What CSP does is make it possible for a web site author, via HTTP headers, to specify what types of scripts can be loaded and from where. For developers who are setting a policy, it adds a layer of protection where even if they make a mistake it is likely to be mitigated by this additional layer of policy.

Clickjacking attacks are where someone embeds a page into a transparent iframe and “steals” user clicks to activate something dangerous. One particular attack allows a browser to be turned into a remote surveillance device. CSP includes the ability for a page to tell the browser that it never wants to be ever included in an iframe.

And last is the ability for a web site to tell the browser that it never wants resources from that page to be loaded over unencrypted HTTP. Banking and other commerce sites will find this particularly useful.

CSP is very powerful and flexible, allowing you to specify whether or not you want to load different kinds of media, different kinds of script methods, css, can be used to set up loading only from specific other hosts and a large number of other things. It’s meant to be very easy to set up for simple cases but will scale up to pretty complex infrastructure where different resources might be spread out over a large number of machines.

Here are four examples that show common use cases. Each of these examples is a header that’s delivered as a header over HTTP and it affects how the page is rendered.

A site wants all of its content to come from its own domain:

X-Content-Security-Policy: allow 'self'

Example 2: An auction site wants to be able to load images from anywhere, plugin content from a list of trusted media providers and a CDN network and scripts only from its server hosting sanitized JavaScript:

X-Content-Security-Policy: allow 'self'; img-src *; \
object-src media1.com media2.com *.cdn.com; \
script-src trustedscripts.example.com

Example 3: Server administrators want to deny all third-party scripts for the site, and a given project group also wants to disallow media from other sites (header provided by sysadmins and header provided by project group are both present):

X-Content-Security-Policy: allow *; script-src 'self'
X-Content-Security-Policy: allow *; script-src 'self'; media-src 'self';

Example 4: An online payments site wants to ensure that all of the content in its page is loaded over SSL to prevent attackers from eavesdropping on requests for insecure content:

X-Content-Security-Policy: allow https://*:443

The implementation isn’t quite complete yet, but it’s pretty close. There’s more information on the demo page for CSP, read the overview or read the spec itself.

Comments

vitamine d said…
In case you don’t know what our Content Security Policy code is – and based on anecdotal evidence a lot of people don’t – it’s a set of easy to use tools that allow a web site owner to tell the browser where it should or should not load resources from. In particular it aims to prevent three different classes of common attacks we see on the web today: cross-site scripting, clickjacking and packet sniffing attacks.

Popular posts from this blog

Ardilla- New tool for finding SQL Injection and XSS

Three Researchers -- MIT's Adam Kiezun , Stanford's Philip Guo , and Syracuse University's Karthick Jayaraman -- has developed a new tool ' Ardilla ' that automatically finds and exploits SQL injection and cross-site scripting vulnerabilities in Web applications. It creates inputs that pinpoint bugs in Web applications and then generates SQL injection and XSS attacks. But for now Ardilla is for PHP -based Web app only. The researchers say Ardilla found 68 never-before found vulnerabilities in five different PHP applications using the tool -- 23 SQL injection and 45 XSS flaws. More information is awaited. For their attack generation techniques refer to their document at: http://www.cs.washington.edu/homes/mernst/pubs/create-attacks-tr054.pdf

File Upload through Null Byte Injection

Sometimes, during file upload we come across situation wherein there would be check on the file extension at the client side as well as server side too. If the application does allow only .jpeg extension to be uploaded, the client side java script checks for the extension of the file before passing the request. We all know that how easily this can be defeated. Some applications, checks for the extension at the server side also. That's not easy to bypass. However there are some ways with which it still can be bypassed. Most of server side scripts are written in high level languages such as Php, Java etc who still use some C/C++ libraries to read the file name and contents. That leads to the problem. In C/C++ a line ends with /00 or which is called Null Byte. So whenever the interpreter sees a null byte at the end of the a string, it stops reading thinking it has reached at the end of the string. This can be used for the bypass. It works for many servers, specially php servers. T

Combining power of Fiddler with Burp

Both are pretty powerful tools when it comes to intercept and modify http communications. But at some point of time, they become even more powerful combo if tied with each other. They complement each other. In a recent pentest I came across a similar situation where in Burp was not able to intercept a specific kind of traffic and Fiddler came to rescue. The application was designed to upload video. The initial communication was straight forward, I mean logging into application, filling up the video details etc. And all these were easily captured by Burp except the point where you hit the Upload Video and it connects to a different server and surprisingly it was not captured by Burp, not sure why, even after repeated attempts. So, I fired Fiddler to see if the it sees this request. But it's a;ways to play with requests using Burp due to it's various functionalities like, Intruder, Repeaters etc. But it was necessary to capture this request in Burp. So the below steps can be