Inside the Firesheep code: how it steals your identity

You may have heard about Firesheep, a new Firefox browser add-on that lets anyone easily snoop over Wi-Fi and hijack your identity for services such as Facebook and Twitter. This is rather scary; if you're using Wi-Fi in a coffee shop and access one of these sites, the guy in the corner with a laptop could just go click-click and be logged in as you. He could then start updating your Facebook status and feed for instance. Even if you log in securely over SSL, you're not protected.

The quick explanation

Bad guy at computer
The Firesheep site gives an overview of its operation: after you log into a website, the website gives your browser a cookie. By snooping on the Wi-Fi network, Firesheep can grab this cookie, and with the cookie the Firesheep user can hijack your session just as if they are logged in as you.

You may be wondering what these mysterious cookies are. Basically, a cookie is a short block of characters. The cookie consists of a name (e.g. "datr") and a value (e.g. "QKvHTCbufakBOZi5FOI8RTXQ"). For a login cookie, the website makes up a unique value each time someone logs in and sends it to the browser. Every time you load a new page, your browser sends the value back to the website and the website knows that you're the person who logged on. This assumes a couple things: first, that a bad guy can't guess the cookie (which would be pretty hard for a long string of random characters), and second, that nobody has stolen your cookie.

Web pages usually use https for login pages, which means SSL (Secure Socket Layer) is used to encrypt the data. When using SSL, anyone snooping will get gibberish and can't get your userid and password. However, because https is slower than regular http (because all that encryption takes time), websites often only use the secure https for login, and use insecure http after that. Banking sites and other high-security sites typically use https for everything, but most websites do not.

The consequence is that if you're using unencrypted Wi-Fi, and the website uses insecure http, it's very easy for anyone else on the Wi-Fi network to see all that data going to and from your computer, including the cookies. Once they have your cookie for a website, they can impersonate you on that website.

This insecurity has been known for a long time, and it's easy for moderately knowledgeable people to use a program such as tcpdump or wireshark to see your network traffic. What Firesheep does is makes this snooping so easy anyone can do it. (I would recommend you don't do it, though.)

The detailed explanation

A few things about Firesheep still puzzled me. In particular, how do other people's network packets get into your browser for Firesheep to steal?

To get more information on how Firesheep works, I took a look at the source code. Since it's open source, anyone can look at the code at

The packet sniffing code is in the firesheep/backend/src directory. This code implements a little program called "firesheep-backend" that uses the pcap library to sniff network traffic and output packets as JSON.

pcap is a commonly-used packet capture library that will capture data packets from your network interface. Normally, a network interface ignores network packets that aren't intended to be received by your computer, but network interfaces can be put into "promiscuous mode" (note: I didn't invent this name) and they will accept any incoming network data. Normally packet capture is used for testing and debugging, but it can also be used for evil snooping. (As an aside, the unique MAC address - the number such as 00:1D:72:BF:C9:55 on the back of a network card - is used by the network interface to determine if the packet is meant for it or not.)

Going back to the code, the http_sniffer.cpp gets a data packet from the pcap library, looks for TCP packets (normal internet data packets), and then http_packet.cpp uses http-parser to parse the packet if it's an HTTP packet. This breaks a HTTP packet into its relevant pieces including the cookies. Finally, the relevant pieces of the packet are output in JSON format (a JavaScript-based data format that can be easily used by the JavaScript plugin in the browser).

That explains how the packets get captured and converted into a format usable by the Firefox add-on. Next I will show how Firesheep knows how to deal with the cookies for a particular website.

The xpi/handlers directory has a short piece of JavaScript code for each website it knows how to snoop. For instance, for Flickr:

// Authors:
//   Ian Gallagher 
  name: 'Flickr',
  url: '',
  domains: [ '' ],
  sessionCookieNames: [ 'cookie_session' ],

  identifyUser: function () {
    var resp = this.httpGet(this.siteUrl);
    var path =;
    this.userName = path.split('/')[2];
    this.userAvatar = resp.body.querySelector('.Buddy img').src;
This code gives the name of the website (Flickr), the URL to access, the domain of the website, and the name of the session cookie. The session cookie is the target of the attack, so this is a key line. Next is a four line function that is used to fetch the user's name and avatar (i.e. picture) from the website once the cookie is obtained.

Firesheep currently has handlers for about 25 different websites. By writing a short handler similar to the above, new websites can easily be hacked (if their cookie is accessible).

The visible part of the extension that appears in the browser is in firesheep/xpi/chrome. The most interesting parts are in the content subdirectory. ff-sidebar.js implements the actual sidebar and displays accounts as they are sniffed.

The "meat" of the JavaScript plugin is in firesheep/xpi/modules. Firesheep.js implements the high-level operations such as startCapture() and stopCapture(). FiresheepSession.js is the glue between the plugin and the firesheep-backend binary that does the actual packet collection. Finally FiresheepWorker.js does the work of reading the packet summary from firesheep-backend (via JSON) and processing it by checking the appropriate website-specific handler and seeing if the desired cookie is present.

Finally, how do the pieces all get put together into the add-on that you can download? Firefox extensions are explained on the developer website. The install.rdf file (in firesheep/xpi) gives the Firefox browser the main information about the extension.

Well, that summarizes how the Firesheep plugin works based on my analysis of the code. Hopefully this will help you realize the risk of using unsecured Wi-Fi networks!


tribe said...

Does Firesheep only work on open wifi networks?

Or does it also work on WEP and WPA2-PSK password protected wifi hotspots?

An example: My local coffee shop has a password protected hotspot, but they give the password to anyone who asks. So if you were logged into the hotspot would someone else in the coffeshop who is also logged in and running firesheep be able to sniff your cookies?

Ken Shirriff said...

Hi tribe,
Firesheep only works on open networks, not encrypted networks. Note that Wi-Fi networks where you log in through your browser (e.g. airports) don't provide encryption.

I've read that WEP encryption can be cracked fairly easily link), so I'm sure it's only a matter of time until someone packages up WEP cracking with Firesheep into an easy-to-use package.

Anonymous said...

Hi, Ken, I'm curious if the attacker doesn't do the ARP poisoning first, even his NIC is in promiscuous mode, his NIC still can't "see" packets between AP(or wired router) and the victim. This applies to Wireshark, too.


Anonymous said...

Couldn't you do this on non-switched ethernet too?

More than a few places use ethernet 'hubs' as opposed to switches, I've always wondered if the were a security risk.

Unknown said...

@Anonymous Non-switched networks, as you suspected, are a big security risk. And what's worse: Non-encrypted wifi networks (without client isolation) are the same.

Anonymous said...

absolutely agreed with roland.