A Naked Security reader just drew our attention to a recently released extension in the Chrome Web Store called AccessURL that is being talked about positively on mailing lists and online lifestyle websites.
AccessURL offers to share access to your online accounts temporarily without giving away your password, so it sounds like just the ticket if you occasionally want to let colleagues tweet on your behalf, or to let a friend use your video streaming service while you’re off-grid for a few days.
Our reader wanted to know, “Is this a good idea, or in fact a really bad one?”
Our answer is simple: it’s a bad idea.
AccessURL works by copying and sharing your session cookies, and here’s why you shouldn’t do that.
Web servers deal with keeping track of who and where you are using cookies, a standard way of remembering settings and configuration data while you move around on on a website.
For example, Naked Security articles include a banner that looks like this, asking you if you would like to subscribe to our daily email newsletter:
If you don’t want to subscribe, and you don’t want to be asked again, you can just click on
The banner vanishes because we set a cookie called
nakedsecurity-hide-newsletter to have the value
(By “having the value true”, we mean quite literally that the cookie consists of the ASCII characters
e to make up the word
true as a string of text – the signal that you don’t want to see the banner.)
In technical jargon, cookies provide a way of maintaining state between one web request and another.
Without cookies, you wouldn’t be able to login to a website at all, because there would be no way of remembering your identity from one page to the next, so you’d need to put in your username and password over and over again.
So, logging in is usually handled with what’s called a session cookie or session token, a bit like our
hide-newsletter cookie, but holding a unique (and time-limited) cryptographic secret that your browser includes in future requests to the same website, so the server knows it’s you returning for more.
If someone steals your session cookie, they can maliciously add it into their own web requests; in many cases, that’s enough to masquerade as you.
That’s known as a session hijack, and it means that they end up authenticated in your name, but without ever needing to know your username and password.
In fact, just over six years ago, a free hacking tool called Firesheep appeared that automated the process of session hijacking.
In those days, many sites encrypted your password, but didn’t encrypt the session after that, thus transmitting your session cookies in plain text where they could easily be eavesdropped by network sniffing tools such as Firesheep.
Firesheep got its name from the fact that it ran as a Firefox extension, and because it added you to what hacker conventions call the wall of sheep – a list of users whose authentication data has been spotted on the network unencrypted and then displayed for everyone to jeer at.
One good side effect of Firesheep is that it proved to us all that web encryption (where you use URLs beginning with
https instead of just
http) is important all the time, not just for the part of the session where you actually supply your password.
Indeed, in the next few years after Firesheep, all major web properties switched over to using HTTPS all the time, thus making session hijacking much harder.
These days, the easiest way to steal someone’s session cookies when they are using HTTPS is to grab their traffic before it’s encrypted or after it’s decrypted, for example by sneaking malware into the browser or onto the server.
A malicious browser plug-in, for instance, can inspect web content before it gets HTTPSified, extracting private data such the selfsame session tokens that Firesheep stole in the days before your whole session was encrypted.
Stealing session tokens instead of passwords can actually make things easier for the crooks: a session token denotes an already logged-in connection, which neatly sidesteps any two-factor authentication (2FA) that might be turned on.
Firesheep by consent
You’ve probably guessed where this is going.
AccessURL works by installing a Chrome extension that basically performs what you might call “Firesheep by consent”.
It extracts session cookies for web logins you select and shares them via the cloud with chosen friends.
The idea is that you can quickly and easily expire any session cookie you’ve already shared simply by logging out on your computer, causing the server to consider the old cookie invalid.
If you’ve shared your password with other people, the only way to “unshare” it and lock them out is to change it, assuming one of those other people hasn’t changed it already, thus locking you out instead.
So, it’s easy to say to yourself, “This cookie sharing system is better than sharing my actual password, so I’ll take the risk, even though I’m giving someone else open slather on my account, at least for a while.”
We strongly urge you not to do this.
Even if the AccessURL servers handle the cryptographic details so that the service can’t decrypt the session tokens itself (which would amplify the risk even further), it’s cloning an already-authenticated session, bypassing any 2FA and putting you at much the same short-term risk as if you handed over your account entirely.
There are typically a few things you can’t do with a session cookie alone: most online services, for example, make you enter your password again if you try to change your profile information, including updating your recovery email address or resetting your password.
But if a cloned session cookie works at all, it will probably work for most parts of your account…
…so if your friends turn out to be infected with cookie-stealing malware themselves, or occasionally leave their computers unlocked for a moment, they’re putting your online identity at direct risk, too, even though they don’t know your password.
In short: sharing session cookies sounds better than sharing passwords, but you need to treat them as two side of the same coin.
We recommend: don’t share passwords, and don’t share session cookies either.
10 comments on “The Chrome extension that “Firesheeps” you by choice”
Another solid article Duck; thanks again. Oh, but for hindsight: stateless HTTP, open SMTP relaying, and the IPv4 bucket dripping dry. oh well….
Sending ‘HTTPSified’ to Anu Garg for hopeful addition to AWAD lexicon
Couple typos (feel free to trim this from my swollen comment):
– Stealing sessions tokens instead of passwords
– you can quickly and easil invalidate the cookie
Fixed the typos (I think :-). Thanks.
I recently came across a website used by a lot of schools which showed how not to do online security, the session id wasn’t even in a cookie but in the URL, bad but it gets worse, when a parent signs up to view their child’s progress, the user name and password are initially put straight into the URL in clear text and both were sent in plain text in an email. The login was over HTTPS but nothing else was. Oh and lets not forget the old favourite passwords are stored in plain text and displayed to you when you access your user profile.
I have to wonder what possible “positive” comments could be said about this.
Unless you are someone spinning this app for a fee.
I found the website a bit… well, opaque.
There’s no indication of who is behind it, or why, and I couldn’t find any technical information to explain why you would consider your session cookies to be safe when handled by the service.
The only technical detail I could see says that the service “uses AES encryption”, though why that helps on its own is not explained.
Thanks, Paul! As always, clear and useful 🙂
I read your Naked Security posts daily! 🙂 This one was epic. In other news anybody want some of my cookies?
I agree with the article, but there’s another risk.
Many sites use encrypted tokens as their session keys. They provide the state, such as principal name, id, roles, and expiry time in an encrypted and authenticated Base64’d blob in a session cookie.
These are closer in spirit to security assertions, and they can’t be revoked by logging out of the system. If they’re shared between people, they can remain valid for an unexpected length of time.
Similarly, how many users of the AccessURL system know that logging out may invalidate the shares given to fellow users? Would they see this as a bug or a feature?
IIRC, the AccessUTL website explicitly claims the “you can logout and therefore force the others to logout implicitly” cookie invalidation situation as a feature.
(Of course, it cuts backwards, too. If your buddy clicks logs out, you have to log back in yourself 🙂
Owch, I think most websites created with the Flask framework would inadvertantly break this feature of AccessURL. I think there may be other frameworks that use the same design.
In this case, sessions aren’t remembered on the server, so there’s no “log-out” feature, your browser just forgets the cookie. If you log out, your buddy would still be able to use the cookie on those sites.