Friday, February 1, 2008

Sidejacking, Forced Sidejacking, and Gmail

The following is not original, just an analysis and summary of other work and the current security state of Gmail's login cookie and sidejacking. Its been well known (Errata Security) for months that Google's cookie management has some serious problems, even when you use SSL. Mike Perry's Bugtraq posting from over half a year ago explains it well.

Executive summary: Google's failure to properly use the secure attribute on Gmail's cookies means that Gmail over SSL offers effectively no security against an active attacker when compared with Gmail without SSL. This is a well known problem.

First, some general purpose background on cookies and sidejacking:

What is a cookie? A cookie is a small value stored in the user's web browser that is sent to the web server. Whenever a webpage is fetched, the browser sends all cookies associated with that web site. Likewise, the web site can set cookies in the reply.

Cookies are often used as "login cookies" or "session cookies", a way for the server to know who a user is and that he's allowed to log in. Often, these cookies are marked as secure, which means they will only be sent over an encrypted (https) session. But if they aren't marked as secure, they will be sent with every page request made to the server, regardless of encryption.

What is sidejacking? Sidejacking is simply stealing the session cookies when they are sent over an insecure (normal HTTP) session and reusing them. If you can grab someone's login cookie, you can impersonate them on whatever website the cookie is for, such as read their email on Gmail or Hotmail, or whatever other impersonation is desired.

What is forced sidejacking? (Also known as a cross site request forgery attack, but that's not as catchy a name). If a session is unencrypted, someone able to observe packets (such as the guy sitting next to you in Starbucks) can obviously see, intercept, and reuse any cookies seen in the classic Sidejack attack. But often an attacker wants to get the cookies associated with a connection which is not currently active, or when the session is encrypted.

In this case, if the attacker can inject packets, instead of being an eavesdropper he can become a man in the middle. And since listening in on packets to capture people's email accounts is already illegal, there is no reason for the attacker who's sitting next to you in starbucks to limit himself to just passive snooping.

By performing packet injection or any one of half a dozen other attacks, the attacker can fool your web browser into going where the attacker wants and revealing all the cookies for the site he wants, by modifying your web browsing.

As an example, the attacker could replace the victim's fetch of any normal web page with an "HTTP 302 Temporarily moved" pointing to the web site he wants to steal the cookies from. Thus the victim's browser then thinks "Oh, I should visit this instead", so it creates a request which includes all cookies which aren't secure. The attacker then replaces the reply with another "HTTP Temporarily moved", back to the page the victim was trying to visit. [1]

As a result, the attacker has learned what the cookies are for the victim's login to the targeted site, but the victim doesn't observe anything (it just looks like the page took a little longer to load, and the address bar might flicker).

Any site which does not allow HTTPS is trivially vulnerable to sidejacking and forced sidejacking. Thus, Hotmail can always be forced sidejacked. However, sites which use SSL to access can set the secure attribute on the login cookie, which should prevent this attack.

Unfortunately, Gmail, although it supports SSL, does NOT set the secure attribute on the login cookie (which is named "GX"). Thus if you manually type, you are safe from a passive eavesdropper (passive sidejacking), but just as vulnerable to an active attacker (forced sidejacking) as you are with the "SSL only for the login" Hotmail or Gmail over plain http! This is because although your session is encrypted, an attacker could still redirect any of your other surfing to "", which reveals your GX cookie to the attacker. And since its just as illegal to sniff a person's cookies to access their email account as it is to modify their browsing, and almost as easy to inject traffic instead of just sniffing traffic, Gmail's SSL offers effectively no security over the non-SSL option [2].

This situation will remain until Google changes the GX cookie to secure. But the situation, as you will see, is more complex then a one-line change.

Naturally, I thought there would be an easy fix: use greasemonkey to change the GX cookie to secure whenever I visit Gmail. (Greasemonkey allows you to inject arbitrary Javascript into a set of pages you view, which can both read and change cookie attributes). Now if the attacker tried to actively sidejack me, my browser just wouldn't send the cookie because its not an HTTPS session.

But that doesn't work, as Google uses a two stage login process for Gmail, Blogger, etc which prevents the quick fix from working. There is a local session cookie (GX) and a global user login cookie group (SID (insecure) and LSID (secure)) which are used by Google's single signon system. The LSID cookie is restricted to, and has the secure attribute set, to the attacker can't grab it directly.

Yet an active attacker can use the global login cookie directly, if he's performing forced sidejacking.

The Gmail login procedure (verified as of 2/1/2008) works as follows:

  • If the GX cookie is set, log in the user.
  • If the GX cookie is NOT set, perform an HTTP redirect to
  • checks if the LSID and SID cookies are valid. If so, it redirects to{auth_info}.. which sets the GX cookie and redirects the user back to (if no SSL was used) or (if was entered manually).
  • Otherwise gives the login prompt

This is why, if you are logged into Gmail, you can log into blogger by just clicking the "log me in" button: blogger redirects you to, which checks the same SID and LSID cookies and then redirects you back to Blogger.

But the result is Greasemonkey can't fix the Gmail cookie, because all the attacker needs to do is redirect the victim's surfing to If the GX cookie is set, the attacker gets it, and redirects the victim back to his original web site.

Otherwise, the attacker lets redirect the victim to which redirects the victim to which sets the GX cookie and gives an HTTP redirect back to which will happily spit out the GX cookie, followed by the attacker redirecting the request back to where the victim was going in the first place.

Since no page ever loaded (it was all "HTTP 302 moved temporarily"), the browser reveals the GX cookie before any page loaded, and therefore before Greasemonkey-injected Javascript can run.

The result is, IF the victim is logged into ANY single-sign-on Google service (even using SSL) and visits ANY web page while still logged in, an active attacker can capture login cookies for any Google service which allows (not requires, just allows) non-SSL use, even if they aren't currently logged into the particular Google service.

This also suggests that attempting to fix other sites with Greasemonkey wouldn't work either if they have any form of "URL for SSL that transitions to non SSL and updates the cookie" that the attacker can inject to extract a session cookie from. And it points out the problem with any site which uses "SSL-only" to check an existing login cookie to generate a session cookie: to an active attacker, although the attacker can't capture the SSL-only login cookie, he can use the login cookie to generate a session cookie.

In the specific case of Google, Google needs to infer and set "paranoid" cookies. [3] If the user logs into a Google service by manually selecting "https", it checks if the local login cookie also has a corresponding local paranoid cookie. If so, it should just proceed normally after ensuring that the login and paranoid cookies have the secure attribute set.

But if the paranoid cookie is not set, but the user explicitly selected https, it should redirect to regardless of the cookie status. In this case, Google should set a GLOBAL paranoid cookie. This paranoid cookie should cause to always redirect to https if the service supports it, and any local service login cookie to be set with the SECURE attribute and a corresponding local service paranoid cookie.

Now only services which don't support HTTPS for all operation (like blogger) could be actively or passively sidejacked, and once a user logs in with https://, all newly authenticated sessions will default to https until the user manually logs out by pressing the logout button.

Finally, this problem is not unique to Google, as there are rumors of other significant sites which fail to properly set secure on their session or login cookies, even when all traffic should be restricted to encrypted links.

[1] There are other ways to inject content as well to force the victim's browser to visit a targeted site, such as iframe injection through ARP cache poisoning.

[2] The only exception is if ONLY gmail is accessed from the web browser (no other browsing or even RSS fetches) and the user clicks the "logout" button when done. Since only the truely paranoid would do such a thing, I believe it is safe to say that Gmail with https really is not more secure than Hotmail in the face of an active attacker.

[3] Google doesn't use HTTPS-only for gmail for several reasons, but the biggest one is that HTTPS has considerably more latency when starting a connection, something which occurs often in web browsing. Yet this is obviously not a concern for anyone who manually selects an encrypted link.

Many thanks to Christian Kreibech, Vern Paxson, and Mike Perry for comments and feedback.

Edit/Extension: Here's how to find out if a site you use is vulnerable.

  • Clear all cookies (in firefox, Preferences->privacy)
  • Go to the site and log in.
  • Close the browser window.
  • Reopen the site, if you become logged in, then there is a saved login cookie.
  • Now close the window. In Firefox, open up "Preferences/Privacy", manually delete EVERY cookie marked as "SECURE"
  • Reopen the site. If you become logged in again, the site is vulnerable!

    NOTE PLEASE: As of Sept 2008, gmail does appear to be inferring that the user wants SECURE access when the user manually inputs, in addition to providing an "always use SSL" preference. Thus the attack on the GX cookie is NOW OBSOLETE, the problem with gmail has been fixed. Many other sites may remain vulnerable, however.


Brian Tierney said...

Great summary! Thanks.

I use the gmail imap service, so this should not affect me, but
I sure would like to know what other sites have this issue.

Nicholas Weaver said...

Thanks for the note Brian. I've added "how to tell if a site is vulnerable"

Nicholas Weaver said...

Also, you may still be vulnerable if you use Blogger or another Google service!

SA said...

Great summary Nicholas. Really a nice read.

Steve Hanna said...

Nice writeup Nick!