After finding an information leakage in Google Search, I’ve been curious to see if there were no other pieces of information that could be gleaned from other Google services. To verify this, I visited my Google Dashboard, replaced my SID cookie and clicked on all the HTTP services that were listed.
My first attempt failed as I was systematically redirected to the account page where I was asked to enter my password. I then tried to also spoof the HSID cookie — also sent clear text — but because HSID cookie is an HTTPOnly cookie [1], it cannot be modified by a script or by the user: the cookie can only be modified by the server.

Spoofing HTTPOnly cookie

The best solution I found was to install a local proxy to intercept the HTTP traffic and then modify the cookies (I recommend Burp free edition which does a good job). It is then quite simple to replace the HSID cookie in the sent requests.
This time it worked, I was able to log into two services under with the spoof account:

  • Google Alerts: I was able to view and edit the mail alerts that were configured for the spoofed account.
  • Google Social Content: This service lists all your Gtalk contacts (that means most of the people you chatted with a couple of times).
  • Google Contacts: This is the Gmail contacts manager, it allows you to view, edit and create Gmail contacts. Quite useful if you want to get a list of persons to spam. An attacker could also attempt to replace the mail address of a contact with its own mail address.
  • Google Reader: You could see and edit RSS subscription.
  • Google Maps : You could see the maps associated to the spoof account.

There might be other vulnerable services but I think this list is already quite exhaustive and each of the listed service is likely to provide sensitive information.

Design flaws

Spoofing an unsecured cookie to hijack a session is nothing new. Nevertheless, there are two design flaws that HSID and SID cookies spoofing more critical:

  • These cookies can be used to provide an access to multiple services: when Google created these services, it did not assign a specific cookie for each of them. Therefore a single pair of cookies provides an access to all these services.
  • SID cookies are still valid even after the user logout: if a user thinks his session has been compromised, there is nothing he can do to revoke it. It seems that this was already pointed out 4 years ago .


Google is working on these issues and they should be fixed soon (users are already redirected to encrypted search [2]). Therefore, a next step would be to check if other major Web service providers have a better cookie policy.

[1] Jeff Atwood, “Protecting Your Cookies: HttpOnly”,
[2] Evelyn Kao, “Making search more secure”,