A while ago I discovered a bug in Mozilla that lets you steal cookies for any domain by convincing the browser to load a specially formatted URL; I have been too busy to get around to making the details known earlier, so here they are. This is similar to holes that have been found, both by myself and by others, previous in IE. Details available at http://alive.znep.com/~marcs/security/mozillacookie/ and are also included below. Update to Netscape 6.2.1 or Mozilla 0.9.7 for a fix. Using open source products doesn't magically make you invulnerable to security problems like those that plague Microsoft. Mozilla Cookie Exploit Marc Slemko <marcsat_private> Last Modified: $Date: 2002/01/22 05:06:04 $ $Revision: 1.6 $ Table of Contents [1]Executive Summary [2]What's New [3]Background [4]Details [5]Example Exploit Executive Summary Cookies are often used to identify and authenticate users to a website. If an attacker can steal a user's cookies, then they can impersonate that user. The completeness of the impersonation and the actions the attacker can perform as that user depend on how the particular site uses the cookies. This bug in Mozilla allows an attacker to, if he can convince the user's browser to load a given URL, steal their cookies for any given domain. It does not require that active scripting is enabled in the browser, and can be done with something as simple as an image tag, allowing for hassle free use in HTML email, web based email services, etc. As expected, this bug is also present in Netscape 6.1. Upgrade to Netscape 6.2.1 or Mozilla 0.9.7 or higher, which fix this bug. The take-away message is that, due to implementation bugs in browser and in web applications, cookies can be stolen. It is critical that any application that depends on cookies does so with an understanding of this fact, and takes appropriate measures to limit the damage that can be done using stolen cookies. What's New * Current Status Summary: (last updated Mon Jan 21 20:48:17 PST 2002) I finally got around to making this vulnerability public. * mid-Jan 2002: Netscape put up a [6]note on their site saying that there was a security hole that they fixed. * Sometime between when I reported this bug to Netscape and when I made it public: This bug was fixed with the release of Netscape 6.2.1 and Mozilla 0.9.7. * November 15, 2001: I reported this bug to Netscape via their security bug submission form. I had trouble finding a documented method for submitting security bugs to mozilla.org, but eventually figured out that securityat_private existed. In any case, both submissions found their way to the same contact at Netscape. Background Cookies are the mechanism used by most websites to identify and authenticate a user. If you can steal someone's cookies, you can trick the server into thinking you are them. Exactly what this gains you depends on the application and how it is designed. It may gain you very little, or it may gain you a whole lot (eg. [7]Microsoft Passport to Trouble). For more information about cookies, see [8]The Unofficial Cookie FAQ. Cookies are set with a specific hostname or a domain, so that they are only sent to that host or domain, with an exception or two that I won't go into here. They can also be set with a specific path, or with the secure flag, which means they will only be sent if the connection is a SSL connection. Normally, this should mean that only the server that set the cookie, or others it is operating in cooperation with (eg. in the same domain) can read it. Mozilla has a bug that lets you bypass this protection and steal cookies for any domain. This is quite similar to bugs found in Microsoft Internet Explorer in the past, such as [9]this one and [10]this one. As has been shown time and time again, there are many security flaws in many Microsoft products. Sadly, they are far from being alone. There is almost certainly no web browser out there that is functional enough to browse a significant percent of current popular websites and that does not have similar security holes. Details The details are very trivial. Loading a URL such as: http://alive.znep.com%00www.passport.com/cgi-bin/cookies ...will cause Mozilla to connect to the hostname specified before the "%00", but send the cookies to the server based on the entire hostname. The "%00" is the URL encoded version of the null character, used in C to terminate strings. This exploit can be used to steal cookies with a specific path set, and can be used to steal cookies with the secure flag set, by using the specific path and SSL in the request URL. Note, however, that cookies set for a specific hostname (eg. "www.passport.com") can not be stolen using this method, but only cookies set for an entire domain (eg. ".passport.com"). This bug was first tested on Netscape 6.1 on Windows 2000 and Mozilla 0.9.5 build 2001111503 and 0.9.5 build 20011012 on Linux. It is expected that all Netscape 6.x and Mozilla versions prior to the recently released fixed versions are vulnerable. Example Exploit An example exploit [11]is available. Very straightforward. _________________________________________________________________ $Id: index.html,v 1.6 2002/01/22 05:06:04 marcs Exp marcs $ _________________________________________________________________ References 1. http://alive.znep.com/~marcs/security/mozillacookie/#executivesummary 2. http://alive.znep.com/~marcs/security/mozillacookie/#history 3. http://alive.znep.com/~marcs/security/mozillacookie/#background 4. http://alive.znep.com/~marcs/security/mozillacookie/#details 5. http://alive.znep.com/~marcs/security/mozillacookie/#example 6. http://home.netscape.com/security/ 7. http://alive.znep.com/~marcs/passport/ 8. http://www.cookiecentral.com/faq/ 9. http://alive.znep.com/~marcs/security/iecookie1/ 10. http://alive.znep.com/~marcs/security/iecookie2/ 11. http://alive.znep.com/~marcs/security/mozillacookie/demo.html
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