Vulnerability Note VU#261869

Vulnerability Note VU#261869
Clientless SSL VPN products break web browser domain-based security models
Overview
Clientless SSL VPN products from multiple vendors operate in a way that breaks fundamental browser security mechanisms. An attacker could use these devices to bypass authentication or conduct other web-based attacks.
I. Description
Web browsers enforce the same origin policy to prevent one site's active content (such as JavaScript) from accessing or modifying another site's data. For instance, active content hosted at http://<example.com>/page1.html can access DOM objects on http://<example.com>/page2.html, but cannot access objects hosted at http://<example.net>/page.html. Many clientless SSL VPN products retrieve content from different sites, then present that content as coming from the SSL VPN, effectively circumventing browser same origin restrictions.

Clientless SSL VPNs provide browser-based access to internal and external resources without the need to install a traditional VPN client. Typically, these web VPNs are used to access intranet sites (such as an internal webmail server), but many have more capabilities, such as providing access to internal fileshares and remote desktop capabilities. To connect to a VPN, a web browser is used to authenticate to the web VPN, then the web VPN retrieves and presents the content from the requested pages.

Web VPN servers interact with clients using a process similar to what is described below:

1. The user presents credentials to the web VPN using a web browser. The authentication can be done through username and password submission, or can involve multi-factor authentication.
2. The web VPN authenticates the user and assigns an ID to the session, which is sent to the user's browser in the form of a cookie.
3. The user can then browse internal resources, such as a webmail server or intranet webserver. URLs as viewed by the user's web browser may be similar to https://<webvpnserver>/www.intranet.example.com

As the web VPN retrieves web pages, it rewrites hyperlinks so that they are accessible through the web VPN. For example, a link to http://<www.intranet.example.com>/mail.html becomes https://<webvpnserver>/www.intranet.example.com/mail.html. Cookies set by the requested webserver are converted into globally unique cookies before being passed to the user's browser, which prevents collision between two identically named cookies from different requested domains. For example, a sessionid cookie set by intranet.example.com could be renamed to intranet.example.com_sessionid before it is sent from the web VPN to the user's browser . Additionally, the web VPN replaces references to specific HTML DOM objects, such as document.cookie. These DOM objects are replaced with script that returns the value for that DOM object as if it had been accessed in the context of the requested site's domain.

If an attacker constructs a page that obfuscates the document.cookie element in such a way as to avoid being rewritten by the web VPN, then the document.cookie object in the returned page will represent all of the user's cookies for the web VPN domain. Included in this document.cookie are the web VPN session ID cookie itself and all globally unique cookies set by sites requested through the web VPN. The attacker may then use these cookies to hijack the user's VPN session and all other sessions accessed through the web VPN that rely on cookies for session identification.

Additionally, an attacker could construct a page with two frames: one hidden and one that displays a legitimate intranet site. The hidden frame could log all keys pressed in the second, benign frame and submit these keypresses as parameters to a XMLHttpRequest GET to the attacker's site, rewritten in web VPN syntax.

Note that if the VPN server is allowed to connect to arbitrary Internet sites, these vulnerabilities can be exploited by any site on the Internet.
II. Impact
By convincing a user to view a specially crafted web page, a remote attacker may be able to obtain VPN session tokens and read or modify content (including cookies, script, or HTML content) from any site accessed through the clientless SSL VPN. This effectively eliminates same origin policy restrictions in all browsers. For example, the attacker may be able to capture keystrokes while a user is interacting with a web page. Because all content runs at the privilege level of the web VPN domain, mechanisms to provide domain-based content restrictions, such as Internet Explorer security zones and the Firefox add-on NoScript, may be bypassed. For additional information about impacts, please see CERT Advisory CA-2000-02.
III. Solution
There is no solution to this problem. Depending on their specific configuration and location in the network these devices may be impossible to operate securely. Administrators are encouraged to view the below workarounds and see the systems affected section of this document for more information about specific vendors.

Limit URL rewriting to trusted domains

If supported by the VPN server, URLs should only be rewritten for trusted internal sites. All other sites and domains should not be accessible through the VPN server.

Since an attacker only needs to convince a user to visit web page being viewed through the VPN to exploit this vulnerability, this workaround is likely to be less effective if there are a large number of hosts or domains that can be accessed through the VPN server. When deciding which sites can be visited through use of the VPN server, it is important to remember that all allowed sites will operate within the same security context in the web browser.

Limit VPN server network connectivity to trusted domains

It may be possible to configure the VPN device to only access specific network domains. This restriction may also be possible by using firewall rules.

Disable URL hiding features

Obfuscating URLs hides the destination page from the end user. This feature can be used by an attacker to hide the destination page of any links they send. For example, https://<vpn.example.com>/attack-site.com vs https://<vpn.example.com>/778928801

Systems Affected
Vendor Status Date Notified Date Updated
3com Inc Unknown 2009-10-19 2009-10-19
ACCESS Unknown 2009-10-19 2009-10-19
aep NETWORKS Unknown 2009-11-06 2009-11-06
Alcatel-Lucent Unknown 2009-10-19 2009-10-19
Avaya, Inc. Unknown 2009-10-19 2009-10-19
Barracuda Networks Unknown 2009-09-16
Check Point Software Technologies Unknown 2009-09-15 2009-09-15
Cisco Systems, Inc. Vulnerable 2009-09-24 2009-11-30
Citrix Unknown 2009-09-22
Computer Associates Not Vulnerable 2009-10-19 2009-11-30
Conectiva Inc. Unknown 2009-10-19 2009-10-19
D-Link Systems, Inc. Unknown 2009-10-19 2009-10-19
Debian GNU/Linux Unknown 2009-10-19 2009-10-19
DragonFly BSD Project Unknown 2009-10-19 2009-10-19
EMC Corporation Unknown 2009-10-19 2009-10-19
Engarde Secure Linux Unknown 2009-10-19 2009-10-19
Enterasys Networks Unknown 2009-10-19 2009-10-19
Ericsson Unknown 2009-10-19 2009-10-19
eSoft, Inc. Unknown 2009-10-19 2009-10-19
Extreme Networks Not Vulnerable 2009-10-19 2009-11-30
F5 Networks, Inc. Unknown 2009-09-16 2009-09-16
Fedora Project Not Vulnerable 2009-10-19 2009-11-30
Force10 Networks, Inc. Unknown 2009-10-19 2009-10-19
Fortinet, Inc. Unknown 2009-10-19 2009-10-19
Foundry Networks, Inc. Unknown 2009-10-19 2009-10-19
FreeBSD, Inc. Unknown 2009-10-19 2009-10-19
Fujitsu Unknown 2009-10-19 2009-10-19
Gentoo Linux Unknown 2009-10-19 2009-10-19
Global Technology Associates Unknown 2009-10-19 2009-10-19
Hewlett-Packard Company Unknown 2009-10-19 2009-10-19
Hitachi Unknown 2009-10-19 2009-10-19
IBM Corporation Unknown 2009-10-19 2009-10-19
IBM eServer Unknown 2009-10-19 2009-10-19
Infoblox Unknown 2009-10-19 2009-10-19
Intel Corporation Unknown 2009-10-19 2009-10-19
Internet Security Systems, Inc. Unknown 2009-10-19 2009-10-19
Intoto Unknown 2009-10-19 2009-10-19
IP Filter Unknown 2009-10-19 2009-10-19
IP Infusion, Inc. Unknown 2009-10-19 2009-10-19
Juniper Networks, Inc. Vulnerable 2009-09-24 2009-11-30
Kerio Technologies Not Vulnerable 2009-09-24 2009-10-01
Luminous Networks Unknown 2009-10-19 2009-10-19
m0n0wall Unknown 2009-10-19 2009-10-19
Mandriva S. A. Unknown 2009-10-19 2009-10-19
McAfee Not Vulnerable 2009-09-15 2009-11-30
Microsoft Corporation Unknown 2009-09-22
MontaVista Software, Inc. Unknown 2009-10-19 2009-10-19
Multitech, Inc. Unknown 2009-10-19 2009-10-19
NEC Corporation Unknown 2009-10-19 2009-10-19
NetApp Unknown 2009-10-19 2009-10-19
NetBSD Unknown 2009-10-19 2009-10-19
netfilter Unknown 2009-10-19 2009-10-19
Netgear, Inc. Unknown 2009-10-20 2009-10-20
Nokia Unknown 2009-10-19 2009-10-19
Nortel Networks, Inc. Unknown 2009-10-19 2009-10-19
Novell, Inc. Not Vulnerable 2009-11-30
OpenBSD Unknown 2009-10-19 2009-10-19
OpenVPN Technologies Unknown 2009-11-13 2009-11-13
Openwall GNU/*/Linux Unknown 2009-10-19 2009-10-19
PePLink Not Vulnerable 2009-10-19 2009-11-30
Process Software Unknown 2009-10-19 2009-10-19
Q1 Labs Unknown 2009-10-19 2009-10-19
QNX Software Systems Inc. Unknown 2009-10-19 2009-10-19
Quagga Unknown 2009-10-19 2009-10-19
RadWare, Inc. Unknown 2009-10-19 2009-10-19
Red Hat, Inc. Not Vulnerable 2009-10-19 2009-11-30
Redback Networks, Inc. Unknown 2009-10-19 2009-10-19
SafeNet Vulnerable 2009-10-19 2009-11-30
Secureworx, Inc. Unknown 2009-10-19 2009-10-19
Silicon Graphics, Inc. Unknown 2009-10-19 2009-10-19
SmoothWall Unknown 2009-10-19 2009-10-19
Snort Unknown 2009-10-19 2009-10-19
Soapstone Networks Unknown 2009-10-19 2009-10-19
SonicWall Vulnerable 2009-12-01
Sourcefire Unknown 2009-10-19 2009-10-19
Stonesoft Unknown 2009-10-19 2009-10-19
Sun Microsystems, Inc. Unknown 2009-10-19 2009-10-19
SUSE Linux Unknown 2009-10-19 2009-10-19
Symantec Unknown 2009-09-15 2009-09-15
The SCO Group Unknown 2009-10-19 2009-10-19
Turbolinux Unknown 2009-10-19 2009-10-19
U4EA Technologies, Inc. Unknown 2009-10-19 2009-10-19
Ubuntu Unknown 2009-10-19 2009-10-19
Unisys Unknown 2009-10-19 2009-10-19
VMware Unknown 2009-10-19 2009-10-19
Vyatta Unknown 2009-10-19 2009-10-19
Watchguard Technologies, Inc. Unknown 2009-10-19 2009-10-19
Webmin Not Vulnerable 2009-09-25 2009-10-02
Wind River Systems, Inc. Unknown 2009-10-19 2009-10-19
ZyXEL Unknown 2009-10-19 2009-10-19
References

https://developer.mozilla.org/En/Same_origin_policy_for_JavaScript
https://developer.mozilla.org/en/DOM/document.cookie
http://code.google.com/p/browsersec/wiki/Part2#Same-origin_policy
http://www.owasp.org/index.php/Category:OWASP_Cookies_Database
http://www.owasp.org/index.php/Testing_for_Session_Management_Schema_&#40;OWASP-SM-001&#41;#Black_Box_Testing_and_Examples
http://www.cisco.com/en/US/docs/ios/security/configuration/guide/sec_ssl_vpn.html#wp1404057
http://seclists.org/fulldisclosure/2006/Jun/238
http://seclists.org/fulldisclosure/2006/Jun/416
http://www.blackhat.com/presentations/bh-usa-08/Zusman/BH_US_08_Zusman_SSL_VPN_Abuse.pdf
Credit

This issue was discovered by David Warren and Ryan Giobbi. Much of the original research into this issue was done by Michal Zalewski and Mike Zusman.

This document was written by David Warren and Ryan Giobbi.
Other Information
Date Public: 2009-11-30
Date First Published: 2009-11-30
Date Last Updated: 2009-12-02
CERT Advisory:
CVE-ID(s): CVE-2009-2631
NVD-ID(s): CVE-2009-2631
US-CERT Technical Alerts:
Metric: 45,00
Document Revision: 135