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ASUS router drive-by code execution via XSS and authentication bypass



ASUS router drive-by code execution via XSS and authentication bypass
=====================================================================
The latest version of this advisory is available at:
https://sintonen.fi/advisories/asus-router-auth-bypass.txt


Overview
--------

Various ASUS routers contain reflected Cross-Site Scripting (CWE-79) and 
authentication bypass (CWE-592) vulnerabilities that can be exploited to 
gain remote administrative access to the devices.


Description
-----------

Several ASUS routers include reflected Cross-Site Scripting (CWE-79) and 
authentication bypass (CWE-592) vulnerabilities. An attacker who can lure 
a victim to browse to a web site containing a specially crafted JavaScript 
payload can execute arbitrary commands on the router as administrator 
(root). No user interaction is required.


Impact
------

An attacker can create a JavaScript payload that uses an exploit to unearth
the administrative password from the victim's ASUS router and logs in to 
the device. Once logged in the payload can perform administrative actions, 
including arbitrary command execution as administrator (root).


Details
-------

The CSRF vulnerability CVE-2013-3093 discovered by Jacob Holcomb / 
Independent Security Evaluators (*) affecting various ASUS routers has been 
known for some time. The vulnerability enables an attacker to forge HTML 
forms and execute actions on the behalf of the target user (admin), 
enabling executing administrative functions. Another vulnerability allows 
executing arbitrary commands as administrator (root). ASUS was notified of 
these issues on March 29th 2013.

These vulnerabilities were not considered critical, likely because of the 
seemingly strict prerequisites for the attack:

"- The victim must have an active web application session on their ASUS
   router.
 - The victim must follow a link crafted by an attacker (e.g., by clicking 
   the link directly, or through some other mechanism such as redirection 
   from a malicious site).
 - The victim must have the necessary permissions to render and execute 
   the forged HTTP."
(*) http://securityevaluators.com/knowledge/case_studies/routers/Vulnerability_Catalog.pdf

The two newly discovered vulnerabilities, described in more detail below, 
enable exploiting the earlier vulnerabilities in an automated fashion. The 
attack requires no interaction from the user, other than browsing to a 
website that has been injected with JavaScript code crafted by the attacker. 
The exploit could be embedded into various otherwise benign sites via e.g. 
malicious advertisement banners or by exploiting persistent Cross-Site 
Scripting vulnerabilities. The attacks could also be carried out with 
phishing email campaigns.

The attack utilizes a reflected Cross-Site Scripting vulnerability on the 
unauthenticated error page to bypass the same-origin policy protection. 
Vulnerability number two described below is used to obtain the 
administrator's password. The reflected JavaScript payload executes within 
the context of the ASUS device and is able to utilize the CVE-2013-3093 
CSRF vulnerability to perform actions on the behalf of the user (admin). 
The exploit utilizes the "SystemCmd" arbitrary command execution feature 
to allow remote administrative telnet connectivity from all addresses.


New vulnerabilities
-------------------

1. Reflected Cross-Site Scripting (CWE-79)

There is a Cross-Site Scripting vulnerability on the router error page:

http://192.168.1.1/error_page.htm?flag=%27%2balert(%27XSS%27)%2b%27

The error page is accessible without authentication. This vulnerability 
enables the attacker to bypass same-origin policy restrictions enforced 
by XMLHttpRequest.


2. Authentication bypass (CWE-592)

The router error page http://192.168.1.1/error_page.htm includes the 
current administrative password in clear text.

For example if the administrative password is "Joshua", the page includes 
the following dynamically generated JavaScript:

if('1' == '0' || 'Joshua' == 'admin')

The error page is accessible without authentication. This vulnerability 
enables the attacker with same-origin rights, obtained by utilizing the 
vulnerability above, to read the password by utilizing an XMLHttpRequest 
call. The script can then perform actions as administrator by utilizing 
further XMLHttpRequest calls.


Vulnerable devices
------------------

The vulnerabilities were discovered from an ASUS RT-N16 device, firmware 
version 3.0.0.4.374_979. By sampling a small set of ASUS firmware images 
the following models were also found likely to be vulnerable:

ASUS RT-N10U, firmware 3.0.0.4.374_168
ASUS RT-N56U, firmware 3.0.0.4.374_979
ASUS DSL-N55U, firmware 3.0.0.4.374_1397 *
ASUS RT-AC66U, firmware 3.0.0.4.374_2050 *
ASUS RT-N15U, firmware 3.0.0.4.374_16
ASUS RT-N53, firmware 3.0.0.4.374_311

*) ASUS DSL-N55U and ASUS RT-AC66U did not appear vulnerable to the 
authentication bypass issue. These devices are still vulnerable to the XSS 
and if the default password 'admin' has not been changed, they are easily 
exploitable as well.

This list is by no means comprehensive. It is likely that other devices are 
vulnerable as well.


Vendor recommendations
----------------------

1. Fix the Cross-Site Scripting vulnerabilities, at least from the 
   unauthenticated part of the web interface (error_page.htm)
2. Fix the admin password disclosure on error_page.htm
3. Fix the CSRF (CVE-2013-3093) issue by utilizing anti-CSRF protection


End user mitigation
-------------------

1. Install the latest firmware update, version 3.0.0.4.374.4422 or later.

or

2. If no firmware update has been released, the end users can partially 
   mitigate the vulnerabilities by changing the "Router Login Name" via
   the "Administration - System" menu to be something other than "admin". 
   Changing the router default network to something else than 
   192.168.1.0/24 might also grant some limited protection.


Proof-of-concept
----------------

A proof-of-concept exploit consisting of a small demo web page will be 
released at a later date.


Credits
-------

The vulnerabilities were discovered by Harry Sintonen / nSense Oy.


Previous work
-------------

The Cross-Site Request Forgery and command execution vulnerabilities
(CVE-2013-3093) were discovered by Jacob Holcomb / Independent Security 
Evaluators.


Timeline
--------

15.1.2014  discovered the vulnerabilities
16.1.2014  wrote a preliminary report and PoC
20.1.2014  attempted to send email to security@xxxxxxxx and secure@xxxxxxxx, 
           both addresses bounced.
20.1.2014  opened a ticket at vip.asus.com on how to report security
           vulnerabilities on ASUS hw.
20.1.2014  emailed netadmin@xxxxxxxxxxx asking how to report security
           vulnerabilities on ASUS hw.
20.1.2014  emailed CERT-FI asking for help contacting ASUS.
21.1.2014  CERT-FI was able to figure out ASUS security contact details.
21.1.2014  reported the security vulnerabilities to ASUS.
21.1.2014  reported the security vulnerabilities to CERT-FI vulncoord.
22.1.2014  got a response from representative of the vendor, report forwarded to 
           R&D for review. (ASUS CASEID=RPTM20140121202264-976)
27.1.2014  got a response from the vendor, CWE-592 is fixed in the latest firmware
           version (for some devices?), CWE-79 is still being fixed.
08.2.2014  the vendor reported that the updated firmware will be released 
           during week 8.
13.2.2014  ASUS released firmware updates for RT-N16, RT-N10U, RT-AC66U and 
           RT-N15U.
13.2.2014  requested CVE IDs from MITRE.
19.2.2014  ASUS released a firmware update for RT-N56U.
21.2.2014  released the security advisory.