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.:: Fun with the Spanning Tree Protocol ::.

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Current issue : #61 | Release date : 2003-08-13 | Editor : Phrack Staff
IntroductionPhrack Staff
LoopbackPhrack Staff
LinenoisePhrack Staff
Toolz ArmoryPhrack Staff
Phrack Prophile on digitPhrack Staff
Advanced Doug Lea's malloc exploitsjp
Hijacking Linux Page Fault Handlerbuffer
The Cerberus ELF interfacemayhem
Polymorphic Shellcode EngineCLET team
Infecting Loadable Kernel Modulestruff
Building IA32 'Unicode-Proof' Shellcodesobscou
Fun with the Spanning Tree ProtocolVladislav V. Myasnyankin & Oleg K. Artemjev
Hacking the Linux Kernel Network Stackbioforge
Kernel Rootkit Experiences & the Futurestealth
Phrack World NewsPhrack Staff
Title : Fun with the Spanning Tree Protocol
Author : Vladislav V. Myasnyankin & Oleg K. Artemjev
                            ==Phrack Inc.==

                Volume 0x0b, Issue 0x3d, Phile #0x0c of 0x0f

|=---------------=[ Fun with the Spanning Tree Protocol ]=---------------=|
|=-----------------------------------------------------------------------=|
|=-----------=[ Oleg K. Artemjev, Vladislav V. Myasnyankin ]=------------=|

 
Introduction.
*=*=*=*=*=*=*

Developed in the 1st part of 80th by International Standards Organization (ISO) 
seven-layer model of Open System Interconnection (OSI) presents a hierarchical 
structure, where each level has strictly assigned job & interface to upper & 
lower levels. Due to business needs modern equipment currently supports on the 
2nd OSI layer not only traditional frame forwarding & hardware address
resolution, but also provides redundancy, multiplexing, load balancing &
separation of information flows. Unfortunately, security issues at this layer
are often left without attention. Here we'll show weakness in implementation
and algorithm of one of the second OSI layer (``channel'' (MAC+LLC))
protocols - Spanning Tree Protocol (STP). This work uses our materials
published in Russian: [2], [4].
 
   Since we're publishing an information about security vulnerabilities before
a fix is ready on the market & since these information may be used by a
malicious person we'll write our article in such a way, so newbies (also known
as ``script kiddies'' or ``black hats'' -  see [1]) would be unable to use
this paper as a step-by-step  ``howto''. We understand that different people
have different opinion to this issue, but feel that this is almost single
possible way to stimulate vendors to fix bugs much faster. Of course we
already notified some vendors (Cisco, Avaya) about these vulnerabilities, but
an answer was  alike: ``unless this gives money we won't make investments''.
Well, since  we're interested in high level of security in switches & routers
we use, we have to publish our investigations - thus we 'll make some pressure
on hardware vendors to implement real security in their devices. Also we note,
 that vendors should be already informed via bugtraq & some - Cisco & Avaya -
directly. Our first publication in Russian concerning STP vulnerabilities was
made about one year ago.
 
   The volume of our materials written while analyzing STP protocol is too big
to be published in one magazine article. Full information is available in the
Internet at the project's web page ([3]) and with the same restrictions which
apply also to this publication (see license below).

License.
*=*=*=*=

   As a complain against trends to inhibit publications of security
vulnerabilities in software (these tendencies are widely known to the public
as a DMCA law in U$ [Digital Millennium Copyright Act]), these materials are a
subject to the following license:

--------------------------------------------------------------------------------
License agreement.

This paper is an intellectual property of it's authors: Oleg Artemjev and
Vladislav Myasnyankin (hereinafter - writers). This paper may be freely used
for the links, but its content or its part cannot be translated into foreign
languages or included into any paper, book, magazine, and other electronic or
paper issues without prior WRITTEN permissions of both writers. Moreover, in
case of using materials of this research or refer to it, according given
license you must provide complete information: full title, authorship and this
license. You can freely distribute this paper electronically, if, and only if,
all of the following conditions are met:
  
 1. This license agreement and article are not modified,  including its PGP
    digital signature. Any reformatting of the text is prohibited. 
 2. The distribution does not contradict the given license. 
 
   Distribution of this paper in the countries with the legislation containing
limitations similar to American DMCA contradicts the given license. At the
moment of publication this includes United States of America (including
embassies,naval vessels, military bases and other areas of  US jurisdiction.
Moreover, reading this paper by citizens of such a country violates this
license agreement and may also violate their law. Nevertheless, distribution
of any links to this document is not a violation of the given license. 
 
   This paper is provided by the authors ``as is'' and any express or implied
warranties, including, but not limited to, the implied warranties of
merchantability and fitness for a particular purpose are disclaimed. In no
event shall the writers be liable for any direct,indirect, incidental,
special, exemplary, or consequential damages (including, but not limited to,
procurement of substitute goods or services; loss of use, data, or profits; or
business interruption). 
 
   Writers claim this article for educational purposes only. You should not
read this paper, if you disagree not to use it any other way. 
 
   The given license agreement is subject to change without warning in the
consent of both writers. 
--------------------------------------------------------------------------------

What is STP?
*=*=*=*=*=*=

   Main task of STP protocol is automated management of network topology with
redundant channels. In general, almost all type of networks are unable to
accept loops(rings) in their structure. Really, if network equipment is
connected with superfluous lines, then without additional measures frames
would be delivered to recipient as a several one - this would result in a
fault. But business require redundancy, thus there is an STP - it takes care
that all physical loops are logically disabled unless one of lines gives a
fault - in this case STP enables line that is currently in reserve. STP should
guarantee that at each point of time only one of several duplicate links is
enabled & should automatically switch between them on demand (fault or
physical topology change).

How STP works?
*=*=*=*=*=*=*=

   STP begin its work from building a tree-alike graph, which begins at 
``root''. One of STP-capable devices becomes a root after winning elections.
Each STP-capable device (it could be a switch, router or other equipment,
hereby & later for simplicity called ``bridge'') starts from power-up claiming
that it's root one by sending special data named Bridge Protocol Data Unit
(BPDU - see [9]) through all ports.  The receiver's address in a BPDU packets
is a group (multicast) address -  this allows BPDUs pass through
non-intellectual (dumb) equipment like hubs and non STP-aware switches.
 
   Here as we say ``address'', we mean MAC-address, since STP is working at 
the level of Media Access Control (MAC). Thereby all issues about STP & its
vulnerabilities apply equal to the different transmission methods, i.e. 
Ethernet, Token Ring & others.
 
   After receiving BPDU from other device the bridge compares received
parameters with its own & depending to result decide to stop or keep insisting
on its root status.  At the end of elections the device with the lowest value
of the bride identifier becomes a root one. The bridge identifier is a
combination of bridge MAC address & defined bridge priority. Obviously in a
network with single STP compatible device it 'll be a root one.
 
   Designated root (or ``Designated Root Bridge'', as named by standard)
doesn't have any additional responsibilities - it only used as a beginning
point to start building topology graph. For all other bridges in a network STP
defines the ``Root Port'' - the nearest to the root bridge port. From other
ports connected to the bridge it differs by its identifier - combination of
its MAC address & defined for the port priority.
 
   The Root Path Cost is also a value meaningful for STP elections - it is
being build as a sum of path costs: to the root port of given bridge & all
path costs to root ports of all other bridges on the route to Root one.
 
   In addition to the ``main'' Root Bridge STP defines a logical entity called
``Designated Bridge'' - owner of this status becomes main bridge in serving of
given LAN segment. This is also a subject of elections.
 
   Similarly STP defines for each network segment the Designated Port (which
serving given network segment) & corresponding to it ``Designated Cost''.
 
   After all the elections are finished, network goes into stable phase. This
state is characterized by the following conditions:
  
 - There is only one device in a network claiming itself as a Root one, all
   others are periodically announcing it. 

 - The Root Bridge periodically sends BPDU through all its ports. The sending
   interval is named ``Hello Time''. 

 - In each LAN segment there is a single Designated Root Port and all traffic 
   to the Root Bridge is going through it. Compared to other bridges, it has
   lowest  value of path cost to the Root Bridge, if these values are
   identical - the port with a lowest port identifier (MAC plus priority) is
   assigned.  

 - BPDUs are being received & sent by STP-compatible unit on each port, even
   those that are disabled by STP protocol. Exceptionally, BPDUs are not
   operationing on ports that are disabled by administrator. 

 - Each bridge forwards frames only between Root Port & Designated Ports for
   corresponding segments. All other ports are blocked. 

   As follows from the last item, STP manages topology by changing port states
within following list: 
  
Blocking:   The port is blocked (discards user frames), but accepts STP BPDUs.
   
Listening:  1st stage before forwarding. STP frames (BPDUs) are OK, but user 
            frames are not processed. No learning of addresses yet, since it 
            may give wrong data in  switching table at this time; 
Learning:   2nd stage of preparation for forwarding state. BPDUs are processed
            in full,  user frames are only used to build switching table and not
            forwarded; 
Forwarding: Working state of ports from user view - all frames are processed
            - STP & user ones. 
 
   At time of network topology reconfiguration all bridge ports are in one of
three states - Blocking, Listening or Learning, user frames are not delivered
& network is working only for itself, not for user.
 
   In stable state all bridges are awaiting periodical Hello BPDUs from Root
Bridge. If in the time period defined by Max Age Time there was no Hello BPDU,
then bridge decides that either Root Bridge is Off, either the link to is
broken. In this case it initiates network topology reconfiguration. By
defining corresponding parameters it is possible to regulate how fast bridges
will find topology changes & enable backup links.

Lets look closer.
*=*=*=*=*=*=*=*=*

Here is a structure of STP Configuration BPDU according to 802.1d standard:

 ----------------------------------------------
 |Offset  |Name                       |Size   |
 ----------------------------------------------
 ----------------------------------------------
 |1       |Protocol Identifier        |2 bytes|
 ----------------------------------------------
 |        |Protocol Version Identifier|1 byte |
 ----------------------------------------------
 |        |BPDU type                  |1 byte |
 ----------------------------------------------
 |        |Flags                      |1 byte |
 ----------------------------------------------
 |        |Root Identifier            |8 bytes|
 ----------------------------------------------
 |        |Root Path Cost             |4 bytes|
 ----------------------------------------------
 |        |Bridge Identifier          |8 bytes|
 ----------------------------------------------
 |        |Port Identifier            |2 bytes|
 ----------------------------------------------
 |        |Message Age                |2 bytes|
 ----------------------------------------------
 |        |Max Age                    |2 bytes|
 ----------------------------------------------
 |        |Hello Time                 |2 bytes|
 ----------------------------------------------
 |35      |Forward Delay              |2 bytes|
 ----------------------------------------------

In a C language:

typedef struct {

Bpdu_type  type;
Identifier root_id;
Cost       root_path_cost;
Identifier bridge_id;
Port_id    port_id;
Time       message_age;
Time       max_age;
Time       hello_time;
Time       forward_delay;
Flag       topology_change_acknowledgement;
Flag       topology_change;

} Config_bpdu;


Here is how it look like in a tcpdump:
---------------------screendump----------------------------
[root@ws002 root]# tcpdump -c 3 -t -i eth0 stp
tcpdump: listening on eth0
802.1d config 8000.00:50:e2:bd:58:40.8002 root 8000.00:50:e2:bd:58:40 pathcost 0 age 0 max 20 hello 2 fdelay 15 
802.1d config 8000.00:50:e2:bd:58:40.8002 root 8000.00:50:e2:bd:58:40 pathcost 0 age 0 max 20 hello 2 fdelay 15 
802.1d config 8000.00:50:e2:bd:58:40.8002 root 8000.00:50:e2:bd:58:40 pathcost 0 age 0 max 20 hello 2 fdelay 15 
3 packets received by filter
0 packets dropped by kernel
[root@ws002 root]#
---------------------screendump----------------------------

And with extra info:

---------------------screendump----------------------------
[root@ws002 root]# tcpdump -vvv -e -l -xX -ttt -c 3 -i eth0 stp
tcpdump: listening on eth0
000000 0:50:e2:bd:58:42 1:80:c2:0:0:0 0026 64: 802.1d config \
8000.00:50:e2:bd:58:40.8002 root 8000.00:50:e2:bd:58:40 pathcost 0 \
age 0 max 20 hello 2 fdelay 15 
0x0000   4242 0300 0000 0000 8000 0050 e2bd 5840        BB.........P..X@
0x0010   0000 0000 8000 0050 e2bd 5840 8002 0000        .......P..X@....
0x0020   1400 0200 0f00 0000 0000 0000 0000 7800        ..............x.
0x0030   0c00                                           ..
2. 002912 0:50:e2:bd:58:42 1:80:c2:0:0:0 0026 64: 802.1d config \
8000.00:50:e2:bd:58:40.8002 root 8000.00:50:e2:bd:58:40 pathcost 0 \
age 0 max 20 hello 2 fdelay 15 
0x0000   4242 0300 0000 0000 8000 0050 e2bd 5840        BB.........P..X@
0x0010   0000 0000 8000 0050 e2bd 5840 8002 0000        .......P..X@....
0x0020   1400 0200 0f00 0000 0000 0000 0000 7800        ..............x.
0x0030   0c00                                           ..
2. 046164 0:50:e2:bd:58:42 1:80:c2:0:0:0 0026 64: 802.1d config \
8000.00:50:e2:bd:58:40.8002 root 8000.00:50:e2:bd:58:40 pathcost 0 \
age 0 max 20 hello 2 fdelay 15 
0x0000   4242 0300 0000 0000 8000 0050 e2bd 5840        BB.........P..X@
0x0010   0000 0000 8000 0050 e2bd 5840 8002 0000        .......P..X@....
0x0020   1400 0200 0f00 0000 0000 0000 0000 7800        ..............x.
0x0030   0c00                                           ..
3 packets received by filter
0 packets dropped by kernel
[root@ws002 root]# 
---------------------screendump----------------------------

Generally the same is achieved by multicast alias of tcpdump syntax (if you
've no other multicast traffic in the target network:

---------------------screendump----------------------------
[root@ws002 root]# tcpdump -vvv -e -l -xX -ttt -c 3 -i eth0 multicast
tcpdump: listening on eth0
000000 0:50:e2:bd:58:42 1:80:c2:0:0:0 0026 64: 802.1d config \
8000.00:50:e2:bd:58:40.8002 root 8000.00:50:e2:bd:58:40 pathcost 0 \
age 0 max 20 hello 2 fdelay 15 
0x0000   4242 0300 0000 0000 8000 0050 e2bd 5840        BB.........P..X@
0x0010   0000 0000 8000 0050 e2bd 5840 8002 0000        .......P..X@....
0x0020   1400 0200 0f00 0000 0000 0000 0000 7800        ..............x.
0x0030   0c00                                           ..
2. 004863 0:50:e2:bd:58:42 1:80:c2:0:0:0 0026 64: 802.1d config \
8000.00:50:e2:bd:58:40.8002 root 8000.00:50:e2:bd:58:40 pathcost 0 \
age 0 max 20 hello 2 fdelay 15 
0x0000   4242 0300 0000 0000 8000 0050 e2bd 5840        BB.........P..X@
0x0010   0000 0000 8000 0050 e2bd 5840 8002 0000        .......P..X@....
0x0020   1400 0200 0f00 0000 0000 0000 0000 7800        ..............x.
0x0030   0c00                                           ..
2. 006193 0:50:e2:bd:58:42 1:80:c2:0:0:0 0026 64: 802.1d config \
8000.00:50:e2:bd:58:40.8002 root 8000.00:50:e2:bd:58:40 pathcost 0 \
age 0 max 20 hello 2 fdelay 15 
0x0000   4242 0300 0000 0000 8000 0050 e2bd 5840        BB.........P..X@
0x0010   0000 0000 8000 0050 e2bd 5840 8002 0000        .......P..X@....
0x0020   1400 0200 0f00 0000 0000 0000 0000 7800        ..............x.
0x0030   0c00                                           ..
3 packets received by filter
0 packets dropped by kernel
[root@ws002 root]#
---------------------screendump----------------------------

As you see here, normally STP frames are arriving approximately within 
Hello Time (here is 2 seconds).



STP & VLANs.
*=*=*=*=*=*=

   We 'd like to say some words about STP functioning specific to networks
with virtual LANs (VLANs). Enabling this mode on a switch is logically
equivalent to replacing it with a few (by number of VLANs) switches, even when
physically there's no separation between VLANs media. It 'd be obvious to find
there different STP trees, but this option is supported by only some
equipment(i.e. Intel 460T supports only one STP tree for all VLANs; with
Avaya's Cajun switches family you'll find separate Spanning Tree only in high
models). These facts are  destroying a hope to localize possible STP attacks
in one VLAN. But there are threats existing even with separate spanning trees
per VLAN.
 
   Some vendors realize in their devices extended STP-related futures,
enhancing their abilities, like Spanning Tree Portfast in Cisco  (see [11]) &
STP Fast Start in some 3Com switches  (see [12]). We'll show essence of them
below. Also, some companies support their own implementation of STP, i.e. Dual
Layer STP from Avaya. Plus, STP modifications functioning for other network
types (i.e. DECnet). Here we'd like to point on their principle similarity and
differ only in details and extended abilities (so, in Avaya Dual Layer STP
trees could be terminated at the 802.1q-capable ports). All these
implementation suffer from the same defects as their prototypes. Unpublished
proprietary protocols give one more problem - only developers could solve
their problems, since full reverse engineering (needed to provide good
bug-fixing solutions) is much harder then small one required to realize
attacks & by publishing results some would make an evidence of reverse
engineering, which may be illegal.

Possible attack schemes
*=*=*=*=*=*=*=*=*=*=*=*

   An idea of 1st group of attacks lies practically ``on the surface''.
Essentially the principle of STP allows easily organize Denial of Service
(DoS) attack. Really, as defined by standard, on Spanning Tree reconfiguration
all ports of involved devices does not transfer user frames. Thus, to drop a
network (or at least one of its segments) into unusable state it's enough to
master STP-capable device(s) to do infinite reconfiguration. It could be
realized by initiating elections of, for example, root bridge, designated
bridge or root port - practically any of electional object. ``Fortunately''
STP has no any authentication  allowing malicious users easily reach this by
sending fake BPDU.

   A program building BPDU could be written in any high level language having
raw-socket interface (look at C sample and managing shell script at our
project home page -  [5], [6]).  Another way - one may use standard utilities
for managing Spanning Tree, i.e. from Linux Bridge project([13]),  but in this
case its not possible to manipulate STP parameters with  values that doesn't
fit into standard specification. Below we will examine base schemes of
potentially possible attacks.

Eternal elections.
*=*=*=*=*=*=*=*=*=

   Attacker monitors network with a sniffer (network analyzer) & awaits for
one of periodical configuration BPDUs from the root bridge (containing its
identifier). After that he sends into a network a BPDU with identifier that is
lower then received one (id=id-1) - thus it has pretensions to be a root
bridge itself & initiates elections. Then it decrement identifier by 1 and
repeat procedure. Each step initiates new elections wave. When identifier
reach its lowest value attacker return to the value calculated at beginning of
the attack. As a result network will be forever in elections of the root
bridge and ports of STP-capable devices will never reach forwarding state
while attack is in progress.

Disappearance of root.
*=*=*=*=*=*=*=*=*=*=*=

    With this attack there is no need to get current root bridge identifier -
the lowest possible value is a starting one. This, as we remember, means
maximum priority. At the end of elections attacker stops sending BPDUs, thus
after a timeout of Max Age Time gives new elections. At new elections attacker
also acts as before (and wins). By assigning minimum possible Max Age Time it
is possible to get situation when all the network will spend all time
reconfigurating, as it could be in previous algorithm. This attack may occur
less effective, but it has simpler realization. Also, depending to network
scale and other factors (i.e. Forward Delay value, that vary speed of
switching into a forwarding state) the ports of STP-capable devices may never
start forwarding the user frames - so we cannot consider this attack as less
dangerous.

 Merging-splitting of the trees.
*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=

  In a network with VLAN support it may be possible to lunch a modification
of discussed above attack by connecting segments with different vlans & STP
trees. This may be realized without  software, by hands, by linking ports
together with a cross-over cable. This may become a pain for NOC, since it's
hard to detect.

 Local Denial of Service.
*=*=*=*=*=*=*=*=*=*=*=*=*

  Attacker may make Denial of Service not for the entire network, but just on
a part of it. There could be many motivations, i.e. it may isolate victim
client from real server to make ``fake server'' attack. Lets look for
realization of this type of attack on example. 

 ----------------------------------------------------------------
                                         
 .------------------------.                 .------------------.
 | Switch w/ STP #1       |-----------------| Switch w/ STP #2 |
 .________________________.                 '__________________'
    |                |                               |   
    |                |                               |   .___.
    |                |                               |   |   |
    |.....           |  ._                           |   | ==|
  .------,~          |  ||                           |   | ==|
  |Client|'          |  ||                            \_ |  -|
  | PC   ||           \ |....                            |   | 
  \------ /            '=====|                           |   |
   ======/             Attackers                        -------
                       Notebook                          Server

--------------------------Picture 1-----------------------------
On the picture 1 server is connected to one switch & victim is connected 
to another one (connectivity to the bridge may include hubs). Attacker 
needs to fool nearest switch & make it think that he(she) has better way
to the bridge that serves server computer.

In terms of STP, attacker must initiate & win elections of designated bridge
for server segment. As a result of winning such elections the channel between
bridges would be disabled by setting corresponding ports to the blocked state.
By destroying connectivity between segments attacker may either try to fool
client claiming itself as a real server (compare with well known Mitnick
attack) or just feel satisfied if mischief is a subject.

BPDU filter.
*=*=*=*=*=*=

Obvious way to attack is to set a loop that is undetectable by STP by
organizing physical ring with filtering there of all BPDU frames. 

Man In the Middle.
*=*=*=*=*=*=*=*=*=

    Next two attacks have principal difference from already discussed - the
goal of them not to achieve denial of service, but data penetrating, that
impossible in the normal network operation mode. In short, this attack uses
STP to change logical structure of network to direct sensitive traffic via
attacker's station. Let's look at the 2nd picture.


 ----------------------------------------------------------------
     Clients segment                          Server segment           
 .------------------------.                 .------------------.
 | Switch w/ STP #1       |------X X--------| Switch w/ STP #2 |
 .________________________.                 '__________________'
    |                |                        |      |   
    |                |                        |      |   .___.
    |                |                        |      |   |   |
    |.....           |   .------.             |      |   | ==|
  .------,~          |   |      |             |      |   | ==|
  |Client|'          |   |Attacking           ;       \_,|  -|
  | PC   ||           \  | PC   |            /           |   | 
  \------ /            \_========_,         /            |   |
   ======/              |_________|--------'            -------
                                                         Server

--------------------------Picture 1-----------------------------


As against mentioned above partial denial of service attack, suppose that
attackers station is equipped with two NICs, one Network Interface Card 
is connected to the ``client's'' segment, and another - to the ``server's''
segment. By sending appropriate BPDU attacker initiates elections of the 
designated bridge for both segments and wins them. As a result, existing
link between switches (marked as "-X X-" ) will shut down (will switch to 
the blocking state) and all inter-segment traffic will be directed via 
attacker's station. If intruder's plans does not include denial of service,
he(she) MUST provide frame forwarding between NICs. It's a very simple 
task if attacker doesn't needed to change traffic in some manner. This
may be done by either creating simple program module or using built-in STP
functions of the  operating system, for example with Linux Bridge Project (see
 [13]), which contribute complete bridge solution. Of course, an intruder must
take in account ``bottle neck'' problem - inter-segment link  may work at
100Mb (1Gb) speed while client's ports may provide only 10Mb (100Mb) speed,
which lead to the network productivity degradation and  partial data loss (but
software realization of back pressure shouldn't be a  big deal). Of course, if
attacker wants to ``edit'' traffic on the fly on a heavy loaded link, he(she)
may need more powerful computer (both CPU and RAM). Fortunately, this attack
is impossible in networks with single switch - try to realize it in these
conditions and you will get partial DoS. Also note, that realization is
trivial only when attacker is connected to neighbored switches. If connections
are made to the switches without direct link, there is additional task -
guessing at least one Bridge ID, because STP-capable devices never forward
BPDU, sending on the base of received information its own, instead.

Provocated Sniffing.
*=*=*=*=*=*=*=*=*=*=

  In general, sniffing is data penetrating by switching network interface
into promiscuous mode. In this mode NIC receives all the frames, not only
broadcasts and directed to it. There're well known attack on networks 
based on switches, these are either poison targets MAC address table by fake
ARP replies, either over-full bridge switching table and thus making it behave
like a hub, but with splitting collision domains. Almost the same results may
be achieved using STP.

  According specification after tree reconfiguration (for example, after
designated bridge elections) STP-capable device MUST remove from the
switching table all the records (except those statically set by
administrator), included before switch  gone into listening and learning
state. As a result switch will go into hub mode for some time while it refill
switching table. Of course, you already noted weakness of this theory: switch
learns too fast. After  receiving first frame from victim it writes its MAC
address into switching table and stops to broadcast them to all ports.
However, we must not  ignore this attack. This is because manufacturers
include in their products some ``extensions'' to core STP. Just after
elections network is unreachable. To reduce down time some manufacturers
(Cisco, Avaya, 3Com, HP, etc) include an ability to discard listening and
learning states on the ``user'' ports (ports with servers and workstations
connected to). In other words, port is switching from ``blocked'' state
directly to ``forwarding'' state. This ability has different names: Spanning
Tree Portfast (Cisco - [11]), STP Fast Start (3Com - [12]) etc. If this
ability turned on, eternal elections would lead not to DoS, but to periodical
resets of the switching table, that means hub-mode. Note, that this function
should not be turned ON on the trunk ports, because STP convergence
(finalization of elections to a stable state) not guaranteed in this case.
Fortunately, to achieve its goal an intruder must clear switching table at
least two times fast than interesting packets are received, that is
practically impossible. Anyway it allows collecting of some sensitive data.
Also note, that this attack allows to catch all frames, because it works on
the channel level of OSI and redirects all protocols (including IPX, NETBEUI 
etc), not only IP (as ARP-poisoning).

Other possible attacks.
*=*=*=*=*=*=*=*=*=*=*=*

  These attacks are unchecked, but we suppose, that them are possible.

STP attack on the neighbor VLAN.
*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=

   According 802.1q a bridge with VLAN support can receive on the given
channel either all the frames, or the frames with appropriate tags. In
VLAN-divided networks frames containing STP packets will be transmitted via
trunk link with appropriate tags. So, there is an ability to attack VLAN by
sending STP packets in tagged frames to the port, which doesn't support tags.
Fortunately, according 802.1q a bridge may filter out those frames. For
example, Cisco devices drop down tagged frames on the tag-incompatible ports
(at least, users), that makes this attack impossible. But note, that bridge
MAY, not MUST drop these frames.

STP on WAN links.
*=*=*=*=*=*=*=*=*

   We also must understand, that WAN links are vulnerable to STP attacks too.
This because BCP specification declare STP over PPP support. Surprising
consequence of this fact is an ability to attack ISP network via dial-up
connection. According RFC2878 (BCP description, see [RFC2878]) STP turned on
on the PPP link if both sides requesting it, that never takes place in
practice. Nevertheless, STP supported by default on the majority Cisco
routers, at least models, capable to combine virtual interfaces into bridge
group.

This applies to GARP.
*=*=*=*=*=*=*=*=*=*=*=

   As you may read in the Generic Attribute Registration Protocol (GARP)
specification by 802.1d the STP is a subset of GARP. Some of discussed above
attack work against GARP and, in particular, Generic VLAN Registration
Protocol (GVRP). Therefore VLANs cannot be used as single security measure in
network. 802.1q standard originated from 802.1d and inherits all its defects.
 
   We may continue our research of non-standard using STP. All new materials
will be available on the project web-page (see  [3]).
 

Brief resume.
*=*=*=*=*=*=*

So, we shown that unfortunately all networks supporting 802.1d and, 
with some restrictions, those that support 802.1q are all vulnerable. 

While some devices support STP only if administrator turned on appropriate
option during configuration process, others support STP by default, ``from
the box'' (most of current vendors enable STP by default). Ask your admin:
does our network need STP support? Is STP support turned off on our hardware?

Detection and protection.
*=*=*=*=*=*=*=*=*=*=*=*=*

   What is the main difficulty with STP-based attacks detection? The problem
is that for this attack used standard C-BPDU packets, so presence STP packets
on the network is not strong characteristic of attack. Other difficulty is
that Intrusion Detection System (IDS) must have in its disposal information
about network scheme, at least, list of network devices (with bridges IDs) to
distinguish usual STP traffic from intruder's packets. Moreover, as a main
goal of attack is network availability, IDS must have its own alarm channel.
But note that in this case there possible false negatives - attack will not
detected if malicious BPDUs affect network hardware before IDS disclose them.
Each real network normal state can be described in STP terms. For example, in
a network which normally doesn't use STP appearance of STP packets most likely
signify an STP attack attempt. Series of Root Bridge elections with sequential
lowering Root Bridge ID may signify ``eternal election'' attack. In a network
with fixed list of device IDs appearance of BPDUs with new ID in most cases
may signify an attack (except, of course some ridiculous cases like
installation of new device by ones of poor-coordinated administration team).
We suppose, that most effective solution is adaptive self-learning IDS using
neural networks technology, because the can dynamically compare actual network
state with ``normal'' state. One of most significant measure is STP fraction
in total traffic amount.

Quick fix?
*=*=*=*=*=*

What can network administrators do while problem exists?
 
 - If STP is not barest necessity for your network, it must be disabled. As
   we noted above, in most devices STP is enabled by default.
 - In many cases backup links can be controlled using other mechanisms like
   Link Aggregation. This feature supported by many devices, including Intel,
   Avaya etc.
 - If hardware supports individual STP settings on each port then STP must be
   switched off on all ports except tagged port connected to other network
   hardware, but not user workstations. Especially this must be taken in
   account by ISP, because malicious users may attempt to make DoS against
   either ISP network either other client's networks.
 - If possible administrators must to segment STP realm, i.e. create several
   independent spanning trees. Particularly, if two network segment (offices)
   connected via WAN link, STP on this link must be switched off.


Conclusion
*=*=*=*=*=
 
   Each complicated system inevitably has some errors and communications is
not an exclusion. But this fact is not a reason to stop evolution of
information technologies - we can totally escape mistakes only if we do
nothing. Meanwhile increasing complexity of technologies demand new approach
to development, an approach, which takes in account all conditions and
factors, including information security. We suppose that developers must use
new methods, like mathematical simulation of produced system, which takes in
account not only specified controlling and disturbing impacts on the system,
but also predicts system behavior when input values are outside of specified
range.
 
   It is no wonder that developers in first place take in account primary goal
of system creation and other questions gives little consideration. But if we
don't include appropriate security measures while system development, it is
practically impossible to ``make secure'' this system when it is already
created. At least, this process is very expensive, because core design lacks
are hard to detect and too hard (some times - impossible) to repair  in
contrast to implementation and configuration errors.
  

References
*=*=*=*=*=
  
 [2]  Our article in Russian in LAN-magazine:
      http://www.osp.ru/lan/2002/01/088.htm , also there, in paper:
      Russia, Moscow, LAN, #01/2002, published by ``Open Systems'' publishers.
 [3]  Other materials of this research are published in full at
      http://olli.digger.org.ru/STP
 [4]  Formatted report of our research
      http://olli.digger.org.ru/STP/STP.pdf
 [5]  C-code source of BPDU generation program
      http://olli.digger.org.ru/STP/stp.c
 [6]  Shell script to manipulate STP parameters
      http://olli.digger.org.ru/STP/test.sh
 [7]  ANSI/IEEE 802.1d (Media Access Control, MAC) and ANSI/IEEE 802.1q
      (Virtual Bridged Local Area Networks) can be downloaded from
      http://standards.ieee.org/getieee
 [8]  RFC2878 (PPP Bridging Control Protocol)
      http://www.ietf.org/rfc/rfc2878.txt
 [9]  Description of BPDU
      http://www.protocols.com/pbook/bridge.htm#BPDU
 [10] Assigned Numbers (RFC1700) http://www.iana.org/numbers.html
 [11] Cisco STP Portfast feature
      http://www.cisco.com/warppublic/473/65.html
 [12] Description of STP support on 3Com SuperStack Switch 1000
      http://support.3com.com/infodeli/tools/switches/s_stack2/3c16902/man
      ual.a02/chap51.htm
 [13] Linux Bridge Project
      http://bridge.sourceforge.net/
 [14] Thomas Habets. Playing with ARP
      http://www.habets.pp.se/synscan/docs/play_arp-draft1.pdf
  
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