GNUTELLA: veicolo per virus ed intrusi?

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Self Replication Using Gnutella
Team RAZOR - Seth McGann <>

Remember the game telephone? Yeah, you know…someone whispers something to you,
and you’re supposed to whisper that to
someone else, and so on and so on? And the guy at the end of the chain got a
whisper that sounded nothing like the original
whisper. Well, Gnutella is similar in many ways.

— Excerpt from Gnutella homepage (emphasis added)

Gnutella is a protocol recently re-released after a brief period with no
official home. Now at a new domain and back under
official development, Gnutella is poised to revolutionize media distribution.
Along with its new method of media distribution
it provides some interesting routes for abuse. In light of the recent “ILOVEYOU”
worm, and general rekindling of public
interest in worm design, here is another potential vector that may soon be used
by to distribute malicious content.

=Protocol Overview

Alice: A user searching for a file.
Bob: A user who has the file Alice wants.
Carl: A user who has the file Alice wants (behind firewall).
Mallory: A malicious user.

The key component of Gnutella is a piece of software dubbed a “servent” by the
designers. This servent both accepts and
sends Gnutella messages. When a user wants to join the network, he picks a few
known hosts from a seed list provided by the
Gnutella web site and connects. The user could also create a private Gnutella
network with no exterior connections (remember
no centralization). All that is needed for this subset to become a part of the
larger public GnutellaNet is a single

A --> (GnutellaNet) <-- B <-- | Firewall | <-- C <-- (Other Servents)

Each servent has a unique 128-bit identifier that is used to determine which
packets should be forwarded. Discovery of
hosts is done both actively and passively. Passive discovery occurs when Alice
recovers hosts from the packets she is
routing. Active discovery occurs when Alice sends out a “ping” packet. This
ping elicits a “pong” containing the IP address
and port of every servent that receives it. Searching for content is conducted
as follows:

(1) Alice generates a “query” packet and forwards to all her connections.
(2) Each recipient searches its local database, decrements TTL and forwards to
each of its connection.
(3) Bob receives the packet, and replies with a “query-reply” containing his IP
address and port.
(4) These packets are forwarded back along the search path eventually returning
to Alice.
(5) On receipt of the response, Alice attempts to initiate a direct connection
with Bob and transfer the file.
(6) In the event that she cannot connect for some reason (perhaps Carl has the
file instead of Bob) a “push-request”
containing Alice’s IP address and port is sent by Alice.
(7) On receipt of this packet, Carl will attempt to create an outbound connection
to Alice to transfer the file, circumventing
the firewall.

For further information on the protocol see the Gnutella homepage [1], but this
should be sufficient to understand the
following attacks.


=Self Replicating Servents

The most significant problem is that there is no way to verify the source or
contents of a message. A particularly nasty
attack is for Mallory (a servent attached to GnutellaNet in an arbitrary
location) to simply respond to all query packets.
This results spoofing attack is a known issue covered in the Knowbuddy FAQ [2]
and is conducted as follows:

(1) Alice generates a query packet and forwards.
(2) Mallory receives one of these packets and responds with a query-reply.
(3) If Alice decides to pick the reply provided by Mallory she will receive
potentially malicious content.

This content could be an executable containing a trojan horse, such as BO2K [3].
A more sinister payload would be to create a
trojan that when executed attaches to the GnutellaNet. Once attached to the net,
it would respond to all queries and provide
itself as the content. To increase the probability of Alice executing the file,
it would be renamed to something containing
the original query. For instance, a search for “crack” would elicit a reply of
“”. Other extensions could also be
used besides “.zip” and “.exe”. A Winamp “.pls” file exploiting an overflow in
Winamp could be used for example. With source
code available for both Windows and *nix servents, creating a self-propagating
servent is trivial. The worm could even update
itself using GnutellaNet to distribute signed updates in a manner similar to the
“wormnet” described in Michael Zalewski’s “I
don’t think I really love you” post.
What is disturbing here is the combination of low accountability and trust of
the individual servents. In a web, ftp, or
email transaction if malicious content is discovered on a server the
administrator can be notified. On Napster [4], if a user
is distributing malicious content, his account can be disabled. If removal was
not possible, at the very least a warning
could be posted regarding a particular site or Napster user. With Gnutella
anyone can attach to the network and provide
malicious content tailored to specific search requests with relatively small
chance of detection. The IP address of each
transaction could be recorded, as Zeropaid [5] has done with their listing of
people attempting to retrieve child pornography.
The official servent does not display IP addresses. In addition, Mallory only
has to infect one host and the infection will
spread automatically. Once victims begin executing the content the GnutellaNet
can be used to mask the interactions with the
victims. In most traditional backdoors, a direct connection is required between
the attacker and the victim. With Gnutella,
no direct connection is needed for a bi-directional channel.
The most obvious covert channel for controlling these rogue servents is already
being used to allow chatting among Gnutella

(1) Alice and Bob both turn on their search monitors (which display all the
queries passing through them.
(2) When Alice wants to talk she searches for the chat message (“Hi Bob!”).
(3) Bob will see this query appear in his monitor. He then searches for the
reply (“Hi Alice”).

Another stealthier channel is the 128-bit GUID. This would allow relatively
undetectable signaling between Mallory and her
rouge servents. There are several other fields available but queries and GUIDs
are the most significant.

==Man in the Middle Attacks

There are two flavors of man in the middle attacks. For these to work, Mallory
has to be in the path between Alice and Bob
(or Carl). The first attack goes as follows:

(1) Alice generates a query packet and Bob responds.
(2) Mallory receives one of these query-reply packets and rewrites it with her IP
address and port instead of Bob’s.
(3) Alice receives Mallory’s reply first.
(4) Alice decides to download the content but connects to Mallory instead of Bob.
(5) Mallory downloads the original content from Bob, infects it and passes I on
to Alice.

The second attack relies on push-request interception:

(1) Alice generates a query packet and Carl responds.
(2) Alice attempts to connect but Carl is firewalled, so she generates a
(3) Mallory intercepts the push-request and resends with her IP address and port.
(4) Carl connects to Mallory and transfers his content.
(5) Mallory connects to Alice and provides the modified content.

=Gnutella Port Scanner

If Mallory wants to check what ports on a given host are accessible she can
perform the following steps:

(1) Alice sends a query and Bob responds with a query-reply.
(2) Mallory rewrites the query-reply passing through to point at the IP address
and port to be scanned.
(3) Since most likely the port is not listening, Alice will generate a
(4) If the port happens to be listening and is not running the Gnutella service,
no push-request will be generated.
(5) Mallory repeats the process several times to make sure she has an accurate
determination of the port’s state.

Using this procedure a network can be slowly scanned using servents. This
process will generate a significant amount of
noise, which can be used to decoy another attack. Another way to generate
Gnutella noise is for Mallory to send pong packets
with the IP address of hosts she wants others to attempt to connect to. Each
servent receiving her pong will add these to its
table of potential hosts. As servents exhaust their host tables these entries
will be tried eventually resulting in spurious
connections to a given network.


Gnutella is gaining popularity quickly and has already been featured in several
mass media outlets. As it stands now it
provides an almost ideal environment for the spread of self-replicating malicious
agents with the additional bonus of
providing anonymous control. With full source available, parties previously
unable to craft a worm of their own now have a
robust framework to build on.


-1- - Gnutella Homepage -
-2- - Knowbuddy FAQ -
-3- - Back Orifice 2000 -
-4- - Napster -
-5- - Zeropaid -

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