
The contributions of this work include the following:
• A stochastic model of peer-to-peer botnet formation
• Insight on how anti-malware companies can target
their efforts to fight botnet formation efficiently
• Insight on the necessary level of anti-malware perfor-
mance to fight botnet formation effectively
2. History of the Storm Worm
The first widespread peer-to-peer botnet appeared in Jan-
uary 2007 and is most commonly known as the Storm
Worm, even though the malware is more accurately labeled
a Trojan horse, not a worm. Antivirus firms have also
given the malware other names including Small.DAM, Pea-
comm, Nuware, and Zhelatin [7]. The probable attackers
are thought to be a well-organized group in Russia [17].
The Storm Worm has evolved significantly in the first
year after its initial appearance. The attack process relies
heavily on social engineering tactics. When the botnet was
originally created, the main attack vector was through in-
fected email attachments. Victims would receive an email
with a subject line such as “A killer at 11, he’s free at 21
and kill again,” “U.S. Secretary of State Condolezza Rice
has kicked German Chancellor Angela Merkel,” “Naked
teens attack home director,” “230 dead as storm batters Eu-
rope,” and others [12]. The Storm Worm name refers to the
weather-related subject in some of the emails. The body of
the email was empty.
The Storm Worm email contained an attachment
with names such as “FullVideo.exe,” “Full Story.exe,”
“Video.exe,” “Read More.exe,” or “FullClip.exe”; however,
the attachment was a Trojan horse program, not a video
clip [12]. When victims attempted to open the attached
file, the executable inserted a kernel mode driver compo-
nent called wincom32.sys and an initialization file compo-
nent called peers.ini [4]. The malware also inserted itself
into the services.exe process [12]. After the initial Trojan
had been installed, it attempted to connect with peers in the
Storm Worm botnet and, subsequently, to download the full
payload and begin executing code under the control of the
botnet. Section 3 will further describe the role of peer-to-
peer networking in the Storm Worm.
For the antivirus firms, one of the most challenging as-
pects of the Storm Worm is the sheer number of vari-
ants. Titillating new email subject lines and attachment file
names are added sometimes daily [16]. In addition, the
back-end servers for Storm Worm re-encode the malware
binary twice per hour using polymorphic techniques to min-
imize effectiveness of signature-based detection [16].
Over the course of the year, Storm Worm has also ex-
panded its infection mechanisms from email attachments
to email with links to infected sites and e-card spam that
installs rootkits in the infected computer [10]. The links
to infected sites are sometimes disguised as fake log-in
links [15].
In response to the Storm Worm, antivirus firms have
worked diligently on detection and removal. The biggest
improvement occurred when the Microsoft Malicious Soft-
ware Removal tool issued a patch in September 2007 that
was correlated with a significant 20% drop in the Storm
Worm botnet size [19]. This shows that efforts to remove
machines from a botnet can make progress in reducing the
size of the botnet.
Estimates of the size of the Storm Worm botnet vary
widely. Reports in March 2007 claimed that the size of
the Storm Worm botnet was between 20,000 and 100,000
computers [16]. In September 2007, others claimed the
size of the Storm Worm botnet was between one and
two million machines, although only 10% of the compro-
mised machines were active at one time [9]. In November
2007, Storm was considered the world’s largest botnet with
230,000 active members per 24-hour period [13]. The high-
est estimates say that the Storm Worm botnet could contain
as many as 50 million compromised machines [9].
Regardless of its exact size, researchers agree that the
Storm Worm botnet is currently the world’s largest botnet.
Due to its size, the Storm Worm botnet contributes signifi-
cantly to the malware traffic on the Internet. On August 22,
2007, 57 million virus-infected messages crossed the Inter-
net, and 99% were from the Storm Worm [9].
The most recent reports show that the Storm Worm bot-
net is being divided up because it may be more economi-
cally viable for the operators of the Storm Worm botnet to
run several smaller botnets [13]. Larger botnets are more
conspicuous and easier to detect. The Storm Worm botnet
has begun the use of 40-bit encryption keys, which could
be used to segment the botnet [23]. According to security
researcher Joe Stewart, “This could be a precursor to sell-
ing Storm to other spammers, as an end-to-end spam botnet
system” [23].
The threat of peer-to-peer botnets does not stop with the
Storm Worm. Researchers indicate that a new peer-to-peer
botnet is emerging that could some day surpass the size and
sophistication of the Storm Worm botnet [13].
3. Peer-to-Peer Functionality in the Storm
Worm Botnet
The Storm Worm botnet is the first major botnet to use
peer-to-peer networking for command and control of the
bots. As described earlier in Section 2, the initial infection
involves a Trojan installing some files onto a vulnerable ma-
chine. However, a compromised machine is not part of the
botnet until it joins the peer-to-peer network that controls
the botnet.
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