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computer science technology
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Posts: 740
Joined: Jan 2010
24-01-2010, 10:03 PM



What is a computer virus?
Of primary concern is as to what a computer virus is. A virus is a
computer program that by your help or by attaching itself to some other
program is able to move from one computer to another. Typically these
programs are often malicious rather than beneficial even if they have
no payload associated with them as they snatch away the system
resources. There are several classes of code that fall under the
category virus. Not all of them are strictly virus in technical
terms; some of them are Worms and Trojan horses.
What is a computer worm?
Worms are self replicating programs that do not infect other programs
as viruses do; however they create copies of themselves which in turn
create copies again, thus hogging the memory resources and clogging the
network. Worms are usually seen on networks and multiprocessing OSâ„¢s.
What is a Trojan horse?
Trojan Horses are named after the mythical horse that delivered
soldiers into the city of Troy. Thus they are programs acting as
delivery vehicles; programs that do something undocumented, something
that they conceal about at the time of delivery. Thus they disguise
their real motive behind some seemingly showy one and ask user to do
something at the execution of which some actions are taken which are
desperately unwanted by the user.
Slammer Worm: A glance onto the facts.
Slammer (sometimes called Sapphire) was the fastest computer worm in
history. As it began spreading throughout the Internet, the worm
infected more than 90 percent of vulnerable hosts within 10 minutes,
causing significant disruption to financial, transportation, and
government institutions and precluding any human-based response. In
this seminar and presentation, I wish to describe how it achieved its rapid growth,
dissect portions of the worm to study some of its flaws, and look at
the defensive effectiveness against it and its successors.
Slammer began to infect hosts on Saturday, 25 January 2003, by
exploiting buffer-overflow vulnerability in computers on the Internet
running Microsoft's SQL Server or Microsoft SQL Server Desktop Engine
(MSDE) 2000. David Litchfield of Next Generation Security Software
discovered this underlying indexing service weakness in July 2002;
Microsoft released a patch for the vulnerability before the
vulnerability was publicly disclosed. Exploiting this vulnerability,
the worm infected at least 75,000 hosts, perhaps considerably more, and
caused network outages and unforeseen consequences such as canceled
airline flights, interference with elections, and ATM failures (see

Figure1. The geographical spread of Slammer in the 30 minutes after its
release. The diameter of each circle is a function of the logarithm of
the number of infected machines, so large circles visually under
represent the number of infected cases in order to minimize overlap
with adjacent locations. For some machines, we can determine only the
country of origin rather than a specific city.
Slammer's most novel feature is its propagation speed. In approximately
three minutes, the worm achieved its full scanning rate (more than 55
million scans per second), after which the growth rate slowed because
significant portions of the network had insufficient bandwidth to
accommodate more growth.
Although Stuart Staniford, Vern Paxson, and Nicholas Weaver had
predicted rapid-propagation worms on theoretical grounds, Slammer
provided the first real-world demonstration of a high-speed worm's
capabilities. By comparison, Slammer was two orders of magnitude faster
than the Code Red worm, which infected more than 359,000 hosts on 19
July 2001, and had a leisurely 37 minutes of population doubling time.
While Slammer had no malicious payload, it caused considerable harm by
overloading networks and disabling database servers. Many sites lost
connectivity as local copies of the worm saturated their access
bandwidths. Although most backbone providers appeared to remain stable
throughout the epidemic, there were several reports of Internet
backbone disruption.
If the worm had carried a malicious payload, attacked a more widespread
vulnerability, or targeted a more popular service, its effects would
likely have been far more severe.
How Slammer chooses its victims
The worm's spreading strategy uses random scanning--it randomly selects
IP addresses, eventually finding and infecting all susceptible hosts.
Random-scanning worms initially spread exponentially, but their rapid
new-host infection slows as the worms continually retry infected or
immune addresses. Thus, as with the Code Red worm shown in Figure 2,
Slammer's infected-host proportion follows a classic logistic form of
initial exponential growth in a finite system. We label this growth
behavior a random constant spread (RCS) model.
Figure 2. The Code Red worm was a typical random-scanning worm. This
graph shows Code Red's probe rate during its re-emergence on 1 August,
2001, as seen on one Internet sub network, matched against the random
constant spread worm behavior model.
Slammer's spread initially conformed to the RCS model, but in the later
stages it began to saturate networks with its scans, and bandwidth
consumption and network outages caused site-specific variations in its
observed spread. Figure 3 shows a data set from the Distributed
Intrusion Detection System project and implimentation (Dshield) compared to an RCS model.
The model fits extremely well up to a point where the probe rate
abruptly levels out.
Bandwidth saturation and network failure (some networks shut down under
the extreme load) produced this change in the probe's growth rate.

Figure 3. The early moments of the Distributed Intrusion Detection
System data set, matched against the behavior of a random-scanning
Why Slammer was so fast?
While Slammer spread nearly two orders of magnitude faster than Code
Red, it probably infected fewer machines. Both worms use the same basic
scanning strategy to find vulnerable machines and transfer their
exploitive payloads; however, they differ in their scanning
constraints. While Code Red is latency-limited, Slammer is bandwidth-
limited, enabling Slammer to scan as fast as a compromised computer can
transmit packets or a network can deliver them. Slammer's 376 bytes
comprise a simple, fast scanner. With its requisite headers, the
payload becomes a single 404-byte user datagram protocol (UDP) packet.
Contrast Slammer's 404 bytes with Code Red's 4 Kbytes or Nimda's 60
Previous scanning worms, such as Code Red, spread via many threads,
each invoking connect() to open a TCP session to random addresses.
Consequently, each thread's scanning rate was limited by network
latency. After sending a TCP SYN packet to initiate the connection,
each thread must wait to receive a corresponding SYN/ACK packet from
the target host or time-out if no response is received. During this
time, the thread is blocked and cannot infect other hosts. In
principle, worms can compensate for this latency by invoking a
sufficiently large number of threads. In practice, however, operating
system limitations, such as context-switch overhead and kernel stack
memory consumption, limit the number of active threads a worm can use
effectively. So, a worm like Code Red quickly stalls and becomes
latency limited, as every thread spends most of its time waiting for
In contrast, Slammer's scanner is limited by each compromised machine's
Internet bandwidth. Because a single packet to UDP port 1434 could
exploit the SQL server's vulnerability, the worm was able to broadcast
scans without requiring responses from potential victims. Slammer's
inner loop is very small, and with modern servers' I/O capacity to
transmit network data at more than 100 Mbits per second, Slammer
frequently was limited by Internet access bandwidth rather than its
ability to replicate copies of itself.
In principle, an infected machine with a 100-Mbps Internet connection
could produce more than 30,000 scans per second. In practice, bandwidth
limitations and per-packet overhead limit the largest probe rate we
directly observed to 26,000 scans per second, with an Internet-wide
average of approximately 4,000 scans per second per worm during its
early growth phase.
Slammer's scanning technique is so aggressive that it quickly
interferes with its own growth. Subsequent infections' contribution to
its growth rate diminishes because those instances must compete with
existing infections for scarce bandwidth. Thus, Slammer achieved its
maximum Internet-wide scanning rate in minutes.
Any simple-loop approach creates a bandwidth-limited UDP scanner, so
any future single-packet UDP worm probably would have the same property
unless its author deliberately limited its spread. While a TCP-based
worm, such as Code Red, also could use a bandwidth-limited scanner by
sending TCP SYNs at maximum rate and responding automatically to any
replies in another thread, this would require more effort to implement
correctly, as it requires crafting raw TCP packets instead of simply
using existing system calls.
What Slammer's author did wrong?
For a random-scanning worm to be effective, it needs a good source of
random numbers to select new attack targets. Slammer's random-number
generator has some interesting deficiencies that make its analysis
difficult and, perhaps, have implications for future worms. Several
disassembled versions of the worm's source code are available at the
URLs shown in the "Worm guts".
Slammer uses a linear congruent or power residue, pseudo random number
generation (PRNG) algorithm. These algorithms take the form: x' = (x *
a + b) mod m, where x' is the new pseudo random number to be generated,
x is the last pseudo random number generated, m represents the range of
the result, and a and b are carefully chosen constants. Linear
congruent generators are very efficient and, with appropriate values of
a and b have reasonably good distributional properties (although they
are not random from a sequential standpoint). Typically, you "seed" a
generator's initial value with a source of high-quality random numbers
to ensure that the precise sequence is not identical between runs.
Slammer's author intended to use a linear congruent parameterization
that Microsoft popularized, x' = (x * 214013 + 2531011) mod 232.
However, it was found containing two implementation mistakes. First,
the author substituted a different value for the 2531011 increment
value: hex 0xFFD9613C. This value is equivalent to ¬2531012 when
interpreted as a twos-complement decimal. So, it seems likely that the
conversion to a negative number was an error (the author seems to have
forgotten that creating a negative number in twos complement requires
inverting and adding 1, not simply inverting), and probably that the
author intended to use the SUB instruction to compensate for the
resulting negative number, but mistakenly used ADD instead. The
negative constant would be more desirable in the code, as this would
eliminate any null (all-zero) characters from the worm's code. The
result is that the increment is always even.
The author's second mistake was to misuse the OR instruction, instead
of XOR, to clear an important register. This error left the register's
previous contents intact. As a result, the increment inadvertently
XORâ„¢s with the contents of a pointer contained in SqlSort's import
address table (IAT). This "salt" value differs, depending on the
SqlSort DLL's version, although two common values we observed are
0x77F8313C and 0x77E89B18 seeing 0x77EA094C, which is what the register
value needs to be to get what eEye says is their result.
These mistakes significantly reduce the generator's distribution
quality. Because b is even and the register is always 32-bit aligned,
the least-significant two bits are always zero. Interpreted as a big-
endian IP address (the most significant value in the sequence is stored
at the lowest storage address), this ensures that the 25th and 26th
bits in the scan address (the upper octet) remain constant in any worm
execution instance. Similar weaknesses extend to the 24th bit of the
address, depending on the unclear register's value. Moreover, with the
incorrectly chosen increment, any particular worm instance cycles
through a list of addresses significantly smaller than the actual
Internet address space. Thus, many worm instances will never probe our
monitored addresses because none of them are contained in the worm's
scan cycle. Combined with the size of monitored address space, these
mistakes prevent from accurately measuring the number of infected hosts
during the first minutes of the worm's spread.
Slammer will or will not include entire /16 address blocks (large
contiguous address ranges) in a cycle, because the last two bits of the
first address byte never change. That enabled the assembling lists of
the address blocks in each cycle for each value of the salt (cycle
structure depends on salt value). Fortunately, the probability of
choosing a particular cycle is directly proportional to the size of the
cycle if the initial seed is selected uniformly at random. If we looked
at many randomly seeded worms, it is likely that all Internet addresses
would be probed equally. Thus, we can accurately estimate the worm's
scanning rate during the infection process by monitoring relatively
small address ranges. We can estimate the percentage of the Internet
that is infected because the probing will cover all Internet addresses.
If not for the higher-quality numbers in the initial seed, these flaws
would prevent the worm from reaching large portions of the Internet
address space, no matter how many hosts were infected. For the same
reason, these flaws also could bias measurements because even though
data come from several different networks, there is a small chance that
those particular networks were disproportionately more or less likely
to be scanned. However, the worm uses an operating system service, Get
-Tick-Count, to seed each generator with the number of milliseconds
since a system booted, which should provide sufficient randomization to
ensure that across many instances of the worm, at least one host will
probe each address at some point in time. We feel confident that the
risk of bias in our measurements is similarly minimized. Additionally,
the "best" random bits produced by Get-Tick-Count are in the least-
significant bits, which determine which cycle is selected for a given
An interesting feature of this PRNG is that it makes it difficult for
the Internet community to assemble a list of the compromised Internet
addresses. With earlier worms, we could collect a list of all addresses
that probed into a large network. With Slammer, we would need to
monitor networks in every cycle of the random-number generator for each
salt value to have confidence of good coverage.
How the Internet responded?
By passively monitoring traffic (either by sniffing or sampling packets
or monitoring firewall logs) on a set of links providing connectivity
to multiple networks, each responsible for about 65,000 IP addresses,
the worm's overall scanning behavior over time was inferred.
The most-accurate Slammer early-progress data was obtained from the
University of Wisconsin Advanced Internet Lab (WAIL), which logs all
packet traffic into an otherwise unused network, a "tarpit" (see Figure
4). Because this data set represents a complete trace of all packets to
an address space of known size, it lets us accurately extrapolate the
worm's global spread. Unfortunately, a transient failure in data
collection temporarily interrupted this data set approximately 2
minutes and 40 seconds after Slammer began to spread. Other sampled
data sets are not sufficiently precise for accurate evaluation over
short durations.
Figure 4. Slammer's early progress as measured at the University of
Wisconsin Advanced Internet Lab (WAIL) tarpit, an unused network that
logs packet traffic. Scanning rate is scaled to estimate the Internet-
wide scanning rate. (A transient data-collection failure temporarily
interrupted this data set approximately two minutes and 40 seconds
after Slammer began to spread.)
In general, people responded quickly to Slammer. Within an hour, many
sites began filtering all UDP packets with a destination port of 1434
via router or firewall configuration changes. Slammer represents the
idealized situation for network-based filtering: its signature easily
distinguishes it, it is readily filterable on current hardware, and it
attacked a port that is not generally used for critical Internet
communication. Thus, almost all traffic blocked by these filters
represents worm-scanning traffic. If the worm had exploited a commonly
used service vulnerability (for example, DNS at UDP port 53 or HTTP at
TCP port 80), filtering could have caused significant disruption to
legitimate traffic,
with resulting denial of services(DoS) more harmful than the worm
itself. Figure 5 illustrates the results of filtering.

Figure 5. The response to Slammer during the 12 hours after its
release, measured in several locations. Scanning rate is scaled to
estimate the Internet-wide scanning rate.
Regardless of the optimal filter efficacy conditions in this instance,
we must recognize that while filtering controlled the unnecessary
bandwidth consumption of infected hosts, it did nothing to limit the
worm's spread. The earliest filtering began long after Slammer had
infected almost all susceptible hosts.
All distinct infected IP addresses recorded were seen by monitors in
the first 30 minutes of worm spread. We noticed 74,856 distinct IP
addresses, spread across a wide range of domains and geographic
locations. Slammer infected most of these machines in the first few
minutes, but monitoring limitations prevent us from telling precisely
when they were infected. We cannot observe all infected machines due to
the flaws in Slammer's PRNG, but we can document a lower bound on the
number of compromised machines based on the IP addresses we have
recorded--the actual infection is undoubtedly larger. Tables 1 and 2
summarize these distributions.
able 1. Slammer's geographical distribution.
United States 42.87
South Korea 11.82
Unknown 6.96
China 6.29
Taiwan 3.98
Canada 2.88
Australia 2.38
United Kingdom 2.02
Japan 1.72
Netherlands 1.53
Table 2. Slammer's top-level domain distribution.
Unknown 59.49
.net 14.37
.com 10.75
.edu 2.79 1.29
.au 0.71
.ca 0.71
.in 0.65
.br 0.57
.uk 0.57
Why Slammer caused problems?
Although Slammer did not contain an explicitly malicious payload, there
were widely reported incidences of disruption, including failures of
Bellevue, Washington's 911 emergency's data-entry terminals and
portions of Bank of America's ATM network. The 911 and ATM failures
were widely reported: Inadvertent internal DoS attacks caused the large
majority of these disruptions: as one or more infected machines sent
out packets at their maximum possible rates. This traffic either
saturated the first shared bottleneck or crashed some network
equipment. The bottleneck effects are obvious, as a site's outgoing
bandwidth is usually significantly less than a Slammer's instance can
consume. Thus, the worm's packets saturated Internet links, effectively
denying connectivity for all computers at many infected sites.
Equipment failures tended to be a consequence of Slammer's traffic
patterns generated by infected machines, although any given equipment's
failure details varied. Slammer's scanner produced a heavy load in
three ways: a large traffic volume, lots of packets, and a large number
of new destinations (including multicast addresses). We feel this
combination probably caused most network-equipment failures by
exhausting CPU or memory resources.
If attackers can control a few machines on a target network, they can
perform a DoS attack on the entire local network by using a program
that mimics Slammer's behavior. Because these are "normal" UDP packets,
special privileges (such as root or system administrator abilities) are
not required. Instead, they need only the ability to execute the
attacker's program. Thus, critical networks should employ traffic
shaping, fair queuing, or similar techniques, to prevent a few machines
from monopolizing network resources.
Although some had predicted the possibility of high-speed worms,
Slammer represents the first super-fast worm released into the wild.
Microsoft's SQL Server vulnerability was particularly well suited for
constructing a fast worm (because the exploit could be contained in a
single UDP packet). However, techniques exist to craft any worm with a
reasonably small payload into a bandwidth-limited worm similar to
Slammer. Thus, we must not view Slammer's extreme speed as an
aberration of the exploit's nature or the particular protocol used (UDP
versus TCP).
One implication of this worm's emergence is that smaller susceptible
populations are now more attractive to attacks. Formerly, attackers did
not view small populations (20,000 machines or less on the Internet) as
particularly vulnerable to scanning worms because the probability of
finding a susceptible machine in any given scan is quite low. However,
a worm that can infect 75,000 hosts in 10 minutes could infect 20,000
hosts in under an hour. Thus, vulnerabilities of less popular software
present a viable breeding ground for new worms.
Because high-speed worms are no longer a theoretical threat, we need to
automate worm defenses; there is no conceivable way for system
administrators to respond to threats of this speed. Human-mediated
filtering provides no benefit for actually limiting the number of
infected machines. While the filtering could mitigate the overhead of
the worm's continuing scan traffic, a more sophisticated worm might
stop scanning once the entire susceptible population was infected,
leaving itself dormant on more than 75,000 machines to do harm at some
future point. Had the worm's propagation lasted only 10 minutes, it
would likely have taken hours or days to identify the attack, and many
compromised machines might never have been identified. Thus, it is
critical that we develop new techniques and tools that automatically
detect and respond to worms.
Though very simple, Slammer represents a significant milestone in the
evolution of computer worms. It is sobering that worms such as Slammer
preclude human-time response and, to date, deter attribution and
prosecution. It clearly demonstrates that fast worms are a reality and
we should consider them as standard tools in an attacker's arsenal.
Who wrote Slammer?
After much study, very few clues were found about Slammer's author's
identity, location, or motive. No one has claimed authorship and the
codes do not contain identifying signatures or comments. There also are
no variable names in the code, making it impossible to determine the
author's native tongue. Similarly, the author has decent, but not
remarkable, x86 coding skill; much of the code was borrowed from a
published exploit, the additional code is not very complicated, and the
author made three minor mistakes in the random-number generator.
Finally, no one has discovered patient 0, the initial point of
infection. Thus, it is impossible to determine where the worm was
released, in hopes of tracing the release back to the author.
The above points do suggest that even if we take giant leap in the
technological advancement and make virtually everything secure itâ„¢s
always possible for human mind to explore beyond the unobvious. The
firewalls and other security measures can be got away from; the safest
site can be hacked; and even the most intricate encryption can be
decrypted. Thus in this world of fictitious reality making anything
invulnerable is not viable. The need however is to anticipate anything.
The most sites hacked are government owned. Despite all this the
facilities they provide are unquestionable. Security or no security the
Internet will perpetuate and itâ„¢s usersâ„¢ responsibility to fill his own
system with added measures.
The time demands to learn from past mistakes because most of the
malicious programmers use those codes that have been used previously;
so to provide functions against them will undoubtedly be of help.
Lastly, to live in this crude world is impossible without being aware.
To know that there can be thwarts and there have been thwarts certainly
help. Only the curiosity toward knowledge can bring knowledge and only
a beforehand preparation can make the threats less precarious. For, the
quest for wisdom will never end and nor will bow down the thus produced
monster. Attention is only needed for warding them off.
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khandelwal kalpesh
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25-01-2010, 06:29 PM

genetic programming
computer science technology
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Joined: Jan 2010
25-01-2010, 11:57 PM

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