3.3. DoS and DDoS Evolution
| There are many security problems in today's Internet. E-mail viruses lurk to infect machines and spread further, computer worms sometimes silently swarm the Internet, competitors and your neighbor's kids attempt to break into company machines and networks and steal industrial secrets, DDoS attacks knock down online services. The list goes on and on. None of these problems have been completely solved so far, but many have been significantly lessened through practical technological solutions. Firewalls have greatly helped reduce the danger from the intrusions by blocking all but necessary incoming traffic. Antivirus programs prevent the execution of known worms and viruses on the protected machine, thus defeating infection and stopping the spread. Applications and operating systems check for software updates and patch themselves automatically, greatly reducing the number of vulnerabilities that can be exploited to compromise a host machine. However, the DoS problem remains largely unhandled, in spite of great academic and commercial efforts to solve it. Sophisticated firewalls, the latest antivirus software, automatic updates, and a myriad of other security solutions improve the situation only slightly, defending the victim from only the crudest attacks. It is, however, strikingly easy to generate a successful DDoS attack that bypasses these defenses and takes the victim down for as long as the attacker wants. And there is often nothing a victim can do about it. How did these tools develop, and how are they being used? We will now look at a combined history of the development of network-based DoS and DDoS attack tools, in relation to the attacks waged with them. 3.3.1. History of Network-Based Denial of ServiceThe developmental progression of DoS and DDoS tools and associated attacks can give great insight into likely trends for the future, as well as allowing an organization to gauge the kinds of defenses they need to consider based on what is realistic to expect from various attackers, from less skilled up to advanced attackers. These are only some representative tools and attacks, not necessarily all of the significant attacks. For more stories, see http://staff.washington.edu/dittrich/misc/ddos/. The Late 1980sThe CERT Coordination Center (CERT/CC, which was originally founded by DARPA as the Computer Emergency Response Team) at Carnegie Mellon University's Software Engineering Institute was established in 1988 in response to the so-called Morris worm, which brought the Internet to its knees.[5] The CERT Coordination Center has a long-established expertise in handling and responding to incidents in the Internet, analyzing and reporting vulnerabilities within systems, as well as research in computer and network security, and networked system survivability. Figure 3.2 presents a CERT Coordination Center created summary of the trends in attacker tools over the last few years.
Figure 3.2. Over time, attacks on networked computer systems have become more sophisticated, but the knowledge attackers need has decreased. Intruder tools have become more sophisticated and easy to use. Thus, more people can become "successful intruders" even though they have less technical knowledge. (Reprinted with permission of the CERT Coordination Center.)The Early 1990sAfter the story of the Morris worm incident died out, the Internet continued to grow through the early 1990s into a fun place, with lots of free information and services. More and more sites were added, and Robert Metcalf stated his now famous law: The usefulness, or utility, of a network equals the square of the number of users. But as we saw earlier in Section 3.1, some percentage of these new users will not be nice, friendly users. In the mid-1990s, remote DoS attack programs appeared and started to cause problems. In order to use these programs, one needed an account on a big computer, on a fast network, to have maximum impact. This led to rampant account theft at universities in order for attackers to have use of stolen accounts to run DoS programs as they were easy to identify and shut down, they were often considered "throwaway" accounts which drove a market for installing sniffers.[6] These accounts would be traded for pirated software, access to hard-to-reach networks, stolen computers, credit card numbers, cash, etc. At the time, flat thick-wire and thin-wire Ethernet networks were popular, as was the use of telnet and ftp (both of which suffered from a clear-text password problem). The result of this architectural model, combined with vulnerable network services, were networks that were easy prey to attackers running network sniffers.
1996In 1996, a vulnerability in the TCP/IP stack was discovered that allowed a flood of packets with only the SYN bit set (known as a SYN flood; see Chapter 4 for detailed description). This became a popular and effective tool to use to make servers unavailable, even with moderate bandwidth available to an attacker (which was a good thing for attackers, as modems at this time were very slow). Small groups used these tools at first, and they circulated in closed groups for a time. 1997Large DoS attacks on IRC networks began to occur in late 1996 and early 1997. In one attack, vulnerabilities in Windows systems were exploited by an attacker who took out large numbers of IRC users directly by crashing their systems [Val]. DoS programs with names like teardrop, boink, and bonk allowed an attacker to crash unpatched Windows systems at will. In another attack, a Romanian hacker took out portions of IRC network Undernet's server network with a SYN flood [KC]. SYN flood attacks on IRCg networks are still prevalent today. A noteworthy event in 1997 was the complete shutdown of the Internet due to (nonmalicious) false route advertisement by a single router [Bon97] For the most part, these DoS vulnerabilities are simple bugs that were fixed in subsequent releases of the affected operating systems. For example, there were a series of bugs in the way the Microsoft Windows TCP/IP stack handled fragmented packets. One bug in Windows' TCP/IP stack didn't handle fragmented packets whose offset and length did not match properly, such that they overlapped. The TCP/IP stack code authors expected packets to be properly fragmented and didn't properly check start/end/offset conditions. Specially crafted overlapping packets would cause the stack to allocate a huge amount of memory and hang or crash the system.[7] As these bugs were fixed, attackers had to develop new means of performing DoS attacks to up the ante and increase the disruption capability past the point where it can be stopped by simple patches.
Another effective technique that appeared around 1997 was a form of reflected, amplified DoS attack, called a Smurf attack. Smurf attacks allowed for amplification from a single source. By bouncing packets off a misconfigured network, attackers could amplify the number of packets destined for a victim by a factor of up to 200 or so for a so-called Class C or /24 network, or by a factor of several thousands for a moderately populated Class B or /16 network. The attacker would simply craft packets with the return address of the intended victim, and send those packets to the broadcast address of a network. These packets would effectively reach all available and responsive hosts on that particular network and elicit a response from them, Since the return address of the requests was forged, or spoofed, the response would be sent to the victim.[8]
Attackers next decided to explore another avenue for crashing machines. Instead of exploiting a vulnerability, they just sent a lot of packets to the target. If the target was on a relatively slow dial-up connection (say, 14.4Kbps), but the attacker was using a stolen university computer on a 1Mbps connection, she could trivially overwhelm the dial-up connection and the computer would be lagged, or slowed down to the point of being useless (each keystroke typed could take 10 seconds or more to be received and echoed). 1998As the bandwidth between attacker and target became more equal, and more network operators learned to deal with simple Smurf attacks, the ability to send enough traffic at the target to lag it became harder and harder to achieve. The next step was to add the ability to control a large number of remotely located computers someone else's computers and to direct them all to send massive amounts of useless traffic across the network and flood the victim (or victims). Attackers began organizing themselves into coordinated groups, performing an attack in concert on a victim. The added packets, whether in sheer numbers or by overlapping attack types, obtained the desired effect. Prototypes of DDoS tools (most notably fapi, see [CER99]) were developed in mid-1998, serving as examples of how to create client/server DDoS networks. Rather than relying on a single source, attackers could now take advantage of all the hosts they can compromise to attack with. These early programs had many limitations and didn't see widespread use, but did prove that coordination of computers in an attack was possible. Vulnerability-based attacks did not simply go away, and in fact continued to be possible due to a constant stream of newly discovered bugs. Successful DoS attacks using the relatively simple fragmented packet vulnerability targeting tens to hundreds of thousands of vulnerable hosts have been waged in the past. For example, the attacks in 1998, took advantage of fragmented packet vulnerabilities but added scripts to prepare the list of vulnerable hosts in advance and then rapidly fired exploit packets at those systems. This attacker prepared the list by probing potential victims for the correct operating system (by a technique called OS fingerprinting, which can be defeated by packet normalization [SMJ00]) and for the existence of the vulnerability, and creating a list of those that "pass the test." The University of Washington network was one of the victims of the attack. In computer labs across campus, some with hundreds of PCs being busily used by students doing their homework, the sound of keystrokes turned to dead silence as every screen in the lab went blue, only to be replaced seconds later with the chatter of "Hey, did your computer just crash too?" A large civilian U.S.-based space agency was a similar victim of these attacks during the same time period. Users at many locations were subjected to immediate patching, since subsequent controlled tests clearly turned unpatched systems' screens (and their respective users) blue. The next step to counter the patching issue, which made it harder for an attacker to predict which DoS attack would be effective, was to combine multiple DoS exploits into one tool, using Unix shell scripts. This increased the speed at which an effective DoS attack could be waged. One such tool, named rape, (according to the code, written in 1998) integrates the following DoS attacks into a single shell script: echo "Editted for use with www.ttol.base.org" echo "rapeing $IP. using weapons:" echo "latierra " echo -n "teardrop v2 " echo -n "newtear " echo -n "boink " echo -n "bonk " echo -n "frag " echo -n "fucked " echo -n "troll icmp " echo -n "troll udp " echo -n "nestea2 " echo -n "fusion2 " echo -n "peace keeper " echo -n "arnudp " echo -n "nos " echo -n "nuclear " echo -n "ssping " echo -n "pingodeth " echo -n "smurf " echo -n "smurf4 " echo -n "land " echo -n "jolt " echo -n "pepsi " A tool like this has the advantage of allowing an attacker to give a single IP address and have multiple attacks be launched (increasing the probability of a successful attack), but it also means having to have a complete set of precompiled versions of each individual exploit packaged up in a Unix "tar" format archive for convenient transfer to a stolen account from which to launch the attack. To still allow multiple DoS exploits to be used, but with a single precompiled program that is easier to store, transfer, and use quickly, programs like targa.c by Mixter were developed (this same strategy was used again in 2003 by Agobot/Phatbot). Targa combines all of the following exploits in a single C source program: /* targa.c - copyright by Mixter <mixter@gmx.net> version 1.0 - released 6/24/98 - interface to 8 multi-platform remote denial of service exploits */ . . . /* bonk by route|daemon9 & klepto * jolt by Jeff W. Roberson (modified by Mixter for overdrop effect) * land by m3lt * nestea by humble & ttol * newtear by route|daemon9 * syndrop by PineKoan * teardrop by route|daemon9 * winnuke by _eci */ Even combined DoS tools like targa still only allowed one attacker to DoS one IP address at a time, and they required the use of stolen accounts on systems with maximum bandwidth (predominantly university systems). To increase the effectiveness of these attacks, groups of attackers, using IRC channels or telephone "voice bridges" for communication, could coordinate attacks, each person attacking a different system using a different stolen account. This same coordination was being seen in probing for vulnerabilities, and in system compromise and control using multiple backdoors and "root kits." The years 1998 and 1999 saw a significant increase in the ability to attack computer systems, which came as a direct result of programming and engineering efforts, the same ones that were bringing the Internet to the world: automation, increases in network bandwidth, client/server communications, and global chat networks. 1999In 1999, two major developments grew slowly out of the computer underground and began to take shape as everyday attack methods: distributed computing (in the forms of distributed sniffing, distributed scanning, and distributed denial of service), and a rebirth of the worm (the worm simply being the integration and automation of all aspects of intrusion: reconnaissance scanning, target identification, compromise, embedding, and attacker control). In fact, many worms today (e.g., Nimda, Code Red, Deloder, Lion, and Blaster) either implement a DDoS attack or bundle in DDoS tools. The summer of 1999 saw the first large-scale use of new DoS tools: trinoo [Ditf], Tribe Flood Network (TFN) [Dith], and Stacheldraht [Ditg]. These were all simple client/server style programs handlers and agents, as mentioned earlier that performed only DDoS-related functions of command and control, various types of DoS attacks, and automatic update in some cases. They required other programs to propagate them and build attack networks, and the most successful groups using these tools also used automation for scanning, target identification, exploitation, and installation of the DDoS payload. The targets of nearly all the attacks in 1999 were IRC clients and IRC servers. One prominent attack against the IRC server at the University of Minnesota and a dozen or more IRC clients scattered around the globe was large enough to keep the University's network unusable for almost three full days. This attack used the trinoo DDoS tool, which generated a flood of UDP packets with a 2-byte payload and did not use IP spoofing. The University of Minnesota counted 2,500 attacking hosts, but the logs were not able to keep up with the flood, so that count was an underestimate.[9] These hosts were used in several DDoS networks of 100 to 400 hosts each, in rolling attacks that activated and deactivated groups of hosts. This made locating the agents, identification, and cleanup take several hours to a couple of days. The latency in cleanup contributed to the attack's duration.
The change to using distributed tools was inevitable. The growth of network bandwidth that came about through the development of Internet2 [Con] (officially launched in 2000) made simple point-to-point tools less effective against well-provisioned networks, and attacks that used single hosts for flooding were easy to filter out and easy to track back to their source and stop. Scripting of vulnerability scans (which are also made faster because of the same bandwidth increase) and scripting of attacks made it much easier to compromise hundreds, thousands, even tens of thousands of hosts in just a few hours. If you control thousands of computers, why not program them to work in a coordinated manner for leverage? It wasn't a large leap to automate the installation of attack tools, back doors, sniffers whatever the attackers wanted to add. Private communications with some of the authors of the early DDoS tools[10] indicated the motivation for this automation was for a smaller group of attackers to counter attacks wielded at it by a large group (using the classic DoS tools described above, and manual coordination). The smaller group could not wield the same number of manual tools and thus resorted to automation. In almost all cases, these were first coding efforts, and even with bugs they were quite effective at waging large-scale attacks. The counterattacks were strikingly effective, and the smaller group was able to retake all their channels and strike back at the larger group.
Similar attacks, although on a slightly smaller scale, continued through the late fall of 1999, this time using newer tools that spoofed source addresses, making identification of attackers even harder. Nearly all of these attacks were directed at IRC networks and clients, and there was very little news coverage of them. Tribe Flood Network (or TFN), Stacheldraht, and then Tribe Flood Network 2000 (TFN2K) saw popular use, while Shaft showed up in more limited attacks. In November of 1999, CERT Coordination Center sponsored (for the first time ever) a workshop to discuss and develop a response to a situation they saw as a significant problem of the day, in this case Distributed System Intruder Tools (including distributed scanners, distributed sniffers, and distributed denial of service tools.) The product of the workshop was a report [CER99], which covered the issue from multiple perspectives (those of managers, system administrators, Internet service providers, and incident response teams). This report is still one of the best places to start in understanding DDoS, and what to do about it in the immediate time frame (< 30 days), medium term (30 180 days) and long term (> 180 days). Ironically, only days after this workshop, a new tool was discovered that was from a different strain of evolution, yet obeyed the same attack principles as trinoo, TFN, and Stacheldraht [Ditf, Dith, Ditg]. The Shaft [DLD00] tool, as discussed in Chapter 4, was wreaking havoc with only small numbers of agents across Europe, the United States, and the Pacific Rim, and caught the attention of some analysts. This next generation of tools included features such as encryption of communications (to harden the DDoS tools against counterattack and detection), multiplicity of attack methods, integrated chat functions, and reporting of packet flood rates. This latter function is used by the attacker to make it easier to determine the yield of the DDoS attack network, and hence when it is large enough to obtain the desired goal and when (due to attrition) it needs to be reinforced. (The attackers, at this point, knew more about how much bandwidth they were using than many network operators knew was being consumed. Refer to the discussion of Shaft in Chapter 4 for more on this statistics feature, and the OODA Loop in Chapter 4 to understand how this affects the attacker/defender balance of power.) As more and more groups learned of the power of DDoS to engage in successful warfare against IRC sites, more DDoS tools were developed. As Y2K Eve approached, many security professionals feared that widespread DDoS attacks would disrupt the telecommunications infrastructure, making it look like Y2K failures were occurring and panicking the public [SSSW]. Thankfully, these attacks never happened. That did not mean that attack incidence slowed down, however. 2000On January 18, 2000, a local ISP in Seattle, Washington, named Oz.net was attacked [Ric]. This appeared to be a Smurf attack (or possibly an ICMP Echo reply flood with a program like Stacheldraht). The unique element of this attack was that it was directed against not only the servers at Oz.net, but also at their routers, the routers of their upstream provider Semaphore, and the routers of the upstream's provider, UUNet. It was estimated that the slowdown in network traffic affected some 70% of the region surrounding Seattle. Within weeks of this attack, in February 2000, a number of DDoS attacks were successfully perpetrated against several well-provisioned and carefully run sites many of the major "brand names" of the Internet. They included the online auction company eBay (with 10 million customers), the Internet portal Yahoo (36 million visitors in December 1999), online brokerage E*Trade, online retailer Buy.com (1.3 million customers), online bookselling pioneer Amazon.com, Web portal Excite.com, and the widely consulted news site CNN [Har]. Many of these sites were used to high, fluctuating traffic volume, so they were heavily provisioned to provide for those fluctuations. They were also frequent targets of other types of cyberattacks, so they kept their security software up to date and maintained a staff of network administration professionals to deal with any problems that arose. If anyone in the Internet should have been immune to DDoS problems, these were the sites. (A detailed timeline of some of the lesser known events surrounding the first public awareness of DDoS, leading up to the first major DDoS attacks on e-commerce sites, is provided in the side bar at the end of this chapter.) However, the attacks against all of them were quite successful, despite being rather unsophisticated. For example, the attack against Yahoo in February 2000 prevented users from having steady connectivity to that site for three hours. As a result, Yahoo, which relies on advertising for much of its revenue, lost potentially an estimated $500,000 because its users were unable to access Yahoo's Web pages and the advertisements they carried [Kes]. The attack method used was not sophisticated, and thus Yahoo was eventually able to filter out the attack traffic from its legitimate traffic, but a significant amount of money was lost in the process. Even the FBI's own Web site was taken out of service for three hours in February of 2000 by a DDoS attack [CNN]. 2001In January 2001, a reflection DDoS attack (described in Section 2.3.3) [Pax01] on futuresite.register.com [el] used false DNS requests sent to many DNS servers throughout the world to generate its traffic. The attacker sent many requests under the identity of the victim for a particularly large DNS record to a large number of DNS servers. These servers obediently sent the actually undesired information to the victim. The amount of traffic inbound to the victim's IP address was reported to be 60 to 90 Mbps, with one site reporting seeing approximately 220 DNS requests per minute per DNS server. This attack lasted about a week, and could not be filtered easily at the victim network because simple methods of doing so would have disabled all DNS lookup for the victim's customers. Because the attack was reflected off many DNS servers, it was hard to characterize which ones were producing only attack traffic that should be filtered out. Arguably, the DNS servers responding to this query really shouldn't have done so, since it would not ordinarily be their business to answer DNS queries on random records from sites not even in their domain. That they did so could be considered a bug in the way DNS functions, a misconfiguration of DNS servers, or perhaps both, but it certainly underscores the complexity of Internet protocols and protocol implementations and inherent design issues that can lead to DoS vulnerabilities. The number and scope of DDoS attacks continued to increase, but most people (at least those who didn't use IRC) were not aware of them. In 2001 David Moore, Geoffrey Voelker, and Stephan Savage published a paper titled, "Inferring Internet Denial-of-Service Activity" [MVS01]. This work investigates Internet-wide DDoS activity and is described in detail in Appendix C. The technique for measuring DDoS activity underestimates the attack frequency, because it cannot detect all occurring attacks. But even with these limitations, the authors were able to detect approximately 4,000 attacks per week for the three-week period they analyzed. Similar techniques for attack detection and evaluation were already in use among network operators and network security analysts in 1999 and 2000. What many operators flagged as "RST scans" were known to insiders as "echoes" of DDoS attacks [DLD00]. Microsoft's prominent position in the industry has made it a frequent target of attacks, including DDoS attacks. Some DDoS attacks on Microsoft have failed, in part because Microsoft heavily provisions its networks to deal with the immense load generated when they release an important new product or when an upgrade becomes available. But some of the attacks succeeded, often by cleverly finding some resource other than pure bandwidth for the attack to exhaust. For example, in January 2001 someone launched a DDoS attack on one of Microsoft's routers, preventing it from routing normal traffic at the required speeds [Del]. This attack would have had relatively little overall effect on Microsoft's Internet presence were it not for the fact that all of the DNS servers for Microsoft's online properties were located behind the router under attack. While practically all of Microsoft's network was up and available for use, many users could not get to it because their requests to translate Microsoft Web site names into IP addresses were dropped by the overloaded router. This attack was so successful that the fraction of Web requests that Microsoft was able to handle dropped to 2%. 2002Perhaps inspired by this success, in October 2002 an attacker went a step further and tried to perform a DDoS attack on the complete set of the Internet's DNS root servers. DNS is a critical service for many Internet users and many measures have been taken to make it robust and highly available. DNS data is replicated at 13 root servers, that are themselves well provisioned and maintained, and it is heavily cached in nonroot servers throughout the Internet. An attacker attempted to deny service to all 13 of these root servers using a very simple form of DDoS attack. At various points during the attack, 9 of the 13 root servers were unable to respond to DNS requests, and only 4 remained fully operational throughout the attack. The attack lasted only one hour, after which the agents stopped. Thanks to the robust design of the DNS and the attack's short duration, there was no serious impact in the Internet as a whole. A longer, stronger attack, however, might have been extremely harmful. 2003It wasn't until 2003 that a major shift in attack motivations and methodologies began to show up. This shift coincided with several rapid and widespread worm events, which have now become intertwined with DDoS. First, spammers began to use distributed networks in the same ways as DDoS attackers to set up distributed spam networks [KP]. As antispam sites tried to counter these "spambots," the spammers' hired guns retaliated by attacking several antispam sites [AJ]. Using standard DDoS tools, and even worms like W32/Sobig [JL], they waged constant attacks against those who they perceived were threatening their very lucrative business. Second, other financial crimes began to involve the use of DDoS. In some cases, online retailers with daily gross sales on the order of tens of thousands of dollars a day (as opposed to larger sites like the ones attacked in February 2000) were also attacked, using a form of DDoS involving seemingly normal Web requests. These attacks were just large enough to bring the Web server to a crawl, yet not large enough to disrupt the networks of the upstream providers. Similarly to the reflected DNS attacks described earlier, it was difficult (if not impossible) to filter out the malicious Web requests, as they appeared to be legitimate. Similar attacks were waged against online gambling sites and pornography sites. Again, in some cases extortion attempts were made for sums in the tens of thousands of dollars to stop the attacks (a new form of protection racketeering) [CN]. DDoS had finally become a component of widespread financial crimes and high dollar e-commerce, and the trend is likely to only increase in the near future. DDoS continued to also be used as in prior years, including for political reasons. During the Iraq War in 2003, a DDoS attack was launched on the Qatar-based Al-Jazeera news organization, which broadcast pictures of captured American soldiers. Al-Jazeera attempted to outprovision the attackers by purchasing more bandwidth, but they merely ratcheted up the attack. The Web site was largely unreachable for two days, following which someone hijacked their DNS name, redirecting requests to another Web site that promoted the American cause. In May 2003, then several more times in December, SCO's Web site experienced DDoS attacks that took it offline for prolonged periods of time. Statements by SCO management indicated the belief that the attacks were in response to SCO's legal battle over Linux source code and statements criticizing the Open Source community's role in these lawsuits [Lem]. In mid-2003, Clickbank (an electronic banking service) and Spamcop (a company that filters e-mail to remove spam) were subjected to powerful DDoS attacks [Zor. The attacks seemingly involved thousands of attack machines. After some days, the companies were able to install sophisticated filtering software that dropped the DDoS attack traffic before it reached a bottleneck point. 2004Financially motivated attacks continue, along with speculation that worm attacks in 2004 were used to install Trojan software on hundreds of thousands of hosts, creating massive bot networks. Two popular programs Agobot, and its successor Phatbot [LTIG] have been implicated in distributed spam delivery and DDoS; in some cases networks of these bots are even being sold on the black market [Leya]. Phatbot is described in more detail in Chapter 4. Agobot and Phatbot both share many of the features that were predicted in 2000 by Michal Zalewski [Zal] in his write-up on features of a "super-worm." This paper was written in response to the hype surrounding the "I Love You" virus. Zalewski lists such features as portability (Phatbot runs on both Windows and Linux), polymorphism, selfupdate, anti-debugging, and usability ( Phatbot comes with usage documentation and online help commands). Since Phatbot preys on previously infected hosts, other parts of Zalewski's predictions are also possible (via programming how Phatbot executes) and thus Phatbot is one of the most advanced forms of automation seen to date in the category of DDoS (or blended threat, actually) tools. Like all histories, this history of DDoS attacks does not represent a final state, but is merely the prelude to the future. In the next chapter, we will provide more details on exactly how today's attacks are perpetrated, which will then set the stage for discussing what you must do to counter them. Remember, however, that the history we've just covered suggests, more than anything else, continued and rapid change for the future. Early analyses of DDoS attack tools like trinoo, Tribe Flood Network, Stacheldraht, and Shaft all made predictions about future development trends based on past history, but attackers proved more nimble in integrating new attack methods into existing tools than those predictions suggested. We should expect both the number and sophistication of attack tools to grow steadily, and perhaps more rapidly than anyone predicts. Therefore, the tools attackers will use in upcoming years and the methods used to defend against them will progress from the current states we describe in the upcoming chapters, requiring defenders to keep up to date on new trends and defense methods.
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