Over the past few years, denial-of-service (DoS) and distributed denial-of-service (DDoS) attacks have become more of a threat than ever. These attacks are aimed at denying or degrading service for a legitimate user by any means necessary. The need to propose and research novel methods to mitigate them has become a critical research issue in network security. Recently, client puzzle protocols have received attention as a method for combating DoS and DDoS attacks. In a client puzzle protocol, the client is forced to solve a cryptographic puzzle before it can request any operation from a remote server or host. This thesis presents the framework and design of two different client puzzle protocols: Puzzle TCP and Chained Puzzles. Puzzle TCP, or pTCP, is a modification to the Transmission Control Protocol (TCP) that supports the use of client puzzles at the transport layer and is designed to help combat various DoS attacks that target TCP. In this protocol, when a server is under attack, each client is required to solve a cryptographic puzzle before the connection can be established. This thesis presents the design and implementation of pTCP, which was embedded into the Linux kernel, and demonstrates how effective it can be at defending against specific attacks on the transport layer. Chained Puzzles is an extension to the Internet Protocol (IP) that utilizes client puzzles to mitigate the crippling effects of a large-scale DDoS flooding attack by forcing each client to solve a cryptographic problem before allowing them to send packets into the network. This thesis also presents the design of Chained Puzzles and verifies its effectiveness with simulation results during large-scale DDoS flooding attacks. / Master of Science

Suen, Ho Yan. / Thesis (M.Phil.)--Chinese University of Hong Kong, 2009. / Includes bibliographical references (leaves 99-104). / Abstract also in Chinese. / Abstract --- p.i / Acknowledgement --- p.iv / Chapter 1 --- Introduction --- p.1 / Chapter 1.1 --- Motivation --- p.1 / Chapter 1.2 --- Organization --- p.4 / Chapter 2 --- Literature Review --- p.6 / Chapter 2.1 --- Related Works --- p.6 / Chapter 2.2 --- Background Study --- p.7 / Chapter 2.2.1 --- World Wide Web --- p.7 / Chapter 2.2.2 --- Distributed Denial of Service Attack --- p.11 / Chapter 2.2.3 --- Tools for Dimension Reduction --- p.13 / Chapter 2.2.4 --- Tools for Anomaly Detection --- p.20 / Chapter 2.2.5 --- Receiver operating characteristics (ROC) Analysis --- p.22 / Chapter 3 --- System Design --- p.25 / Chapter 3.1 --- Methodology --- p.25 / Chapter 3.2 --- System Overview --- p.27 / Chapter 3.3 --- Reference Profile Construction --- p.31 / Chapter 3.4 --- Real-time Anomaly Detection and Response --- p.32 / Chapter 3.5 --- Chapter Summary --- p.34 / Chapter 4 --- Reference Profile Construction --- p.35 / Chapter 4.1 --- Web Access Logs Collection --- p.35 / Chapter 4.2 --- Data Preparation --- p.37 / Chapter 4.3 --- Feature Extraction and Embedding Engine (FEE Engine) --- p.40 / Chapter 4.3.1 --- Sub-Sequence Extraction --- p.42 / Chapter 4.3.2 --- Hash Function on Sub-sequences (optional) --- p.45 / Chapter 4.3.3 --- Feature Vector Construction --- p.46 / Chapter 4.3.4 --- Diffusion Wavelets Embedding --- p.47 / Chapter 4.3.5 --- Numerical Example of Feature Set Reduction --- p.49 / Chapter 4.3.6 --- Reference Profile and Further Use of FEE Engine --- p.50 / Chapter 4.4 --- Chapter Summary --- p.50 / Chapter 5 --- Real-time Anomaly Detection and Response --- p.52 / Chapter 5.1 --- Session Filtering and Data Preparation --- p.54 / Chapter 5.2 --- Feature Extraction and Embedding --- p.54 / Chapter 5.3 --- Distance-based Outlier Scores Calculation --- p.55 / Chapter 5.4 --- Anomaly Detection and Response --- p.56 / Chapter 5.4.1 --- Length-Based Anomaly Detection Modules --- p.56 / Chapter 5.4.2 --- Characteristics of Anomaly Detection Modules --- p.59 / Chapter 5.4.3 --- Dynamic Threshold Adaptation --- p.60 / Chapter 5.5 --- Chapter Summary --- p.63 / Chapter 6 --- Experimental Results --- p.65 / Chapter 6.1 --- Experiment Datasets --- p.65 / Chapter 6.1.1 --- Normal Web Access Logs --- p.66 / Chapter 6.1.2 --- Attack Data Generation --- p.68 / Chapter 6.2 --- ROC Curve Construction --- p.70 / Chapter 6.3 --- System Parameters Selection --- p.71 / Chapter 6.4 --- Performance of Anomaly Detection --- p.82 / Chapter 6.4.1 --- Performance Analysis --- p.85 / Chapter 6.4.2 --- Performance in defending DDoS attacks --- p.87 / Chapter 6.5 --- Computation Requirement --- p.91 / Chapter 6.6 --- Chapter Summary --- p.95 / Chapter 7 --- Conclusion and Future Work --- p.96 / Bibliography --- p.99

The use of Internet has revolutionized the way information is exchanged, changed business paradigms and put mission critical and sensitive systems online. Any dis- ruption of this connectivity and the plethora of services provided results in significant damages to everyone involved. Denial of Service (DoS) attacks are becoming increas- ingly common and are the cause of lost time and revenue. Flooding type DoS attacks use spoofed IP addresses to disguise the attackers. This makes identification of the attackers extremely difficult. This work proposes a new scheme that allows the victim of a DoS attack to identify the correct origin of the malicious traffic. The suggested mechanism requires routers to mark packets using adjusted probabilistic marking. This results in a lower number of packet-markings required to identify the traffic source. Unlike many related works, we use the existing IPv4 header structure to incorporate these markings. We simulate and test our algorithms using real Internet trace data to show that our technique is fast, and works successfully for a large number of distributed attackers.

Over the last couple of years the rise of application layer Distributed Denial of Service (DDoS) attacks has significantly increased. Because of this, many issues have been raised on how organizations and companies can protect themselves from intrusions and damages against their systems and services. The consequences from these attacks are many, ranging from revenue losses for companies to stolen personal data. As the technologies are evolving, application layer DDoS attacks are becoming more effective and there is not a concrete solution that entirely protects against them. This thesis focuses on the available defense mechanisms and presents a general overview of different types of application layer DDoS attacks and how they are constructed. Moreover this report provides a simulation based on one of the defense mechanisms mentioned, named CALD. The simulation tested two different application layer DDoS attacks and showed that CALD can detect and differentiate between the two attacks. This report can be used as a general information source for application layer DDoS attacks, how to detect them and how to defend against them. Furthermore the simulation can be used as a basis on how well a relatively small-scaled implementation of CALD can detect DDoS attacks on the application layer. / Under de senaste åren har ökningen av Distributed Denial of Service (DDoS) attacker på applikationslagret ökat markant. På grund av detta har många frågor uppkommit om hur organisationer och företag kan skydda sig mot intrång och skador mot sina system och tjänster. Konsekvenserna av dessa attacker är många, allt från intäktsförluster för företag till stulen personlig data. Eftersom tekniken utvecklas, har DDoS attacker på applikationslagret blivit mer effektiva och det finns inte en konkret lösning för att hindra dem. Denna rapport fokuserar på de tillgängliga försvarsmekanismer och presenterar en allmän översikt över olika typer av DDoS-attacker på applikationslagret och hur de är uppbyggda. Dessutom bidrar den här rapporten med en redovisning av en simulering baserad på en av de försvarsmekanismer som nämns i rapporten, CALD. Simuleringen testade två olika attacker på applikationslagret och visar att CALD kan upptäcka och skilja mellan de två attackerna. Denna rapport kan användas som en allmän informationskälla för DDoSattacker på applikationslagret och hur man försvarar sig mot och upptäcker dessa. Vidare kan simuleringen användas som utgångspunkt på hur väl en relativt småskalig implementering av CALD kan upptäcka DDoS-attacker på applikationslagret.

Improving end-to-end Quality of Service (QoS) in existing network systems is a fundamental problem, as it can be affected by many factors, including congestion, packet scheduling, attacks, and air-time allocation. This dissertation addresses QoS in two critical environments: home WiFi and cloud networks. In home networks, we focus on improving QoS over WiFi networks, the dominant means for home Internet access. Three major reasons for end-to-end QoS efforts fail in WiFi networks are its: 1) inherent wireless channel characteristics, 2) approach to access control of the shared broadcast channel, and 3) impact on transport layer protocols, such as TCP, that operate end-to-end, and over-react to the loss or delay caused by the single WiFi link. We present our cross-layer design, Virtual Wire, leveraging the philosophy of centralization in modern networking to address the problem at the point of entry/egress into the WiFi network. Based on network conditions measured from buffer sizes, airtime, and throughput, flows are scheduled to the optimal utility. Unlike most existing WiFi QoS approaches, our design only relies on transparent modifications, requiring no changes to the network (including link layer) protocols, applications, or user intervention. Through extensive experimental investigation, we show that our design significantly enhances the reliability and predictability of WiFi performance, providing a ``virtual wire''-like link to the targeted application. In cloud networks, we explore mechanisms to improve availability during DDoS attacks. The availability of cloud servers is impacted when excessive loads induced by DDoS attacks cause the servers to crash or respond too slowly to legitimate session requests. We model and analyze the effectiveness of a shuffling mechanism: the periodic, randomized re-assignment of users to servers. This shuffling mechanism not only complicates malicious users’ abilities to target specific servers but also, over time, allows a system to identify who the malicious users are. We design and evaluate improved classifiers which can, with statistical accuracy and well-defined levels of confidence, identify malicious users. We also propose and explore the effectiveness of a two-tiered system in which servers are partitioned in two, where one partition serves only ”filtered” users who have demonstrated non-malicious behavior. Our results show how shuffling with these novel classifiers can improve the QoS of the system, which is evaluated by the survival probability, the probability of a legitimate session not being affected by attacks.

Η παρούσα διατριβή πραγματεύεται το θέμα των κατανεμημένων επιθέσεων άρνησης υπηρεσιών στο Διαδίκτυο. Αναλύει τα υπάρχοντα συστήματα αντιμετώπισης και τα εργαλεία που χρησιμοποιούνται για την εξαπόλυση τέτοιου είδους επιθέσεων. Μελετά τον τρόπο που οργανώνονται οι επιθέσεις και παρουσιάζει την αρχιτεκτονική και την υλοποίηση ενός πρωτότυπου συστήματος ανίχνευσης των πηγών μιας κατανεμημένης επίθεσης άρνησης υπηρεσιών, καθώς και αντιμετώπισης των επιθέσεων αυτών. Τέλος, ασχολείται με το θέμα της ανεπιθύμητης αλληλογραφίας ως μιας διαφορετικού είδους επίθεση άρνησης υπηρεσιών και προτείνει ένα πρωτότυπο τρόπο αντιμετώπισής της. / In our thesis we deal with the issue of Distributed Denial of Service attacks on the Internet. We analyze the current defense methodologies and the tools that are used to unleash this type of attacks. We study the way that those attacks are constructed and organized and present a novel architecture, and its implementation details, of a system that is able to trace back to the true sources of such an attack as well as effectively filter such attacks in real time. Lastly we deal with the issue of spam e-mail as a different form of a distributed denial of service attack and propose a novel methodology that deals with the problem.

A DDoS attack is an attempt to bring down a machine connected to the Internet. This is done by having multiple computers repeatedly sending requests to tie up a server making it unable to answer legitimate requests. DDoS attacks are currently one of the biggest security threats on the internet according to security experts. We used a qualitative interview with experts in IT security to gather data to our research. We found that most companies are lacking both in knowledge and in their protection against DDoS attacks. The best way to minimize this threat would be to build a system with redundancy, do a risk analysis and revise security policies. Most of the technologies reviewed were found ineffective because of the massive amount of data amplification attacks can generate. Ingress filtering showed promising results in preventing DDoS attacks by blocking packages with spoofed IP addresses thus preventing amplification attacks.

>Magister Scientiae - MSc / Cybersecurity defense tools, techniques and methodologies are constantly faced with increasing challenges including the evolution of highly intelligent and powerful new-generation threats. The main challenges posed by these modern digital multi-vector attacks is their ability to adapt with machine learning. Research shows that many existing defense systems fail to provide adequate protection against these latest threats. Hence, there is an ever-growing need for self-learning technologies that can autonomously adjust according to the behaviour and patterns of the offensive actors and systems. The accuracy and effectiveness of existing methods are dependent on decision making and manual input by human experts. This dependence causes 1) administration overhead, 2) variable and potentially limited accuracy and 3) delayed response time.

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A cyber-physical control system (CPS) typically consists of a set of physical subsystems, their remote terminal units, a central control center (if applicable), and local communication networks that interconnect all the components to achieve a common goal. Applications include energy systems, autonomous vehicles, and collaborative robots. Ensuring stability, performance, and resilience in CPS requires thorough analysis and control design, utilizing robust algorithms to handle delays, communication failures, and potential cyber-attacks. Time delays are a challenge in CPS, particularly in teleoperation systems, where human operators remotely control robotic systems. These delays cause chattering, oscillations, and instability, making it difficult to achieve smooth and stable remote robot control. Applications like remote surgery, space exploration, and hazardous environment operations are highly susceptible to these disruptions. To address this issue, a novel passivity-shortage framework is proposed, that enables systems to maintain stability and transparency despite time-varying communication delays and environmental disturbances. CPS are prone to attacks, particularly Denial-of-Service (DoS) attacks, which disrupt the normal functioning of a network by overwhelming it with excessive internet traffic, rendering the communication channels unavailable to legitimate users. These attacks threaten the stability and functionality of CPS. To enhance resilience in multi-agent systems, novel distributed algorithms are proposed. These graph theory-based algorithms mitigate network vulnerabilities by incorporating strategically placed additional communication channels, thereby increasing tolerance to attacks in large, dynamic networks. The effectiveness of these proposed approaches is validated through simulations, experiments, and numerical examples. The passivity-shortage teleoperation strategies are tested using Phantom Omni devices and they show reduced chattering and better steady-state error convergence. A case study demonstrates how the proposed distributed algorithms effectively achieve consensus, even when some agents are disconnected from the network due to DoS attacks.