This paper presents the design, implementation, analysis, and experimental evaluation of speak-up, a defense against applicationlevel distributed denial-of-service (DDoS), in which attackers cripple a server by sending legitimate-looking requests that consume computational resources (e.g., CPU cycles, disk). With speak-up, a victimized server encourages all clients, resources permitting, to automatically send higher volumes of traffic. We suppose that attackers are already using most of their upload bandwidth so cannot react to the encouragement. Good clients, however, have spare upload bandwidth and will react to the encouragement with drastically higher volumes of traffic. The intended outcome of this traffic inflation is that the good clients crowd out the bad ones, thereby capturing a much larger fraction of the server’s resources than before. We experiment under various conditions and find that speak-up causes the server to spend resources on a group of clients in rough proportion to their aggregate upload bandwidth. This result makes the defense viable and effective for a class of real attacks.

Cellular networks are a critical component of the economic and social infrastructures in which we live. In addition to voice services, these networks deliver alphanumeric text messages to the vast majority of wireless subscribers. To encourage the expansion of this new service, telecommunications companies offer connections between their networks and the Internet. The ramifications of such connections, however, have not been fully recognized. In this paper, we evaluate the security impact of the SMS interface on the availability of the cellular phone network. Specifically, we demonstrate the ability to deny voice service to cities the size of Washington D.C. and Manhattan with little more than a cable modem. Moreover, attacks targeting the entire United States are feasible with resources available to medium-sized zombie networks. This analysis begins with an exploration of the structure of cellular networks. We then characterize network behavior and explore a number of reconnaissance techniques aimed at effectively targeting attacks on these systems. We conclude by discussing countermeasures that mitigate or eliminate the threats introduced by these attacks.

Systems using capabilities to provide preferential service to selected flows have been proposed as a defense against large-scale network denial-of-service attacks. While these systems offer strong protection for established network flows, the Denial-of-Capability (DoC) attack, which prevents new capability-setup packets from reaching the destination, limits the value of these systems. Portcullis mitigates DoC attacks by allocating scarce link bandwidth for connection establishment packets based on per-computation fairness. We prove that a legitimate sender can establish a capability with high probability regardless of an attacker’s resources or strategy and that no system can improve on our guarantee. We simulate full and partial deployments of Portcullis on an Internetscale topology to confirm our theoretical results and demonstrate the substantial benefits of using per-computation fairness.

The transformation of telecommunications networks from homogeneous closed systems providing only voice services to Internetconnected open networks that provide voice and data services presents significant security challenges. For example, recent research illustrated that a carefully crafted DoS attack via text messaging could incapacitate all voice communications in a metropolitan area with little more than a cable modem. This attack highlights a growing threat to these systems; namely, cellular networks are increasingly exposed to adversaries both in and outside the network. In this paper, we use a combination of modeling and simulation to demonstrate the feasibility of targeted text messaging attacks. Under realistic network conditions, we show that adversaries can achieve blocking rates of more than 70 % with only limited resources. We then develop and characterize five techniques from within two broad classes of countermeasures- queue management and resource provisioning. Our analysis demonstrates that these techniques can eliminate or extensively mitigate even the most intense targeted text messaging attacks. We conclude by considering the tradeoffs inherent to the application of these techniques in current and next generation telecommunications networks.

Broadcast authentication is a critical security service in wireless sensor networks. There are two general approaches for broadcast authentication in wireless sensor networks: digital signatures and µTESLA-based techniques. However, both signature-based and µTESLA-based broadcast authentication are vulnerable to Denial of Services (DoS) attacks: An attacker can inject bogus broadcast packets to force sensor nodes to perform expensive signature verifications (in case of signature-based broadcast authentication) or packet forwarding (in case of µTESLA-based broadcast authentication), thus exhausting their limited battery power. This paper presents an efficient mechanism called message specific puzzle to mitigate such DoS attacks. In addition to signaturebased or µTESLA-based broadcast authentication, this approach adds a weak authenticator in each broadcast packet, which can be efficiently verified by a regular sensor node, but takes a computationally powerful attacker a substantial amount of time to forge. Upon receiving a broadcast packet, each sensor node first verifies the weak authenticator, and performs the expensive signature verification (in signature-based broadcast authentication) or packet forwarding (in µTESLA-based broadcast authentication) only when the weak authenticator is valid. A weak authenticator cannot be pre-computed without a non-reusable (or short-lived) key disclosed only in a valid packet. Even

Availability is a critical issue in modern distributed systems. While many techniques and protocols for preventing denial of service (DoS) attacks have been proposed and deployed in recent years, formal methods for analyzing and proving them correct have not kept up with the state of the art in DoS prevention. This paper proposes a new protocol for preventing malicious bandwidth consumption, and demonstrates how game-based formal methods can be successfully used to verify availability-related security properties of network protocols. We describe two classes of DoS attacks aimed at bandwidth consumption and resource exhaustion, respectively. We then propose our own protocol, based on a variant of client puzzles, to defend against bandwidth consumption, and use the JFKr key exchange protocol as an example of a protocol that defends against resource exhaustion attacks. We specify both protocols as alternating transition systems (ATS), state their security properties in alternatingtime temporal logic (ATL) and verify them using MOCHA, a model checker that has been previously used to analyze fair exchange protocols. 1.

Abstract. Recently, we have seen increasing numbers of denial of service (DoS) attacks against online services and web applications either for extortion reasons, or for impairing and even disabling the competition. These DoS attacks have increasingly targeted the application level. Application level DoS attacks emulate the same request syntax and network level traffic characteristics as those of legitimate clients, thereby making the attacks much harder to be detected and countered. Moreover, such attacks usually target bottleneck resources such as disk bandwidth, database bandwidth, and CPU resources. In this paper we propose server-side middleware to counter application level DoS attacks. The key idea behind our technique is to adaptively vary a client’s priority level, and the relative amount of resources devoted to this client, in response to the client’s past requests in a way that incorporates application level semantics. Application specific knowledge is used to evaluate the cost and the utility of each request and the likelihood that a sequence of requests are sent by a malicious client. Based on the evaluations, a client’s priority level is increased or decreased accordingly. A client’s priority level is used by the server side firewall to throttle the client’s request rate, thereby ensuring that more server side resources are allocated to the legitimate clients. We present a detailed implementation of our approach on the Linux kernel and evaluate it using two sample applications: Apache HTTPD micro-benchmarks and TPCW. Our experiments show that our approach incurs low performance overhead and is resilient to application level DoS attacks. 1

Abstract. CPU bound client puzzles have been suggested as a defense mechanism against connection depletion attacks. However, the wide disparity in CPU speeds prevents such puzzles from being globally deployed. Recently, Abadi et. al. [1] and Dwork et. al. [2] addressed this limitation by showing that memory access times vary much less than CPU speeds, and hence offer a viable alternative. In this paper, we further investigate the applicability of memory bound puzzles from a new perspective and propose constructions based on heuristic search methods. Our constructions are derived from a more algorithmic foundation, and as a result, allow us to easily tune parameters that impact puzzle creation and verification costs. Moreover, unlike prior approaches, we address client-side cost and present an extension that allows memory constrained clients (e.g., PDAs) to implement our construction in a secure fashion. 1

In this paper, we present Telex, a new approach to resisting state-level Internet censorship. Rather than attempting to win the cat-and-mouse game of finding open proxies, we leverage censors ’ unwillingness to completely block day-to-day Internet access. In effect, Telex converts innocuous, unblocked websites into proxies, without their explicit collaboration. We envision that friendly ISPs would deploy Telex stations on paths between censors’ networks and popular, uncensored Internet destinations. Telex stations would monitor seemingly innocuous flows for a special “tag ” and transparently divert them to a forbidden website or service instead. We propose a new cryptographic scheme based on elliptic curves for tagging TLS handshakes such that the tag is visible to a Telex station but not to a censor. In addition, we use our tagging scheme to build a protocol that allows clients to connect to Telex stations while resisting both passive and active attacks. We also present a proof-of-concept implementation that demonstrates the feasibility of our system. 1

Several important security protocols require parties to perform computations based on random challenges. Traditionally, proving that the challenges were randomly chosen has required interactive communication among the parties or the existence of a trusted server. We offer an alternative solution where challenges are harvested from oblivious servers on the Internet. This paper describes a framework for deriving “harvested challenges ” by mixing data from various pre-existing online sources. While individual sources may become predictable or fall under adversarial control, we provide a policy language that allows application developers to specify combinations of sources that meet their security needs. Participants can then convince each other that their challenges were formed freshly and in accordance with the policy. We present Combine, an open source implementation of our framework, and show how it can be applied to a variety of applications, including remote storage auditing and non-interactive client puzzles.