Computer Security and Cryptography Reading Group
September 2005 List

V. Paxson

URL: http://www.icir.org/vern/papers/TcpReassembly/TcpReassembly.pdf

There is a growing interest in designing high-speed network devices to perform packet processing at semantic levels above the network layer. Some examples are layer-7 switches, content inspection and transformation systems, and network intrusion detection/prevention systems. Such systems must maintain per-flow state in order to correctly perform their higher-level processing. A basic operation inherent to per-flow state management for a transport protocol such as TCP is the task of reassembling any out-of-sequence packets delivered by an underlying unreliable network protocol such as IP. This seemingly prosaic task of reassembling the byte stream becomes an order of magnitude more difficult to soundly execute when conducted in the presence of an adversary whose goal is to either subvert the higher-level analysis or impede the operation of legitimate traffic sharing the same network path.

We present a design of a hardware-based high-speed TCP reassembly mechanism that is robust against attacks. It is intended to serve as a module used to construct a variety of network analysis systems, especially intrusion prevention systems. Using trace-driven analysis of out-of-sequence packets, we first characterize the dynamics of benign TCP traffic and show how we can leverage the results to design a reassembly mechanism that is efficient when dealing with non-attack traffic. We then refine the mechanism to keep the system effective in the presence of adversaries. We show that although the damage caused by an adversary cannot be completely eliminated, it is possible to mitigate the damage to a great extent by careful design and resource allocation. Finally, we quantify the trade-off between resource availability and damage from an adversary in terms of Zombie equations that specify, for a given configuration of our system, the number of compromised machines an attacker must have under their control in order to exceed a specified notion of "acceptable collateral damage."

URL: http://www.nsa.gov/selinux/papers/inevit-abs.cfm

Although public awareness of the need for security in computing systems is growing rapidly, current efforts to provide security are unlikely to succeed. Current security efforts suffer from the flawed assumption that adequate security can be provided in applications with the existing security mechanisms of mainstream operating systems. In reality, the need for secure operating systems is growing in today's computing environment due to substantial increases in connectivity and data sharing. The goal of this paper is to motivate a renewed interest in secure operating systems so that future security efforts may build on a solid foundation. This paper identifies several secure operating system features which are lacking in mainstream operating systems, argues that these features are necessary to adequately protect general application-space security mechanisms, and provides concrete examples of how current security solutions are critically dependent on these features.

URL: http://www.eecg.toronto.edu/~ashvin/publications/sosp-2005.pdf

Recovery from intrusions is typically a very time-consuming operation in current systems. At a time when the cost of human resources dominates the cost of computing resources, we argue that next generation systems should be built with automated intrusion recovery as a primary goal. In this paper, we describe the design of Taser, a system that helps in selectively recovering legitimate file-system data after an attack or local damage occurs. Taser reverts tainted, i.e. attack-dependent, file-system operations but preserves legitimate operations. This process is difficult for two reasons. First, the set of tainted operations is not known precisely. Second, the recovery process can cause conflicts when legitimate operations depend on tainted operations. Taser provides several analysis policies that aid in determining the set of tainted operations. To handle conflicts, Taser uses automated resolution policies that isolate the tainted operations. Our evaluation shows that Taser is effective in recovering from a wide range of intrusions as well as damage caused by system management errors.

J. R. Crandall

Z. Su

S. F. Wu

F. T. Chong

URL: http://wwwcsif.cs.ucdavis.edu/~crandall/ccsdacoda.pdf

Vulnerabilities that allow worms to hijack the control flow of each host that they spread to are typically discovered months before the worm outbreak, but are also typically discovered by third party researchers. A determined attacker could discover vulnerabilities as easily and create zero-day worms for vulnerabilities unknown to network defenses. It is important for an analysis tool to be able to generalize from a new exploit observed and derive protection for the vulnerability.

Many researchers have observed that certain predicates of the exploit vector must be present for the exploit to work and that therefore these predicates place a limit on the amount of polymorphism and metamorphism available to the attacker. We formalize this idea and subject it to quantitative analysis with a symbolic execution tool called DACODA. Using DACODA we provide an empirical analysis of 14 exploits (seven of them actual worms or attacks from the Internet, caught by Minos with no prior knowledge of the vulnerabilities and no false positives observed over a period of six months) for four operating systems.

Evaluation of our results in the light of these two models leads us to conclude that 1) single contiguous byte string signatures are not effective for content alltering, and token-based byte string signatures composed of smaller substrings are only semantically rich enough to be effective for content alltering if the vulnerability lies in a part of a protocol that is not commonly used, and that 2) practical exploit analysis must account for multiple processes, multithreading, and kernel processing of network data necessitating a focus on primitives instead of vulnerabilities.