Quantitative Security Evaluation for Software System from Vulnerability Database

Abstract

This paper proposes a quantitative security evaluation for software system from the vulnerability data consisting of discovery date, solution date and exploit publish date based on a stochastic model. More precisely, our model considers a vulnerability life-cycle model and represents the vulnerability discovery process as a non-homogeneous Poisson process. In a numerical example, we show the quantitative measures for contents management system of an open source project.

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H. Okamura, M. Tokuzane and T. Dohi, "Quantitative Security Evaluation for Software System from Vulnerability Database," Journal of Software Engineering and Applications, Vol. 6 No. 4A, 2013, pp. 15-23. doi: 10.4236/jsea.2013.64A003.

1. Introduction

From the latter half of 1990s, many security incidents have been reported in enterprise systems and personal computers, such as the denial-of-service attack via computer viruses and the data leak caused by unauthorized accesses.

Generally, most of security incidents are caused by software flaws and bugs called security holes and vulnerabilities. The effective counter measure against security incidents is to validate there is no flaw in the software during design and testing phases. Nowadays, for these purpose, model verification techniques are enhanced to validate the software design. For example, the model checking ensures that the software behaves according to its specification mathematically [1], and several testing techniques are developed to remove software faults as many as possible in the testing phase [2]. However, even if such techniques are applied, it is difficult to remove all the flaws before releasing the software to the market due to external circumstances of software development; development cost, delivery date and unexpected specification changes. For such software systems, a security patching is one of the feasible solutions that do not allow an attacker to exploit vulnerabilities.

A security patch is a small program to fix the software faults causing security holes and vulnerabilities, and is distributed to the end-users through the Internet or other means after the software release. The user can remove a vulnerability by applying a corresponding security patch which is distributed from the vendor. Ideally, the security patch should be distributed whenever one discovers a vulnerability of the software product. However, the development and distribution of security patches incur expenses for the vendor, and a short development time might cause the distribution of a poorly designed patch causing a new problem. Thus, many of the software vendors design a plan to distribute a security patch at a specified period of time, e.g., quarterly distribution, and the patch fixes all the vulnerabilities which have been discovered until the distribution time. On the other hand, from the user perspective, applying a patch involves not only a tedious task but also a risk that the patch causes an error like misconfiguration. Therefore, in practice, users, especially enterprises and firms, also make a plan of what patches are applied at a specified period of time. These strategies for the software patch are called patch management. In [3], Okamura et al. discussed the optimal patch release timing to help the patch management for enterprise based on the stochastic model.

Essentially, it is important to quantify degree of security for the software system to discuss the patch management. In general, there are two perspectives on the quantitative evaluation of security: vendor’s and user’s perspective. From the vendor’s perspective, the risk is that vendor is to release exploitation of a vulnerability before a patch is distributed. On the other hand, users should consider the risk caused by the delay of applying patches as well as the risk of software system itself. In fact, Okamura et al. [4] tried to evaluate the degree of security from user’s perspective by considering user profile of the system. In this paper, we focus on the security risk for vendors.

In the past literature, many researches considered the risk of security in software system from the vendor’s perspective. Wang et al. [5] presented a continuous-time Markov model to evaluate the security in the intrusiontolerant database system. Jonsson et al. [6] discussed the security model based on the analysis of attacker’s behavior. In these papers, they considered the quantitative security for specific systems and it cannot always be applied to any kind of software system. Also Kimura [7] proposed a stochastic model, which is similar to the classical software reliability growth model, and presented a quantitative evaluation for the security of software system. His method focused on the vulnerability discovery process only and therefore it can be applied to many kinds of software system. However, the model derived in [7] is essentially equivalent to testing-domain dependent software reliability growth model [8]. Thus, it cannot represent a variety of patterns for the vulnerability discovery process.

In this paper, we refine the quantitative software security model based on the vulnerability discovery process by using general distributions. Although the model presented here does not exactly include the model in [7], we adopt the similar situation where vendors and attackers compete to make a patch and to find an exploit. In addition, we present an illustrative example of the quantitative security evaluation of contents management system from the vulnerability data.

The rest of this paper is organized as follows. In Section 2, we describe the vulnerability model with respect to its discovery process. Section 3 presents the formulation of a quantitative security measure based on the vulnerability discovery process, patch release distribution and exploitation time distribution. Section 4 is devoted to the experiment for our quantitative security evaluation based on the vulnerability data.

2. Vulnerability Discovery Model

2.1. Vulnerability Life Cycle

Vulnerability is defined as a fault on system requirements or a program that allows an attacker to violate the system integrity. A vulnerability is often caused by flaws on software requirements as well as software bugs, and thus it is more difficult to find vulnerabilities by software testing than to detect usual software bugs.

Arbaugh et al. [9] presented a vulnerability life-cycle model which consists of the following seven states:

• Birth: The birth of a vulnerability, strictly speaking a flaw, occurs at software requirement or software design.

• Discovery: Someone discovers a flaw on software security, and then the flaw becomes a vulnerability.

• Disclosure: The vulnerability is disclosed when the discoverer reveals details of the problem.

• Correction: The vulnerability is correctable by developing and releasing a security patch.

• Publicity: The vulnerability and its problem become known by disclosing them to public medias.

• Scripting: An exploitation of the vulnerability is released. In this state, crackers with little or no skill can exploit the vulnerability to violate the integrity of system.

• Death: The vulnerability dies when one applies a security patch to all the vulnerable systems.

Figure 1 illustrates the state transition of a typical vulnerability in the life-cycle model.

Conflicts of Interest

The authors declare no conflicts of interest.

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