Software Security: Building Security In

Given the limitations of traditional approaches, a more holistic risk management methodology involves thinking about risk throughout the lifecycle (as described in Chapter 2). Starting the risk analysis process early is critical. In fact, risk analysis is even effective at the requirements level. Modern approaches emphasize the importance of an architectural view and of architectural risk analysis.

Security Requirements

In the purest sense, risk analysis starts at the requirements stage because design requirements should take into account the risks that you are trying to counter. The box Back to Requirements briefly covers three approaches to interjecting a risk-based philosophy into the requirements phase. (Do note that the requirements systems based around UML tend to focus more attention on security functionality than they do on abuse cases, which I discuss at length in Chapter 8.)

Whatever risk analysis method is adopted, the requirements process should be driven by risk.

Back to Requirements

SecureUML[*] is a methodology for modeling access control policies and their integration into a model-driven software development process. SecureUML is based on Role-Based Access Control and models security requirements for well-behaved applications in predictable environments.

UMLsec [Jurjens 2001] is an extension of UML to include modeling of security-related features, such as confidentiality and access control.

Sindre and Opdahl [2000] attempt to model abuse cases as a way of understanding how an application might respond to threats in a less controllable environment and to describe functions that the system should not allow.

[*] See http://kisogawa.inf.ethz.ch/WebBIB/publications-softech/papers/2002/0_secuml_uml2002.pdf.

As stated earlier, a key variable in the risk equation is impact. The business impacts of any risks that we are trying to avoid can be many, but for the most part, they boil down into three broad categories:

  1. Legal and/or regulatory risk: These may include federal or state laws and regulations (e.g., the Gramm-Leach-Bliley Act [GLBA], HIPPA, or the now-famous California Senate Bill 1386, also known as SB1386)

  2. Financial or commercial considerations (e.g., protection of revenue, control over high-value intellectual property, preservation of brand and reputation)

  3. Contractual considerations (e.g., service-level agreements, avoidance of liability)

Even at this early point in the lifecycle, the first risk-based decisions should be made. One approach might be to break down requirements into three simple categories: "must-haves," "important-to-haves," and "nice-but-unnecessary-to-haves."

Unless you are running an illegal operation, laws and regulations should always be classed into the first category, making these requirements instantly mandatory and not subject to further risk analysis (although an ROI study should always be conducted to select the most cost-effective mitigations). For example, if the law requires you to protect private information, this is mandatory and should not be the subject of a risk-based decision. Why? Because the government may have the power to put you out of business, which is the mother of all risks (and if you want to test the government and regulators on this one, then go aheadjust don't say that you weren't warned!).

You are then left with risk impacts that need to be managed in other ways, the ones that have as variables potential impact and probability. At the initial requirements definition stage, you may be able to make some assumptions regarding the controls that are necessary and the ones that may not be.

Even application of these simple ideas will put you ahead of the majority of software developers. Then as we move toward the design and build stages, risk analysis should begin to test those assumptions made at the requirements stage by analyzing the risks and vulnerabilities inherent in the design. Finally, tests and test planning should be driven by risk analysis results as well.

A Basic Risk Analysis Approach

To encompass the design stage, any risk analysis process should be tailored. The object of this tailoring exercise is to determine specific vulnerabilities and risks that exist for the software. A functional decomposition of the application into major components, processes, data stores, and data communication flows, mapped against the environments across which the software will be deployed, allows for a desktop review of threats and potential vulnerabilities. I cannot overemphasize the importance of using a forest-level view of a system during risk analysis. Some sort of high-level model of the system (from a whiteboard boxes-and-arrows picture to a formally specified mathematical model) makes risk analysis at the architectural level possible.

Although one could contemplate using modeling languages, such as UMLsec, to attempt to model risks, even the most rudimentary analysis approaches can yield meaningful results. Consider Figure 5-3, which shows a simple four-tier deployment design pattern for a standard-issue Web-based application. If we apply risk analysis principles to this level of design, we can immediately draw some useful conclusions about the security design of the application.

Figure 5-3. A forest-level view of a standard-issue four-tier Web application.

During the risk analysis process we should consider the following:

  • The threats who are likely to want to attack our system

  • The risks present in each tier's environment

  • The kinds of vulnerabilities that might exist in each component, as well as the data flow

  • The business impact of such technical risks, were they to be realized

  • The probability of such a risk being realized

  • Any feasible countermeasures that could be implemented at each tier, taking into account the full range of protection mechanisms available (e.g., from base operating systemlevel security through Virtual Machine security mechanisms, such as use of the Java Cryptography Extensions in J2EE)

This very basic process will sound familiar if you read Chapter 2 on the RMF. In that chapter, I describe in great detail a number of critical risk management steps in an iterative model.

In this simple example, each of the tiers exists in a different security realm or trust zone. This fact immediately provides us with the context of risk faced by each tier. If we go on to superimpose data types (e.g., user logon credentials, records, orders) and their flows (logon requests, record queries, order entries) and, more importantly, their security classifications, we can draw conclusions about the protection of these data elements and their transmission given the current design.

For example, suppose that user logon flows are protected by SSL between the client and the Web server. However, our deployment pattern indicates that though the encrypted tunnel terminates at this tier, because of the threat inherent in the zones occupied by the Web and application tiers, we really need to prevent eavesdropping inside and between these two tiers as well. This might indicate the need to establish yet another encrypted tunnel or, possibly, to consider a different approach to securing these data (e.g., message-level encryption as opposed to tunneling).

Use of a deployment pattern in this analysis is valuable because it allows us to consider both infrastructure (i.e., operating systems and network) security mechanisms as well as application-level mechanisms as risk mitigation measures.

Realize that decomposing software on a component-by-component basis to establish trust zones is a comfortable way for most software developers and auditors to begin adopting a risk management approach to software security. Because most systems, especially those exhibiting the n-tier architecture, rely on several third-party components and a variety of programming languages, defining zones of trust and taking an outsidein perspective similar to that normally observed in traditional security has clear benefits. In any case, interaction of different products and languages is an architectural element likely to be a vulnerability hotbed.

At its heart, decomposition is a natural way to partition a system. Given a simple decomposition, security professionals will be able to advise developers and architects about aspects of security that they're familiar with such as network-based component boundaries and authentication (as I highlight in the example). Do not forget, however, that the composition problem (putting the components all back together) is unsolved and very tricky, and that even the most secure components can be assembled into an insecure mess!

As organizations become adept at identifying vulnerability and its business impact consistently using the approach illustrated earlier, the approach should be evolved to include additional assessment of risks found within tiers and encompassing all tiers. This more sophisticated approach uncovers technology-specific vulnerabilities based on failings other than trust issues across tier boundaries. Exploits related to broken transaction management and phishing attacks[9] are examples of some of the more subtle risks one might encounter with an enhanced approach.

[9] For more on phishing, which combines social engineering and technical subterfuge, see <http://www.antiphishing.org/>.

Finally, a design-level risk analysis approach can also be augmented with data from code reviews and risk-based testing.

Coder's Corner

Avi Rubin, a professor at Johns Hopkins University, and his graduate students spent much effort performing an architectural risk analysis on Diebold electronic voting machines. Their work is collected here <http://avirubin.com/vote/>.

The abstract of their paper <http://avirubin.com/vote.pdf> on one of their more famous (and controversial) analyses says:

With significant U.S. federal funds now available to replace outdated punch-card and mechanical voting systems, municipalities and states throughout the U.S. are adopting paperless electronic voting systems from a number of different vendors. We present a security analysis of the source code to one such machine used in a significant share of the market. Our analysis shows that this voting system is far below even the most minimal security standards applicable in other contexts. We identify several problems including unauthorized privilege escalation, incorrect use of cryptography, vulnerabilities to network threats, and poor software development processes [emphasis added]. We show that voters, without any insider privileges, can cast unlimited votes without being detected by any mechanisms within the voting terminal software. Furthermore, we show that even the most serious of our outsider attacks could have been discovered and executed without access to the source code. In the face of such attacks, the usual worries about insider threats are not the only concerns; outsiders can do the damage. That said, we demonstrate that the insider threat is also quite considerable, showing that not only can an insider, such as a poll worker, modify the votes, but that insiders can also violate voter privacy and match votes with the voters who cast them. We conclude that this voting system is unsuitable for use in a general election. Any paperless electronic voting system might suffer similar flaws, despite any "certification" it could have otherwise received. We suggest that the best solutions are voting systems having a "voter-verifiable audit trail," where a computerized voting system might print a paper ballot that can be read and verified by the voter.

In the paper, the authors present a number of findings. Before presenting the technical information, a concise overview of the system (a forest-level view) is presented. The overview sets the stage for the technical results, many of which focus on the construction of the system and its architecture. Among the technical results is the following finding:

3.2 Casting multiple votes

In the Diebold system, a voter begins the voting process by inserting a smart card into the voting terminal. Upon checking that the card is "active," the voting terminal collects the user's vote and then deactivates the user's card; the deactivation actually occurs by rewriting the card's type, which is stored as an 8-bit value on the card, from VOTER_CARD (0x01) to CANCELED_CARD (0x08). Since an adversary can make perfectly valid smart cards, the adversary could bring a stack of active cards to the voting booth. Doing so gives the adversary the ability to vote multiple times. More simply, instead of bringing multiple cards to the voting booth, the adversary could program a smart card to ignore the voting terminal's deactivation command. Such an adversary could use one card to vote multiple times. Note here that the adversary could be a regular voter, and not necessarily an election insider.

Will the adversary's multiple-votes be detected by the voting system? To answer this question, we must first consider what information is encoded on the voter cards on a per voter basis. The only per voter information is a "voter serial number" (m_VoterSN in the CVoterInfo class). m_VoterSN is only recorded by the voting terminal if the voter decides not to place a vote (as noted in the comments in TSElection/Results.cpp, this field is recorded for uncounted votes for backward compatibility reasons). It is important to note that if a voter decides to cancel his or her vote, the voter will have the opportunity to vote again using that same card (and, after the vote has been cast, m_VoterSN will no longer be recorded).

If we assume the number of collected votes becomes greater than the number of people who showed up to vote, and if the polling locations keep accurate counts of the number of people who show up to vote, then the back-end system, if designed properly, should be able to detect the existence of counterfeit votes. However, because m_VoterSN is only stored for those who did not vote, there will be no way for the tabulating system to distinguish the real votes from the counterfeit votes. This would cast serious doubt on the validity of the election results. The solution proposed by one election official, to have everyone vote again, does not seem like a viable solution.

Notice how the technical result is presented in terms of impact. The key to a good risk analysis is clearly stated impact statements. The only thing missing in the report is a mitigation strategy that is workable. The Diebold people appear to have their software security work cut out for them!

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