Current standard security practices do not provide substantial assurance that the end-to-end behavior of a computing system satisfies important security policies such as confidentiality. An end-to-end confidentiality policy might assert that secret input data cannot be inferred by an attacker through the attacker's observations of system output; this policy regulates information flow.

Security remains a major roadblock to universal acceptance of the Web for many kinds of transactions, especially since the recent sharp increase in remotely exploitable vulnerabilities has been attributed to Web application bugs. Many verification tools are discovering previously unknown vulnerabilities in legacy C programs, raising hopes that the same success can be achieved with Web applications. In this paper, we describe a sound and holistic approach to ensuring Web application security. Viewing Web application vulnerabilities as a secure information flow problem, we created a lattice-based static analysis algorithm derived from type systems and typestate, and addressed its soundness. During the analysis, sections of code considered vulnerable are instrumented with runtime guards, thus securing Web applications in the absence of user intervention. With sufficient annotations, runtime overhead can be reduced to zero. We also created a tool named WebSSARI (Web application Security by Static Analysis and Runtime Inspection) to test our algorithm, and used it to verify 230 open-source Web application projects on SourceForge.net, which were selected to represent projects of different maturity, popularity, and scale. 69 contained vulnerabilities and their developers were notified. 38 projects acknowledged our findings and stated their plans to provide patches. Our statistics also show that static analysis reduced potential runtime overhead by 98.4%. Categories and Subject Descriptors D.2.4 [Software Engineering]: Software / Program Verification – class invariants, formal methods; D.4.6 [Operating Systems]: Security and Protection – information flow controls, correctness proofs, formal methods; K.6.5 [Computing Milieux]: Security and Protection – invasive software, unauthorized access.

Security properties based on information flow, such as noninterference, provide strong guarantees that confidentiality is maintained. However, programs often need to leak some amount of confidential information in order to serve their intended purpose, and thus violate noninterference. Real systems that control information flow often include mechanisms for downgrading or declassifying information; however, declassification can easily result in the unexpected release of confidential information.

We consider a sequential object-oriented language with pointers and mutable state, private fields and classbased visibility, dynamic binding and inheritance, recursive classes, casts and type tests, and recursive methods. Programs are annotated with security levels, constrained by security typing rules. A noninterference theorem shows how the rules ensure pointer confinement and secure information flow.

Most attempts at analysing secure information flow in programs are based on domain-specific logics. Though computationally feasible, these approaches suffer from the need for abstraction and the high cost of building dedicated tools for real programming languages. We recast the information flow problem in a general program logic rather than a problem-specific one. We investigate the feasibility of this approach by showing how a general purpose tool for software verification can be used to perform information ow analyses. We are able to handle phenomena like method calls, loops, and object types for the target language Java Card. We are also able to prove insecurity of programs.

In traditional information-flow type systems, the security policy is often formalized as noninterference properties. However, noninterference alone is too strong to express security properties useful in practice. If we allow downgrading in such systems, it is challenging to formalize the security policy as an extensional property of the system. This paper presents a generalized framework of downgrading policies. Such policies can be specified in a simple and tractable language and can be statically enforced by mechanisms such as type systems. The security guarantee is then formalized as a concise extensional property using program equivalences. This relaxed noninterference generalizes traditional pure noninterference and precisely characterizes the information released due to downgrading.

Bytecode verification is a crucial security component for Java applets, on the Web and on embedded devices such as smart cards. This paper reviews the various bytecode verification algorithms that have been proposed, recasts them in a common framework of dataflow analysis, and surveys the use of proof assistants to specify bytecode verification and prove its correctness.

This paper develops a simple object calculus with harmless aspect-oriented advice. A piece of harmless advice is a computation that, like ordinary aspect-oriented advice, executes when control reaches a designated control-flow point. However, unlike ordinary advice, harmless advice is designed to obey a weak non-interference property. Harmless advice may change the termination behavior of computations and use I/O, but it does not otherwise influence the final result of computations that trigger it. A simple type and effect system related to information-flow type systems helps enforce harmlessness. We have proven that harmless advice does not interfere with the mainline computation.

Access control mechanisms are often used with the intent of enforcing confidentiality and integrity policies, but few rigorous connections have been made between information flow and runtime access control. The Java virtual machine and the .NET runtime system provide a dynamic access control mechanism in which permissions are granted to program units and a runtime mechanism checks permissions of code in the calling chain. We investigate a design pattern by which this mechanism can be used to achieve confidentiality and integrity goals: a single interface serves callers of more than one security level and dynamic access control prevents release of high information to low callers. Programs fitting this pattern would be rejected by previous flow analyses. We give a static analysis that admits them, using permission-dependent security types. The analysis is given for a class-based object-oriented language with features including inheritance, dynamic binding, dynamically allocated mutable objects, type casts and recursive types. The analysis is shown to ensure a noninterference property formalizing confidentiality and integrity.

Non-interference is a high-level security property that guarantees the absence of illicit information leakages through a program execution. A common means to enforce non-interference is to use an information flow type system. However, such type systems are inherently imprecise, and reject many secure programs, even for simple programming languages. The purpose of this paper is to propose a logical formulation of non-interference that allows a more precise analysis or programs, and that is amenable to deductive verification techniques, such as programming logics and weakest precondition calculi, and algorithmic verification techniques such as modelchecking. We illustrate the applicability of our method in several scenarii, including a simple imperative language, a non-deterministic language, and finally a language with shared mutable data structures.