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Section: New Results

Security

Participants : Ilaria Castellani, Francis Doliere Some, Nataliia Bielova, Bernard Serpette, Tamara Rezk [correspondant] .

Hybrid Typing of Secure Information Flow in a JavaScript-like Language

We propose a novel type system for securing information flow in a core of JavaScript. This core takes into account the defining features of the language, such as prototypical inheritance, extensible objects, and constructs that check the existence of object properties. We design a hybrid version of the proposed type system. This version infers a set of assertions under which a program can be securely accepted and instruments it so as to dynamically check whether these assertions hold. By deferring rejection to runtime, the hybrid version can typecheck secure programs that purely static type systems cannot accept.

This work has been published at the 10th International Symposium on Trustworthy Global Computing [11] .

Modular Monitor Extensions for Information Flow Security in JavaScript

Client-side JavaScript programs often interact with the web page into which they are included, as well as with the browser itself, through APIs such as the DOM API, the XMLHttpRequest API, and the W3C Geolocation API. Precise reasoning about JavaScript security must therefore take API invocation into account. However, the continuous emergence of new APIs, and the heterogeneity of their forms and features, renders API behavior a moving target that is particularly hard to capture. To tackle this problem, we propose a methodology for modularly extending sound JavaScript information flow monitors with a generic API. Hence, to verify whether an extended monitor complies with the proposed noninterference property, our methodology requires only to prove that the API satisfies a predefined set of conditions. In order to illustrate the practicality of our methodology, we show how an information flow monitor-inlining compiler can take into account the invocation of arbitrary APIs, without changing the code or the proofs of the original compiler. We provide an implementation of such a compiler with an extension for handling a fragment of the DOM Core Level 1 API. Furthermore, our implementation supports the addition of monitor extensions for new APIs at runtime. This work has been published at the 10th International Symposium on Trustworthy Global Computing [12] .

Relaxed Noninterference

We have began a study concerning the use of gradual typing for down casting or declassification for information flow. The particularity of this work is to use a finite state machine to gradually accept the down casting process.

This work is done with Éric Tanter of University of Santiago de Chile, in the context of the project Conicyt Redes CEV Challenges on Electronic Voting.

Hybrid Monitoring of Attacker knowledge

Enforcement of non-interference requires to prove that an attacker's knowledge about the initial state remains the same after observing a programs public output. We define a powerful hybrid monitoring mechanism which evaluates dynamically the knowledge that is contained in program variables. To get a precise estimate of the knowledge, the monitor statically analyses non-executed branches. We show that our knowledge-based approach can be combined with existing dynamic monitors for non-interference. A distinguishing feature of such a combination is that the combined monitor is provably more powerful than each mechanism taken separately. We demonstrate this by proposing a knowledge-enhanced version of a dynamic monitor based on the no-sensitive-upgrade principle. We show how to use the knowledge computed by our hybrid monitor to quantify information leakage associated to the program output. The monitor and its static analysis has been formalized and proved correct within the Coq proof assistant.

A Taxonomy of Information Flow Monitors

We propose a rigorous comparison of information flow monitors with respect to two dimensions: soundness and transparency.

For soundness, we notice that the standard information flow security definition called Termination-Insensitive Non-interference (TINI) allows the presence of termination channels, however it does not describe whether the termination channel was present in the original program, or it was added by a monitor. We propose a stronger notion of noninterference, that we call Termination-Aware Non-interference (TANI), that captures this fact, and thus allows us to better evaluate the security guarantees of different monitors. We further investigate TANI, and state its formal relations to other soundness guarantees of information flow monitors. For transparency, we identify different notions from the literature that aim at comparing the behaviour of monitors. We notice that one common notion used in the literature is not adequate since it identifies as better a monitor that accepts insecure executions, and hence may augment the knowledge of the attacker. To discriminate between monitors' behaviours on secure and insecure executions, we factorized two notions that we call true and false transparency. These notions allow us to compare monitors that were deemed to be incomparable in the past.

We analyse five widely explored information flow monitors: no-sensitive- upgrade (NSU), permissive-upgrade (PU), hybrid monitoring (HM), se- cure multi-execution (SME), and multiple facets (MF).

This work has been accepted for publication in the International Conference on Principles of Security and Trust (POST 2016).

A Study of JavaScript constructs used in Top Alexa Sites

Several works on JavaScript analysis have shown that including remote scripts can introduce severe security implications in the behavior of the whole web application. To deal with different kinds of attacks, a number of research groups are developing automatic tools to analyze JavaScript programs. However, most of these works rely on one assumption: the scripts are written in a subset of JavaScript language meaning that only certain constructs are used (that are easier to analyse automatically) and others are omitted (for example, eval is impossible to analyze statically). The goal of the internship was to account for the use of each JavaScript construct in real world programs. To achieve that, we first did a large-scale crawl of the top 10,000 Alexa sites, collecting both inlined scripts and remote scripts. Second, we established the popularity of remote scripts. Next, we accounted for the occurrence of JavaScript constructs in the collected programs. Finally, we use the occurrence of different constructs as basis to propose a subset of JavaScript language, which covers most of JavaScript programs found in the wild. One can rely on this evidence-based subset of JavaScript in future works on that language.