Android platform is gaining explosive popularity. This leads developers to invest resources to maintain the upward trajectory of the demand. Unfortunately, as the profit potential grows higher, the chances of these Apps getting attacked also get higher. Therefore, developers improved the security of their Apps, which limits attackers ability to compromise upgraded versions of the Apps. However, developers cannot enhance the security of earlier versions that have been released on the Play Store. The earlier versions of the App can be subject to reverse engineering and other attacks. In this paper, we find that attackers can use these earlier versions as attack vectors, which threatens well protected upgraded versions. We show how to attack the upgraded versions of some popular Apps, including Facebook, Sina Weibo and Qihoo360-Cloud-Driven by analyzing the vulnerabilities existing in their earlier versions. We design and implement a tool named DroidSkynet to analyze and find out vulnerable apps from the Play Store. Among 1,500 mainstream Apps collected from the real world, our DroidSkynet indicates the success rate of attacking an App using an earlier version is 34 percent. We also explore possible mitigation solutions to achieve a balance between utility and security of the App update process.

Crowdsourcing systems which utilize the human intelligence to solve complex tasks have gained considerable interest and adoption in recent years. However, the majority of existing crowdsourcing systems rely on central servers, which are subject to the weaknesses of traditional trust-based model, such as single point of failure. They are also vulnerable to distributed denial of service (DDoS) and Sybil attacks due to malicious users involvement. In addition, high service fees from the crowdsourcing platform may hinder the development of crowdsourcing. How to address these potential issues has both research and substantial value. In this paper, we conceptualize a blockchain-based decentralized framework for crowdsourcing named CrowdBC, in which a requester's task can be solved by a crowd of workers without relying on any third trusted institution, users' privacy can be guaranteed and only low transaction fees are required. In particular, we introduce the architecture of our proposed framework, based on which we give a concrete scheme. We further implement a software prototype on Ethereum public test network with real-world dataset. Experiment results show the feasibility, usability, and scalability of our proposed crowdsourcing system.

Extensive online knowledge sharing can be exploited to solve tasks better, faster and cheaper, and it garners considerable interest in institutional cooperation, learning communities, etc., while many technical problems, such as fair exchange in incentive design and security issues are waiting to be solved. Blockchain technique has high accountability and thus has potential to improve the transparency and security of knowledge sharing. In this paper, to address the above problems, first we propose a Reputation Based Knowledge Sharing system in blockchain, called RBKS. The aim of RBKS is to exploit the copyright protection of the knowledge owner using our proposed fine-grained access control system, and to achieve the paid-for content service which allows bystanders who are interested in the shared knowledge to pay a small fee for the access. Second, a reputation evaluation algorithm is introduced as the core of the incentive, and it may possibly be combined with stake in blockchain to form a hybrid stake for the RBKS system. Third, a blockchain and a trusted storage server are employed together for sharing and storing knowledge, and the main procedures are implemented with smart contract in blockchain to ensure secure execution and fairness. Finally, our analysis shows that the RBKS system is feasible and secure.