Analyzing Ethereum Smart Contract Vulnerabilities at Scale Based on Inter-Contract Dependency

Smart contracts running on public blockchains are permissionless and decentralized,attracting both developers and malicious participants.Ethereum,the world’s largest decentralized application platform on which more than 40 million smart contracts are running,is frequently challenged by smart contract vulnerabilities.What’s worse,since the homogeneity of a wide range of smart contracts and the increase in inter-contract dependencies,a vulnerability in a certain smart contract could affect a large number of other contracts in Ethereum.However,little is known about how vulnerable contracts affect other on-chain contracts and which contracts can be affected.Thus,we first present the contract dependency graph(CDG)to perform a vulnerability analysis for Ethereum smart contracts,where CDG characterizes inter-contract dependencies formed by DELEGATECALL-type internal transaction in Ethereum.Then,three generic definitions of security violations against CDG are given for finding respective potential victim contracts affected by different types of vulnerable contracts.Further,we construct the CDG with 195,247 smart contracts active in the latest blocks of the Ethereum and verify the above security violations against CDG by detecting three representative known vulnerabilities.Compared to previous large-scale vulnerability analysis,our analysis scheme marks potential victim contracts that can be affected by different types of vulnerable contracts,and identify their possible risks based on the type of security violation actually occurring.The analysis results show that the proportion of potential victim contracts reaches 14.7%,far more than that of corresponding vulnerable contracts(less than 0.02%)in CDG.

Risk factors and molecular epidemiology of canine rabies in Beijing

Rabies is a serious public health problem in Asia and Africa.Approximately 99%of cases result from a bite by an infected dog.The high rabies risk countries are generally characterized by rapid urbanization;however,the prevalence and risk factors for rabies in urban dogs in developing countries remain unclear.Beijing,the capital of China,has been undergoing rapid urbanization,and implemented compulsory rabies immunization for dogs on October 1,2014.We evaluated the effects of this immunization policy and investigated the dynamics and determinants of the spread of dog rabies virus(RABV)in Beijing.Between 2013 and 2018,the positive rate of rabies in biting dogs declined significantly;79.6%of these were stray dogs.Reconstructed spatiotemporal diffusion for RABV presents a more central distribution within Beijing and a surrounding dissemination of virus lineage toward suburban areas,indicating that rabies is self-sustaining in Beijing.Continuous RABV phylogeographic reconstructions and logistic regression analysis of rabies cases revealed that rabies tended to remain in commercial areas,high-rise residences,demolition areas,and high-GDP,high-population and high-accessibility areas.Demolition areas,where many residents have abandoned their pets,are recognized as high-risk areas for rabies.Our findings reveal that the implementation of the immunization policy in Beijing has been effective in prevention and control of canine rabies,and because the prevalence of canine rabies results from multiple factors,emphasize the important role of multi-sectoral cooperation in rabies prevention and control.