Sereum: Protecting Existing Smart Contracts Against Re-Entrancy Attacks

Recently, a number of existing blockchain systems have witnessed major bugs and vulnerabilities within smart contracts. Although the literature features a number of proposals for securing smart contracts, these proposals mostly focus on proving the correctness or absence of a certain type of vulnerability within a contract, but cannot protect deployed (legacy) contracts from being exploited. In this paper, we address this problem in the context of re-entrancy exploits and propose a novel smart contract security technology, dubbed Sereum (Secure Ethereum), which protects existing, deployed contracts against re-entrancy attacks in a backwards compatible way based on run-time monitoring and validation. Sereum does neither require any modification nor any semantic knowledge of existing contracts. By means of implementation and evaluation using the Ethereum blockchain, we show that Sereum covers the actual execution flow of a smart contract to accurately detect and prevent attacks with a false positive rate as small as 0.06% and with negligible run-time overhead. As a by-product, we develop three advanced re-entrancy attacks to demonstrate the limitations of existing offline vulnerability analysis tools

EVMPatch: Timely and Automated Patching of Ethereum Smart Contracts

Recent attacks exploiting errors in smart contract code had devastating consequences thereby questioning the benefits of this technology. It is currently highly challenging to fix errors and deploy a patched contract in time. Instant patching is especially important since smart contracts are always online due to the distributed nature of blockchain systems. They also manage considerable amounts of assets, which are at risk and often beyond recovery after an attack. Existing solutions to upgrade smart contracts depend on manual and error-prone processes. This paper presents a framework, called EVMPatch, to instantly and automatically patch faulty smart contracts. EVMPatch features a bytecode rewriting engine for the popular Ethereum blockchain, and transparently/automatically rewrites common off-the-shelf contracts to upgradable contracts. The proof-of-concept implementation of EVMPatch automatically hardens smart contracts that are vulnerable to integer over/underflows and access control errors, but can be easily extended to cover more bug classes. Our extensive evaluation on 14,000 real-world (vulnerable) contracts demonstrate that our approach successfully blocks attack transactions launched on these contracts, while keeping the intended functionality of the contract intact. We perform a study with experienced software developers, showing that EVMPatch is practical, and reduces the time for converting a given Solidity smart contract to an upgradable contract by 97.6 %, while ensuring functional equivalence to the original contract.Comment: A slightly shorter version of this paper will be published at USENIX Security Symposium 202

Two Bitcoins at the Price of One? Double-Spending Attacks on Fast Payments in Bitcoin

Bitcoin is a decentralized payment system that is based on Proof-of-Work. Bitcoin is currently gaining popularity as a digital currency; several businesses are starting to accept Bitcoin transactions. An example case of the growing use of Bitcoin was recently reported in the media; here, Bitcoins were used as a form of fast payment in a local fast-food restaurant. In this paper, we analyze the security of using Bitcoin for fast payments, where the time between the exchange of currency and goods is short (i.e., in the order of few seconds). We focus on double- spending attacks on fast payments and demonstrate that these attacks can be mounted at low cost on currently deployed versions of Bitcoin. We further show that the measures recommended by Bitcoin developers for the use of Bitcoin in fast transactions are not always effective in resisting double-spending; we show that if those recommendations are integrated in future Bitcoin implementations, double-spending attacks on Bitcoin will still be possible. Finally, we leverage on our findings and propose a lightweight countermeasure that enables the detection of double-spending attacks in fast transactions

Tampering with the Delivery of Blocks and Transactions in Bitcoin

Given the increasing adoption of Bitcoin, the number of transactions and the block sizes within the system are only expected to increase. To sustain its correct operation in spite of its ever-increasing use, Bitcoin implements a number of necessary optimizations and scalability measures. These measures limit the amount of information broadcast in the system to the minimum necessary. In this paper, we show that current scalability measures adopted by Bitcoin come at odds with the security of the system. More specifically, we show that an adversary can exploit these measures in order to effectively delay the propagation of transactions and blocks to specific nodes—without causing a network partitioning in the system. We show that this allows the adversary to easily mount Denial-of-Service attacks, considerably increase its mining advantage in the network, and double-spend transactions in spite of the current countermeasures adopted by Bitcoin. Based on our results, we propose a number of countermeasures in order to enhance the security of Bitcoin without deteriorating its scalability

Securing Cloud Data in the New Attacker Model

The world just witnessed the surge of a new and powerful attacker, which was able to coerce operators and acquire the necessary keys to break the privacy of users. Once the encryption key is exposed, the only viable measure to preserve data confidentiality is to limit the adversary’s access to the ciphertext. This may be achieved, for example, using multi-cloud storage systems. These systems spread data across multiple servers in different administrative domains, to cater for availability and fault tolerance. If the adversary can only compromise a subset of these domains, multi-cloud storage systems may prevent the adversary from accessing the entire ciphertext. However, if data is encrypted using existing encryption schemes, spreading the ciphertext on multiple servers does not entirely solve the problem since an adversary which has the encryption key, can still compromise single servers and decrypt the ciphertext stored therein. In this paper, we leverage multi-cloud storage systems to provide data confidentiality against an adversary which has access to the encryption key, and can compromise a large fraction of the storage servers. For this purpose, we first introduce a novel security definition that captures data confidentiality in the new adversarial model. We then propose Bastion, a primitive that is secure according to our definition and, therefore, guarantees data confidentiality even when the encryption key is exposed, as long as the adversary cannot compromise all storage servers. We analyze the security of Bastion, and we evaluate its performance by means of a prototype implementation. Our results show that Bastion incurs less than 5% overhead compared to existing semantically secure encryption modes. We also discuss practical insights with respect to the integration of Bastion in commercial multi-cloud storage systems

Towards Shared Ownership in the Cloud

Cloud storage platforms promise a convenient way for users to share files and engage in collaborations, yet they require all files to have a single owner who unilaterally makes access control decisions. Existing clouds are, thus, agnostic to the notion of shared ownership. This can be a significant limitation in many collaborations because, for example, one owner can delete files and revoke access without consulting the other collaborators. In this paper, we first formally define a notion of shared ownership within a file access control model. We then propose two possible instantiations of our proposed shared ownership model. Our first solution, called Commune, relies on secure file dispersal and collusion-resistant secret sharing to ensure that all access grants in the cloud require the support of an agreed threshold of owners. As such, Commune can be used in existing clouds without modifications to the platforms. Our second solution, dubbed Comrade, leverages the blockchain technology in order to reach consensus on access control decision. Unlike Commune, Comrade requires that the cloud is able to translate access control decisions that reach consensus in the blockchain into storage access control rules, thus requiring minor modifications to existing clouds. We analyze the security of our proposals and compare/evaluate their performance through implementation integrated with Amazon S3

On the Security and Performance of Proof of Work Blockchains

Proof of Work (PoW) powered blockchains currently account for more than 90% of the total market capitalization of existing digital currencies. Although the security provisions of Bitcoin have been thoroughly analysed, the security guarantees of variant (forked) PoW blockchains (which were instantiated with different parameters) have not received much attention in the literature. In this paper, we introduce a novel quantitative framework to analyse the security and performance implications of various consensus and network parameters of PoW blockchains. Based on our framework, we devise optimal adversarial strategies for double-spending and selfish mining while taking into account real world constraints such as network propagation, different block sizes, block generation intervals, information propagation mechanism, and the impact of eclipse attacks. Our framework therefore allows us to capture existing PoW-based deployments as well as PoW blockchain variants that are instantiated with different parameters, and to objectively compare the tradeoffs between their performance and security provisions

PoTS - A Secure Proof of TEE-Stake for Permissionless Blockchains

Proof-of-Stake (PoS) protocols have been actively researched for the past few years. PoS finds direct applicability in permissionless blockchain platforms and emerges as one of the strongest candidates to replace the largely inefficient Proof of Work mechanism that is currently plugged in the majority of existing permissionless blockchain systems. Although a number of PoS variants have been proposed, these protocols suffer from a number of security shortcomings. Namely, most existing PoS variants are either subject to the nothing at stake, the long range, or the stake grinding attacks which considerably degrade security in the blockchain. These shortcomings do not result from a lack of foresight when designing these protocols, but are inherently due to the ease of manipulating stake when compared to other more established variants, such as work . In this paper, we address these problems and propose a secure Proof of Stake protocol, PoTS, that leverages Trusted Execution Environments (TEEs), such as Intel SGX, to ensure that each miner can generate at most one block per height for strictly increasing heights—thus thwarting the problem of nothing at stake and a large class of long-range attacks. In combination with TEEs, PoTS additionally uses cryptographic techniques to also prevent grinding attacks and protect against posterior corruption. We show that our protocol is secure, in the sense of well-established cryptographic notions for blockchain protocols, down to realistic hardware assumptions on TEE and well-established cryptographic assumptions. Finally, we evaluate the performance of our proposal by means of implementation. Our evaluation results show that PoTS offers a strong tradeoff between security of performance of the underlying PoS protocol