CVE-2018-15552 in simplelottery
Summary
by MITRE
The "PayWinner" function of a simplelottery smart contract implementation for The Ethereum Lottery, an Ethereum gambling game, generates a random value with publicly readable variable "maxTickets" (which is private, yet predictable and readable by the eth.getStorageAt function). Therefore, it allows attackers to always win and get rewards.
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Analysis
by VulDB Data Team • 03/21/2020
The vulnerability identified in CVE-2018-15552 resides within a smart contract implementation for an Ethereum-based gambling platform known as The Ethereum Lottery. This particular smart contract contains a critical flaw in its PayWinner function that fundamentally undermines the integrity of the gambling system by compromising the randomness of ticket selection. The issue stems from the contract's reliance on a publicly accessible storage variable named maxTickets which, despite being marked as private in the contract's source code, remains readable through Ethereum's eth.getStorageAt RPC method. This accessibility creates a significant security weakness that directly violates the fundamental principles of cryptographic randomness required for fair gaming operations.
The technical flaw manifests when attackers exploit the predictable nature of the maxTickets variable to manipulate the outcome of the lottery game. Since this variable is stored in the Ethereum blockchain's storage layout and can be accessed via the eth.getStorageAt function, malicious actors can determine the maximum number of tickets that can be purchased in a given lottery cycle. This predictable value allows them to calculate winning combinations in advance, effectively enabling them to always select winning tickets and claim rewards without any genuine chance of winning. The vulnerability represents a classic example of improper entropy generation and predictable random number generation, which falls under CWE-330, indicating insufficient entropy in the generation of random numbers.
The operational impact of this vulnerability extends far beyond simple gaming manipulation, as it fundamentally compromises the trust and integrity of the entire smart contract system. Players who participate in the Ethereum Lottery lose confidence in the fairness of the game, while legitimate operators face potential financial losses and reputational damage. The ability for attackers to systematically win rewards means that the smart contract's economic model becomes unsustainable, as it creates an environment where malicious actors can drain the contract's funds indefinitely. This vulnerability also demonstrates a critical failure in smart contract security auditing practices, as the use of publicly accessible storage variables for critical game mechanics violates basic security principles and makes the system vulnerable to exploitation through standard blockchain exploration techniques.
Mitigation strategies for this vulnerability require immediate remediation of the smart contract code to eliminate the dependency on predictable storage variables for random number generation. The recommended approach involves implementing proper entropy sources such as block timestamps, block hashes, or external random number generators to ensure genuine randomness in ticket selection. Security practitioners should also consider implementing the cryptographic randomness standards outlined in NIST SP 800-90A for generating cryptographically secure random numbers in blockchain environments. Additionally, the contract should be restructured to avoid storing critical game parameters in publicly accessible storage slots, instead utilizing secure random generation techniques that are resistant to blockchain-based exploration attacks. The ATT&CK framework's T1059.006 technique for exploitation of predictable random number generators applies directly to this vulnerability, emphasizing the need for proper entropy implementation in smart contract systems to prevent adversarial manipulation of gaming outcomes and maintain the integrity of decentralized applications.