CVE-2018-17987 in HashHeroes Tiles
Summary
by MITRE
The determineWinner function of a smart contract implementation for HashHeroes Tiles, an Ethereum game, uses a certain blockhash value in an attempt to generate a random number for the case where NUM_TILES equals the number of people who purchased a tile, which allows an attacker to control the awarding of the prize by being the last person to purchase a tile.
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Analysis
by VulDB Data Team • 04/24/2020
The vulnerability identified as CVE-2018-17987 resides within the HashHeroes Tiles smart contract implementation on the Ethereum blockchain, representing a critical flaw in the game's random number generation mechanism. This issue specifically affects the determineWinner function which attempts to utilize blockhash values to create randomness for determining prize distribution when the total number of purchased tiles matches the predefined number of tiles available. The fundamental problem lies in the deterministic nature of Ethereum blockhashes, which can be manipulated by miners or attackers who control the mining process. When an attacker identifies that they can influence the outcome by being the final purchaser of a tile, they gain complete control over the prize allocation process, effectively allowing them to guarantee their own victory regardless of other participants' contributions.
The technical implementation flaw stems from the improper use of blockchain data as a source for random number generation, violating established cryptographic principles and security best practices. According to CWE-330, this represents an insufficient entropy vulnerability where the system fails to generate cryptographically secure random values. The attacker's strategy involves monitoring the transaction queue and strategically timing their purchase to be the last transaction included in a block, thereby ensuring that the blockhash used in the random number calculation favors their position. This attack vector directly correlates with ATT&CK technique T1499.001, which describes the manipulation of system processes to achieve desired outcomes through blockchain transaction control.
The operational impact of this vulnerability extends beyond simple game mechanics to represent a fundamental security failure that undermines the integrity of the entire smart contract system. Players who purchase tiles in anticipation of fair prize distribution find themselves at risk of losing their investments when an attacker exploits this weakness. The vulnerability creates an economic incentive for malicious actors to actively monitor and manipulate the game's transaction flow, potentially leading to significant financial losses for legitimate participants. Additionally, this flaw demonstrates poor understanding of blockchain security principles where developers fail to recognize that blockhashes are not suitable for random number generation due to their predictable nature and the ability of miners to influence their values. The attack can be executed with minimal technical expertise and requires only knowledge of the game's transaction patterns and timing strategies, making it particularly dangerous for decentralized applications that rely on fair randomization for their core functionality.
Mitigation strategies must address both immediate remediation and long-term architectural improvements to prevent similar vulnerabilities in future smart contract implementations. The most effective solution involves replacing the blockhash-based random number generation with a verifiable random function or external oracle service that provides truly unpredictable values. This approach aligns with security recommendations from the Ethereum Foundation and other blockchain security organizations that emphasize the importance of using external randomness sources for critical applications. Additionally, implementing a commitment scheme where players must reveal their intentions before the final transaction can prevent last-minute manipulation attacks. The system should also incorporate multiple sources of entropy and avoid relying on any single blockchain parameter that can be influenced by malicious actors. Proper testing and auditing of smart contracts before deployment, including adversarial testing scenarios, can help identify such vulnerabilities early in the development lifecycle and ensure that the security model remains robust against various attack vectors.